Radio engineering measurements. where W is the specific counteracting moment, depending on the properties of the elastic element. Statistical processing of measurement results

ANALOG ELECTROMECHANICAL INSTRUMENTS

General information

In analog electromechanical measuring instruments for direct assessment, electromagnetic energy supplied to the device directly from the circuit being measured is converted into mechanical energy of the angular movement of the moving part relative to the stationary one.

Electromechanical measuring instruments (EIM) are used to measure current, voltage, power, resistance and other electrical quantities on direct and alternating currents, mainly at an industrial frequency of 50 Hz. These devices are classified as direct action devices. They consist of an electrical transducer (measuring circuit), an electromechanical transducer (measuring mechanism), and a reading device (Fig. 5.1).

Rice. 5.1. Block diagram of analog EIP

Measuring circuit. It ensures the transformation of the electrical measured quantity X into some intermediate electrical quantity Y (current or voltage), functionally related to the measured quantity X. The Y quantity directly affects the measuring mechanism (MM).

According to the nature of the transformation, the measuring circuit can be a set of elements (resistors, capacitors, rectifiers, thermocouples, etc.). Various measuring circuits make it possible to use the same MM when measuring heterogeneous quantities, voltage, current, resistance, varying over a wide range.

Measuring mechanism. Being the main part of the design of the device, it converts electromagnetic energy into mechanical energy necessary for the angle of deflection a of its moving part relative to the stationary one, i.e.

α = f(Y) = F(X).

The moving part of the IM is a mechanical system with one degree of freedom relative to the axis of rotation. Momentum equal to the sum moments acting on the moving part.

The differential equation of moments describing the operation of the IM has the form

J( d 2 α/ dt 2) = Σ M, (5.1)

where J is the moment of inertia of the moving part of the IM; α - angle of deflection of the moving part; d 2 α/ dt 2 - angular acceleration.

The moving part of the MI during its movement is affected by:

torque M , determined for all EIP by the rate of energy change electromagnetic field w e, concentrated in the mechanism, according to the angle of deflection α of the moving part. Torque is a function of the measured quantity X, and therefore Y (current, voltage, product of currents) and α:

M= (∂w e /∂α) = f(α) Y n , (5.2)

counter moment M α, created mechanically using spiral springs, braces, lead wires and proportional to the deflection angle α of the moving part:

M α = - Wα, (5.3)

Where W- specific counteracting moment per unit angle of twist of the spring (depends on the material of the spring and its geometric dimensions);

moment of calm M usp, i.e. the moment of forces of resistance to movement, always directed towards the movement and proportional to the angular velocity of the deflection:

M successful =- R (dα/ d t), (5.4)

Where R- damping coefficient.

Substituting (5.2) - (5.4) into (5.1), we obtain the differential equation for the deflection of the moving part of the mechanism:

J( d 2 α/ dt 2) = M + M α + M usp, (5.5)

J( d 2 α/ dt 2) + R (dα/ d t) + Wα = M. (5.6)

The steady deflection of the moving part of the MI is determined by the equality of the torque and counteracting moments, i.e. M = Mα , if the first two terms of the left side differential equation(5.6) are equal to zero. Substituting into equality M = Mα analytical expressions of the moments, we obtain the equation of the instrument scale, showing the dependence of the deviation angle a of the moving part on the value of the measured quantity and the MI parameters.

Depending on the conversion method electromagnetic energy In the mechanical angular movement of the moving part of the IM, electromechanical devices are divided into magnetoelectric, electrodynamic, ferrodynamic, electromagnetic, etc.

Analog EIP reading device. Most often, it consists of a pointer rigidly connected to the moving part of the IM and a fixed scale. There are arrow (mechanical) and light indicators. The scale is a set of marks that are located along a line and depict a series of sequential numbers corresponding to the values ​​of the quantity being measured. Marks take the form of strokes, dashes, dots, etc.

According to the scale There are rectilinear (horizontal or vertical), arc (for an arc up to 180° inclusive) and circular (for an arc of more than 180°).

By the nature of the location of the marks There are scales that are uniform and uneven, one-sided relative to zero, two-sided and non-zero. Scales are graduated either in units of the measured value (named scale) or in divisions (unnamed scale). The numerical value of the measured quantity is equal to the product of the number of divisions read on the scale and the price (constant) of the device. Division value is the value of the measured quantity corresponding to one division of the scale.

Since EIPs are direct action devices, the sensitivity of the device S p is determined by the sensitivity of the circuit S c and the sensitivity of the measuring mechanism S and:

S p = S c S and (5.7)

Analog EIP accuracy classes: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0.

Units and parts of measuring instruments. For most EIP, despite the diversity of the IM, it is possible to identify common components and parts - devices for installing the moving part of the IM, to create a counteracting moment, balancing and calming

.

Rice. 5.2. Installation of the moving part of the measuring mechanism

Since any EIP measuring mechanism consists of a moving and a fixed part, to ensure free movement of the moving part, the latter is installed on supports (Fig. 5.2, a), guy wires (Fig. 5.2,6), and a suspension (Fig. 5.2, c). During transportation, the moving part of the MI is secured motionless using a lock.

Devices for installing the moving part on supports They are a lightweight aluminum tube into which cores (steel pieces) are pressed. The ends of the cores are sharpened and ground to a rounded cone. The cores are supported on agate or corundum bearings. When installing the moving part of the MI on cores, friction occurs between the core and the thrust bearing, which introduces an error in the instrument readings. In devices high class precision (laboratory) to reduce friction, the scale is installed horizontally and the axis vertically. In this case, the load is concentrated mainly on the lower support.

Devices for installing the moving part on guy wires They are two thin belts made of a bronze alloy on which the movable part of the IM is suspended.

Rice. 5.3. General details of the moving part of the IM on supports

Their presence ensures the absence of friction in the supports, facilitates the moving system, and increases vibration resistance. Stretch bars are used to supply current to the frame winding and create a counteracting torque.

Devices for installing moving parts on suspensions used in particularly sensitive devices. The moving part of the MI is suspended on a thin metal (sometimes quartz) thread. Current is supplied to the frame of the moving part through a suspension thread and a special torque-free current lead made of gold or silver.

To create a counteracting moment in IM with the installation of the moving part on supports (Fig. 5.3), one or two flat spiral springs 5 ​​and 6, made of tin-zinc bronze, are used. The springs also serve as current leads to the winding of the frame of the moving part. One end of the spring is attached to the axle or axle shaft, and the other - to the driver 4 of the corrector. The corrector, which sets the pointer 3 of a device that is not turned on to zero, consists of a screw 9 with an eccentrically located pin 8 and a fork 7 with a leash. The corrector screw 9 is brought out to the front panel of the device body; when rotating, it moves the fork 7, which causes the spring to twist and, accordingly, the pointer 3 to move. Axis 2 ends with cores resting on thrust bearings 1.

To balance the moving part counterweight weights 10 serve.

Rice. 5.4. Schemes of magnetic induction (a) and air (b) dampers

The measuring mechanism is considered balanced when the center of gravity of the moving part coincides with the axis of rotation. A well-balanced measuring mechanism shows the same value of the measured quantity at different positions.

To create the necessary sedation for MI They are equipped with dampers that develop a torque directed towards the movement (soothing time no more than 4 s). In MI, magnetic induction and air calmers are most often used, less often liquid ones (when very high calming is required).

Magnetic induction damper (Fig. 5.4, o) consists of a permanent magnet 1 and an aluminum disk 2, rigidly connected to the moving part of the mechanism and freely moving in the field of the permanent magnet. Calming is created due to the interaction of currents induced in the disk when it moves in the magnetic field of a permanent magnet with the flux of the same magnet.

Air damper (Fig. 5.4, b) is a chamber / in which a lightweight aluminum wing (or piston) 2 moves, rigidly connected to the moving part of the IM. When air moves from one part of the chamber to another through the gap (between the chamber and the wing), the movement of the wing is slowed down and the vibrations of the moving part quickly die out. Air dampers are weaker than magnetic induction dampers.

Logometers

Ratiometers are devices of the electromechanical group that measure the ratio of two electrical quantities Y 1 and Y 2:

α = F(Y 1 / Y2) n, (5.41)

where n is a coefficient depending on the MI system.

The peculiarity of ratiometers is that the rotating M and counteracting M α moments in them are created electrically, therefore the ratiometer has two sensing elements, which are affected by the quantities Y 1 and Y 2 that make up the measured ratio. The directions of the quantities Y 1 and Y 2 must be chosen such that the moments M and M α acting on the moving part are directed towards each other; in this case, the moving part will rotate under the influence of a larger moment. To fulfill these conditions, the moments M and M α must depend differently on the angle of deflection of the moving part of the device.

The sources of logometer error are the non-identical design of the two sensing elements, especially in the presence of ferromagnetic materials; the presence in the ratiometer of additional moments M additional (from friction in the supports, momentless connections, imbalance of the moving part). Hence,

M = M α + M add. (5.42)

The presence of an additional moment M additional makes the ratiometer readings dependent on secondary factors (for example, voltage). Therefore, the logometer scale indicates the operating voltage range within which the scale calibration is valid. The upper voltage limit is determined by the maximum power released in the ratiometer circuits, and the lower limit is determined by M add. The needle, which is not connected to the voltage of the ratiometer, occupies an indifferent position due to the absence of a mechanical counteracting moment.

Rice. 5.18. The mechanism of the magnetoelectric ratiometer

The operation of a magnetoelectric ratiometer is as follows.

The movable part of the IM is placed in the uneven magnetic field of a permanent magnet (Fig. 5.18), containing two frames, rigidly fastened at an angle d = 30°-90° and mounted on a common axis. Currents I 1 and I 2 are supplied to the frames using torqueless current leads. The direction of the currents is such that the current I 1 creates a torque, and I 2 creates a counteracting moment:

M = I 1 (∂Ψ 1 /∂α); M α = I 2 (∂Ψ 2 /∂α), (5.43)

where Ψ 1, Ψ 2 are the flows created by the magnet and coupled to the frames.

The moments M and M α change depending on the change in angle α. The maximum values ​​of the moments will be shifted by an angle d, which makes it possible to obtain a decrease in M ​​and an increase in M ​​α in the working area. At equilibrium, I 1 (∂Ψ 1 /∂α) = I 2 (∂Ψ 2 /∂α), whence

where f 1 (α), f 2 (α) are quantities that determine the rate of change in flux linkage.

From the equality of moments it follows that

α = F(I 1 / I 2) (5.45)

If the ratio of currents is expressed through the desired value X, then

α = F 1 (X). (5.46)

The existence of this functional dependence is possible if the main operating conditions of the ratiometer are met, i.e. at ∂Ψ 1 /∂α ≠ ∂Ψ 2 /∂α, which is ensured by artificially created unevenness of the magnetic field in the air gap of the ratiometer. Magnetoelectric ratiometers are used to measure resistance, frequency and non-electrical quantities,

Electro-radiotechnical measurements

The book discusses the basic methods of measuring electrical and radio engineering quantities on direct current and alternating current in a wide range of frequencies. Measuring circuits, their principles of construction are described and specifications the most widely used measuring instruments. Examples of calculations are given to facilitate the assimilation of the material. The textbook can be used when vocational training workers in production.

Basic definitions. Features and methods of measurements.
A qualitatively common property of many physical objects (physical systems, their states, processes occurring in them) is called a physical quantity. In electrical and radio engineering, physical quantities are electrical voltage, current strength, power, energy, as well as electrical resistance, electrical capacitance, inductance, frequency.

A physical quantity can have different meanings. A certain value is taken as a unit of measurement of a physical quantity. As a rule, this value is one

The measurement of a given physical quantity is the determination of its value experimentally. Quantitative result, i.e. the measurement result is obtained by comparing the found value of a physical quantity with its unit of measurement.

TABLE OF CONTENTS
Introduction
Chapter first. General information about measurements
§1. Basic definitions. Features and measurement methods
§2. Physical quantities and their units of measurement
§3. Measurement errors
§4. Classification and designation system of measuring instruments
Chapter two. Electromechanical measuring instruments
§5. General information
§6. Magnetoelectric system devices
§7. Electromagnetic system devices
§8. Devices of electro-, ferrodynamic and induction systems
§9. Electrostatic system devices
Chapter three. Measurement direct current and voltage
§10. Measuring direct current with a magnetoelectric device
§eleven. Measuring direct current with an electronic microammeter
§12. Measuring DC voltage with a magnetoelectric device
§13. Measuring DC voltage with electronic devices
Chapter Four. Measurement alternating current and voltage
§14. General information
§15. Thermoelectric system devices
§16. Rectifier system devices
§17. Ammeters and voltmeters of the rectifier system
§18. Combined instruments
§19. Electronic voltmeters
§20. Digital voltmeters
Chapter Five. Measuring the parameters of electrical elements radio circuits
§21. General information
§22. Direct reading ohmmeters
§23. Voltmeter - ammeter method
§24. Bridge method
§25. Resonance method
Chapter six. Measurement of parameters of diodes, transistors and vacuum tubes
§26. Diode parameters measurement
§27. Parameter measurement bipolar transistors
§28. Measuring parameters of field-effect transistors
§29. Vacuum tube testing
Chapter seven. Measuring generators
§thirty. General information
§31. Signal generators low frequencies
§32. High Frequency Signal Generators
§33. Microwave Signal Generators
§34. Pulse signal generators
Chapter eight. Electronic oscilloscopes
§35. General information
§36. Cathode-ray tube
§37. Oscilloscope sweeps
§38. Ramp voltage generators
§39. Control channels
§40. Measurement of voltages and time intervals
Chapter Nine. Frequency measurement
§41. General information
§42. Oscillographic frequency comparison method
§43. Comparison of frequencies based on zero beats
§44. Resonant frequency measurement method
§45. Direct-reading analog frequency meters
§46. Direct indicating electronic frequency counters
Chapter ten. Measurement of parameters of modulated oscillations and spectrum
§47. Measuring parameters of modulated oscillations
§48. Spectrum survey
§49. Measurement nonlinear distortion
Chapter Eleven. Measurements in distributed constant circuits
§50. Measuring lines
§51. Power measurement
Literature.

BASIC RADIO-ELECTRONIC MEASUREMENTS AND MEASURING INSTRUMENTS

Radio electronic measurements and radio measuring instruments are widely used in the work of experimental physicists and research engineers of any specialty. Measurement is finding the value of a physical quantity experimentally using special technical means. In radio electronics, the objects of measurement are the parameters and characteristics of radio-electronic circuits and signals, and the means of measurement are radio measuring instruments. Radio-electronic measurements have the following features.

1. Diversity in character.

From this point of view, radio-electronic measuring instruments are divided into four groups:

The first group is measuring generators. They serve to simulate signals during setup and configuration radio-electronic equipment, measuring some signal parameters by methods of comparison, power supply and calibration of measuring equipment.

The second group is instruments for measuring parameters and characteristics of signals. A feature of this group of devices is the need to supply measured signals to the device input. The output of the device is quantitative information about one or another signal parameter. This group includes such measuring instruments as oscilloscopes, electronic voltmeters, frequency meters, phase meters, spectrum analyzers, etc.

The third group is instruments for measuring the characteristics and parameters of quadripole networks, as well as various components of radio-electronic circuits. A feature of the devices of this group is the presence in them of signal generators of a certain shape that supply the two-terminal network or node under study, and measuring devices that make it possible to evaluate the passage of these oscillations through a given four-terminal network or node. Examples of devices of the third group are measuring bridges, Q-meters, meters frequency characteristics(characterographers), etc.

The fourth group is elements of measuring circuits. This includes separately manufactured and calibrated attenuators, phase shifters, instrument transformers, etc.

2. Wide range of measured values, sometimes reaching 10-12 orders of magnitude.

3. Low power of measured signals.

During the measurement process, the value being determined is compared with a known value, taken as a unit and called a standard measure. For this purpose, the scale of measuring instruments is calibrated. When measuring, a count is taken - a number indicated by the device indicator. Indication is a physical quantity corresponding to a reading and obtained by multiplying the reading by a conversion factor.

2.2. Measuring generators.

In a measuring generator, the frequency, shape and voltage of the simulated signal are set equal to the required value and can be adjusted within wide limits. Based on the shape of the output signals, measuring generators are divided into sinusoidal signal generators, pulse signal generators and noise signal generators.

Sine Wave Generators in turn, are divided into low-frequency (sound) with a frequency of 20 Hz ÷ 200 kHz, high-frequency with a frequency of 100 kHz ÷ 30 MHz and ultra-high-frequency.

Sound generators (SG) produce a signal with a voltage ranging from tens of microvolts to 30 volts. These generators are usually made according to a multi-stage circuit (Fig. 1), which makes it possible to eliminate the influence of the load on the stability of the generated signal and obtain sufficient power at the load. The master oscillator is usually a two-stage RC oscillator with a Wien chain feedback. A step change in frequency is carried out by switching the capacitance C, and a smooth change is carried out by changing the resistance R. The broadband amplifier is a push-pull power amplifier connected to the master oscillator through a phase inversion stage.

Rice. 1. Block diagram of a sinusoidal signal generator

Next, the signal goes to the output device, consisting of an attenuator and a matching device. An attenuator is a voltage divider with a signal attenuation coefficient that does not depend on frequency. The output attenuator changes the voltage in steps, and within each step (range) smooth adjustment is carried out in a broadband amplifier. The voltage meter is connected to the output of the amplifier, which greatly simplifies its design, since in this case it operates only in one signal voltage range. The output voltage is equal to the meter voltage multiplied by the attenuator division factor. To ensure stability of the attenuator's division ratio, the load at its output must be constant (usually 600 Ohms). If the load resistance differs from this value, it is matched with the attenuator using a matching device consisting of a transformer and an internal load. The internal load is turned on if the load resistance, taking into account the transformation ratio, significantly exceeds 600 Ohms. The transformer output also allows for easy symmetrical output. In the latter case, the middle of the secondary winding of the output transformer is grounded. When making measurements, it is often not the signal voltage that is used, but its level in decibels, determined by the formula:

U=20 log(U/U 0) (dB).

The zero level is most often taken to be the voltage U0 that creates a dissipated power of 1 mW at a resistance of 600 Ohms. Sometimes a voltage equal to one volt is taken as the zero level.

Generators standard signals(GSS, group G4) produce high-frequency (carrier) sinusoidal signals calibrated in frequency, output voltage and shape, which can be simulated from both an internal and external low-frequency generator. The source of high-frequency voltage is a tunable high-frequency self-oscillator (Fig. 2), which is an LC generator of sinusoidal oscillations.

Rice. 2. Block diagram of a standard signal generator

The amplifier-modulator is a high-frequency amplifier, which in modulation mode also performs the functions of a modulator. The output device consists of a smooth attenuator, then a step and sometimes a remote divider located at the end of the cable. The position of the smooth attenuator is calibrated using a scale. The Carrier Voltage and Modulation Depth Meter is an electronic voltmeter with high frequency (HF) and low frequency (LF) signal detectors. The output impedance of the GSS in most cases is tens of ohms and is matched to the cable.

Pulse generators(GI, group G5) are a source of pulse signals of a certain shape (most often rectangular). A diagram of a typical GI is shown in Fig. 3. The master oscillator generates the pulses necessary to start the pulse generation unit, as well as to output clock pulses from this device. Autogenerators of sinusoidal oscillations with subsequent two-way limitation or relaxation generators can be used as a master oscillator. The main pulse shaper is launched with a settable time delay relative to the output of the synchronization pulse. The delay of the main pulse relative to the synchronization pulse is widely used in the application of generators. So, when using an oscilloscope, a synchronizing pulse triggers the sweep of the oscilloscope, and the main pulse is fed to the circuit under study and through it to the oscilloscope. In this case, the leading edge of the pulse is clearly visible on the oscilloscope screen.

Rice. 3. Block diagram of the pulse generator

The operating principle of the pulse generation unit is as follows. The triggering pulse, arriving at the relaxer and causing it to overturn, forms the leading edge of the measuring pulse. At the same time, the triggering pulse, passing through the internal delay line equal to the pulse duration τ, is applied to the other input of this relaxer, causing it to roll over to its original state and thereby forming the trailing edge of the main pulse with duration τ. The output amplifier is a broadband amplifier that provides the required amplitude of measuring pulses at the output. The output device consists of a phase-inverted cascade to obtain output pulses of the required polarity, an emitter follower to ensure a given value of the internal resistance of the generator and an attenuator. Amplitude meters usually work using the method of comparison with a reference voltage.

2.3. Cathode ray oscilloscopes.

An oscilloscope is designed for visual observation of electrical signals and measurement of their parameters. This is a universal device that allows you to measure voltage, frequency, phase difference, time intervals and other signal parameters. In Fig. Figure 4 shows the block diagram of the oscilloscope. The main unit of the oscilloscope is a cathode ray tube, which forms a narrow electron beam that falls on a luminescent screen and describes the shape of the signal under study, supplied to the vertical deflection plates, provided that a linearly varying voltage is applied to the horizontal deflection plates, ensuring the movement of the electron beam in the horizontal direction with at a constant speed, i.e. directly proportional to time. The formation of the electron beam is carried out by a modulator (M), which operates by analogy with the control grid of an electron tube and regulates the number of electrons in the beam (brightness). Anodes A1 and A2 are designed to focus electrons on the tube screen. The AZ anode serves to increase the speed of electrons in the beam, which is important for exciting the phosphor of the screen.

Rice. 4. Block diagram of a cathode ray oscilloscope

Let's briefly consider the operation and purpose of the remaining components of the oscilloscope. The signal under study is fed via a coaxial cable through an input divider to an emitter follower, the load of which is a delay line (usually a piece of cable). The emitter follower has a high input impedance and low input capacitance, which contributes to the absence of distortion of weak signals. Thanks to its low output impedance it is matched to wave impedance delay lines. The delay in signal arrival relative to the start of the sweep makes it possible to observe the leading edge of the signal, especially in the mode of internal sweep triggering from the signal itself when it reaches a certain level. After amplification, the signal enters the vertical deflection plates of the tube, deflecting the beam on the screen vertically in proportion to the signal voltage. The horizontal displacement of the beam, proportional to time, is carried out by a sawtooth voltage generated by the scan generator and supplied to the horizontal deflection plates. The start mode of the sweep generator can be standby or periodic. The sweep is triggered in standby mode either from an external synchronizing signal from input X (external trigger) or from the signal under study (internal trigger). In periodic mode, the scan generator is started periodically either from the network, or operates automatically with its own, but adjustable frequency. Some oscilloscopes have a horizontal deflection amplifier (X amplifier) ​​that can be connected to the horizontal deflection plates instead of a sweep generator. In this case, the horizontal deflection of the beam becomes proportional to the voltage at the X input. This makes it possible to obtain on the screen the dependence of the Y signal on the X signal, for example, the current-voltage characteristics of devices. Ancillary devices include amplitude and duration calibrators. In multibeam oscilloscopes, the cathode ray tube has several electron beam formers, horizontal deflection plates common to all beams, but vertical deflection plates separate for each beam. In this case, there are several Y inputs and several vertical deflection amplifiers (according to the number of beams). These oscilloscopes allow you to scan multiple signals simultaneously. Multichannel oscilloscopes have a normal, single-beam tube, only signals are supplied to it alternately from several Y inputs using a switch. Storage oscilloscopes have a memory device that stores the signal and then supplies it to the plates after the signal has been applied. This makes it possible to observe the sweep of signals of very short duration (nanoseconds) over a long period of time (minutes).

2.4. Voltage and current measurements.

Current and voltage measurements are fundamental when examining various devices and monitoring their operation. However, in radio engineering, voltage measurement is of predominant importance, and current measurement is resorted to in quite rare cases, trying to replace it by measuring voltage across a known resistance and then determining the current according to Ohm's law. The measured variables voltage and current are evaluated by the following parameters (Fig. 5): amplitude, average, average rectified and effective (effective) values.

Rice. 5. Parameters AC voltage

The amplitude (peak value) U m is defined as the largest voltage value over a period. For a voltage that is asymmetrical with respect to zero, the concepts of peak deviations up U m+ and down U m- are introduced. The average value of alternating voltage U avg is its constant component:

.

The average rectified value of Ust is defined as the constant component of the voltage after its full-wave rectification:

.

The effective or effective value of Ueff is estimated by the root-mean-square value of the measured voltage:

.

The law of voltage change corresponds to certain quantitative relationships between U m, U st, U eff, estimated by the amplitude coefficients K a = U m / U eff and shape K f = U eff / U eff. So, for harmonic voltage K a = 1.41, K f = 1.11.

Rectangular oscillatory voltage - meander - without a constant component is characterized as K a = K f = 1. If the power of the measured voltage and current is sufficiently large, they can be measured by instruments of the magnetoelectric system in combination with additional devices. Thus, direct current and the average value of alternating current (and voltage) can be measured directly by a magnetoelectric device.

The average rectified value is measured using magnetoelectric system devices in combination with a bridge-type diode rectifier.

Rice. 6. Thermoelectric converter

The effective values ​​of currents and voltages are measured by devices of a magnetoelectric system with thermoelectric converters, which are a combination of a thermocouple and a heater through which current flows (Fig. 6). Heater 1 is connected to the working (hot) junction of the thermocouple. A magnetoelectric device is connected to the non-working (cold) junctions. Due to the thermal inertia of the heater, it can be assumed that its temperature in steady state practically does not change when the instantaneous power changes, so that the device measures the effective value of the current. The thermal converter is often placed in a vacuum to reduce heat transfer and increase sensitivity. The frequency range (up to 200 MHz) is limited by the capacitance of the device relative to ground, its own inductance and the skin effect in the heater.

Electronic voltmeters(B2 - direct current, B3 - alternating current, B4 - pulsed, B5 - phase sensitive, B6 - selective, B7 - universal).

In order to increase sensitivity and expand the range of measured voltage values, special devices have been developed - electronic voltmeters. In accordance with the measured parameter, voltmeters of amplitude value (peak), average (constant voltage), average rectified and effective values ​​are distinguished. Electronic voltmeters have a high input resistance, reaching 10 MOhm, have a wide frequency range up to 1-3 GHz, and can withstand heavy loads. Typical block diagrams of electronic voltmeters are shown in Fig. 7. The input device of electronic voltmeters consists of an emitter follower, most often mounted in a remote probe to reduce the influence of wires at high frequencies, and an attenuator, which is a resistive voltage divider.

Rice. 7. Block diagrams of electronic voltmeters:

a) alternating voltage; b) constant voltage;

c) alternating and direct voltage

Amplifiers in electronic voltmeters are designed to increase sensitivity when measuring low voltages. To increase the stability of the amplifier's gain and reduce nonlinear distortion, a multistage amplifier covered by negative feedback is usually used.

The voltmeter detector is designed to convert the measured voltage into a constant or pulsating form, measured by a magnetoelectric device. Depending on the transformation law, detectors are divided into peak (amplitude), effective value detectors and average rectified value detectors.

Rice. 8. Peak detector circuit and voltage graph

In the peak detector, the circuit parameters (Fig. 8) are selected so that the capacitor charging time constant τ 3 = R i * C (R i is the internal resistance of the diode) is much less than the discharge circuit constant τ p = R * C, which is much greater than the oscillation period input voltage: τ r >>T. As a result, after several periods of oscillation, the capacitor will be charged to a voltage U c with an average value U cf close to the amplitude value U m .

The effective value detector must have a quadratic current-voltage characteristic.

Rice. 9. Scheme of a quadratic detector with piecewise smooth approximation of the current-voltage characteristic

Square plot current-voltage characteristics almost all active elements possess: lamps, transistors, diodes; however, the length of this section is short. To increase it, piecewise smooth approximation of a parabolic curve is used on K-sections, each of which is provided by the initial quadratic section of a given active element. In Fig. Figure 9 shows a diagram of such a detector. The number of approximation sections corresponds to the number of diode chains in which a stepwise increasing reverse bias voltage (E cm) is applied to each subsequent diode, which causes the opening of each of them at the input Uin >E cm.

Rice. 10. Circuit of the average rectified value detector

The average-rectified detector is a full-wave rectifier, usually assembled using a bridge circuit (Fig. 10). In order for the current in this detector to be proportional to the average rectified value of the measured voltage, it is necessary that the amplitude of the input voltage supplied to the diodes significantly exceed the quadratic portion of the diode’s current-voltage characteristic, i.e., that the detection be linear and not quadratic. Let's look at some special types voltmeters.

Selective (selective) electronic voltmeter designed to measure sinusoidal voltage of a certain (selected) frequency in the spectrum of other frequencies. The operating principle of such a voltmeter is based on voltage release required frequency from the spectrum of other frequencies, amplification and further measurement of the voltage of the selected frequency.

Digital voltmeters.

The use of a digital readout increases the speed and accuracy of measurement and allows you to automate the measurement process. The main unit of digital instruments is an analog-to-digital converter, which converts a continuous measured value into digital code. Let's consider the block diagram digital voltmeter with a time-pulse converter (Fig. 11).

Rice. 11. Block diagram of a digital voltmeter

At the beginning of the measurement cycle, a pulse from the control device resets the electronic meter to zero and starts the linearly varying voltage generator circuit, simultaneously opening the electronic key. From the moment the electronic key is opened, counting pulses with a repetition frequency f from the counting pulse generator are received at the input of the electronic meter through the electronic key. A linearly varying voltage is supplied to one of the inputs of the comparing device, the second input of which receives the measured voltage. The comparing device, at the moment of equality of the measured and linearly varying voltage, produces a pulse that closes the electronic key. Thus, the measured voltage will be proportional to the time interval Dt of operation of the electronic key, and, consequently, to the number of counting pulses registered by the electronic meter. With a large number of counting pulses (high frequency), the voltage measurement accuracy will be high.

2.5. Frequency measurement.

Frequency measurement is one of the most important problems solved in radio electronics, since, on the one hand, frequency is one of the main characteristics of the signal, and on the other, the technique for measuring frequency is the most accurate compared to the technique for measuring any other quantity, which was the prerequisite to reduce measurements of other physical quantities to measurements of frequency and time intervals. Several methods for measuring frequency have been developed: bridge, charge and discharge of capacitors, resonant, heterodyne, electronic counting.

R 3 /R 4 =(R 1 +1/(iω 0 C 1))/(1/ R 2 + iω 0 C 2) -1 or R 3 /R 4 = R 1 /R 2 +C 1 / C 2 +i(R 1 ω 0 C 2 -1/(R 2 ω 0 C)).

Equating the real and imaginary parts, we get

R 1 / R 2 + C 2 / C 1 = R 3 / R 4 and R 1 ω 0 C 2 -1 / (R 2 ω 0 C 1) = 0.

From the second equality the frequency at which equilibrium of the bridge occurs is determined:

ω 0 =1/(R 1 R 2 C 2 C 1) 1/2.

Rice. 12. Wien bridge circuit for frequency measurement

The equilibrium condition of the bridge R 1 R 2 is fixed at the minimum reading of the indicator device (ID) when the value of the resistances R 1 R 2 and capacitances C 1 C 2 changes. Usually R 1 =R 2 =R, C 1 =C 2 =C, ω 0 =1/(RC), the values ​​of R and C are calibrated in frequency values, and R 1 and R 2 change simultaneously and are smoothly adjustable, and C 1 and C 2 are multipliers for the frequency scale with simultaneous hopping.

Capacitor charge and discharge method is based on measuring the average charge or discharge current of a capacitor, which, at a constant voltage of the signal source, is proportional to its frequency (Fig. 13). A limiter enhances the weak and limits strong signals up to a certain amplitude U 0 , the same for all signals. The charging time constant for capacitor C is chosen to be much less than half the period of the input voltage, so that the capacitor has time to discharge even at the highest frequencies.

Rice. 13. Scheme of periodic charge and discharge of a capacitor

The amount of recharge electricity is Q =СU 0. Thus, the average value of the current i=fQ=cfU 0 passing through the diode and the magnetoelectric device is proportional to the frequency. Frequency counters of this type operate in the range from tens of hertz to units of megahertz. The transition from limit to limit is achieved by changing the capacity.

Rice. 14. Resonance wavemeter:

a) block diagram; b) oscillatory system with a circuit; c) coaxial resonator

Resonance wavemeter is based on obtaining the phenomenon of resonance at a measured frequency in a tunable oscillatory system. This method is used at high and ultra-high frequencies, starting from 50 kHz. At frequencies up to hundreds of megahertz they are used resonant circuits with lumped parameters, and at higher frequencies - resonators or sections of coaxial cable. The state of resonance is determined by a magnetoelectric device based on the maximum voltage. The value of the measured frequency is read from the capacitor scale. In a coaxial resonator, the wavelength is determined by the mechanical movement of the piston. The resonance condition is l=(kλ)/2, where k is an integer. The quality factor of a coaxial resonator is 10 3 -10 4.

Heterodyne frequency meter is based on comparison of the measured frequency with the known frequency of a tunable calibrated oscillator (heterodyne).

Rice. 15. Block diagram of a heterodyne frequency meter

When measuring, the mixer receives the voltage of the measured frequency from the input device and the voltage from the variable frequency local oscillator. By changing the local oscillator frequency, we achieve the appearance of zero beats at the output, registered by an indicator (telephones or dial indicator). Receiving zero beats at the output indicates that the measured frequency is equal to the local oscillator frequency, which is determined on the scale. To calibrate the local oscillator scale, use crystal oscillator, the voltage from the output of which is supplied to the mixer. The local oscillator frequency is set equal to the frequency of the quartz oscillator (or its harmonics) by adjusting using tuning capacitors.

Electronic counting frequency meter.

Typically, the device circuit (Fig. 16) is constructed in such a way that it is possible to directly measure both the frequency and the period of oscillations.

Rice. 16. Block diagram of a digital frequency meter

When measuring frequency f x, a voltage of unknown frequency is applied to input 1. The input device is a voltage divider and a broadband amplifier to amplify the voltage to a value sufficient for the operation of the shaping device. The forming device converts sinusoidal voltage into rectangular pulses with steep edges, constant amplitude, and a frequency equal to the signal frequency. These pulses are sent through an electronic key to an electronic meter. On the other hand, the electronic key receives through the control device pulses of calibrated time intervals of duration Δt, which are formed by ten-day frequency dividers from highly stable frequency oscillations generated by a quartz oscillator. These pulses open the electronic key for a time Δt, during which counting pulses of the measured frequency are supplied to the electronic counter; the latter are counted and displayed on a digital readout device in the form of a readout f x =n/Δt. When measuring the oscillation period, a voltage of unknown frequency is supplied to input 2 and then to the forming device, which generates time intervals Δt=T x, during which the control device opens the electronic key. Counting pulses in in this case are rectangular pulses calibrated in time, received in the forming device after preliminary multiplication of the frequency of a highly stable quartz oscillator. The number of these pulses arriving at the electronic counter during the time Δt will be proportional to the period of the unknown frequency T x =n/f. The higher the period, i.e., the lower the signal frequency, the higher the accuracy of the period measurement, while the higher the higher the signal frequency, the higher the accuracy of the measured frequency at input 1.

2.6. Phase difference measurement.

Measuring the phase difference between two harmonic voltages of the same frequency is widely used in radio electronics when studying various four-terminal networks. Let's look at some methods for measuring phase differences. Oscillographic methods are clearly presented in Fig. 17.

Rice. 17. Oscillographic methods for measuring phase differences:

a) scanning of both signals on a two-beam (two-channel) oscilloscope Δφ=2π Δt/T;

b) use of Lissajous figures with the same gain in X and Y (single-channel oscilloscope with an amplifier in X), sinφ=h/H, tg(φ/2)=b/a,

where a and b are the major and minor semi-axes of the ellipse

Comparison method consists of comparing the measured phase shift at the output of the test quadripole with the phase shift of a calibrated phase shifter, powered from the same source harmonic vibrations(Fig. 18).

Rice. 18. Block diagram of a phase shift meter using the compensation method

The voltage that passed through the two-terminal network under study, and the same voltage that passed through the calibrated phase shifter and amplitude regulator, are supplied to the compensation unit, which is a conventional differential transformer. When the input voltages are equal in phase and amplitude, the voltage at the output of the compensation unit is zero, as evidenced by the zero readings of the voltage indicator. The phase shift is determined by the scale of the phase shifter, the signal attenuation in the quadripole is determined by the scale of the amplitude regulator.

Digital method (discrete counting method) is based on measuring the number of counting pulses of a calibrated frequency during a time Δt=T Δφ/2π, proportional to the phase shift.

Rice. 19. Block diagram of a digital phase meter

The shapers convert harmonic oscillations, between which the phase shift needs to be measured, into sharp-edged unipolar pulses, the leading edge of which corresponds to the moments when the harmonic oscillations pass through zero. The control device opens the electronic key for the shift time Δt between pulses from different inputs, and the counter counts the number of pulses passed during this time.

Note that when measuring the phase difference at high and ultra-high frequencies, the frequency is first reduced using a heterodyne converter that has two identical mixers and one common local oscillator (Fig. 20). Then, in the low-frequency region, the phase difference is measured using one of the methods discussed above.

Rice. 20. Frequency conversion circuit

The phase shift of the voltages at the mixer output is the same as the input voltages:

U 1 = U 1 sin[(ω-ω r)t+φ 1 -φ r ]; U 2 = U 2 sin[(ω-ω r)t+φ 2 -φ r ].

2.7. Spectrum analyzers.

Spectrum analyzers are designed for visual observation of the signal spectrum. The most commonly used sequential analysis spectrum analyzers have two structural circuits: a tunable filter circuit and a superheterodyne circuit.

In a spectrum analyzer with a tunable filter the spectrum of the signal under study is viewed by automatically adjusting the filter, isolating the spectrum components, detecting, amplifying and observing on the CRT tap (Fig. 21).

Rice. 21. Spectrum analyzer with tunable filter

The filter is adjusted by changing the scanning voltage, as a result of which the image of the spectrum on the screen turns out to be motionless. The disadvantage of the scheme is its narrow range.

Superheterodyne circuit(Fig. 22) provides electrical tuning over a wide frequency range. Its operating principle boils down to a linear sequential transfer of the spectrum of the signal under study to the intermediate frequency region and moving it relative to the average filter tuning frequency. In this case, the filter is invariably tuned to the intermediate frequency, and the sequential movement of the signal spectrum is obtained by changing the frequency of the local oscillator, which is a sweep frequency generator (SWG), controlled by the voltage of the sweep generator. During the period of swing of the main frequency generator, the spectrum of the signal under study is observed on the CRT screen in the form of luminous lines, each of which is proportional to the average power for a given harmonic of the spectrum of the signal under study.

Rice. 22. Superheterodyne type spectrum analyzer circuit

2.8. Amplitude-frequency characteristics meters (characteristic meters).

The use of characterographs makes it possible to replace the rather lengthy and labor-intensive process of taking point-by-point amplitude-frequency characteristics using a measuring generator and voltmeter by direct observation of the amplitude-frequency characteristic (AFC) on the screen of a cathode ray tube. The advantage of curve tracers is especially obvious when used for tuning quadripole networks, since the effect of changing certain parameters during the setup process is immediately visible on the curve tracer screen by changing the shape of the amplitude-frequency characteristic.

Rice. 23. Circuit diagram of an amplitude-frequency characteristics meter

The frequency swing of the self-oscillator is usually carried out using a varicap or magnetic modulator. Since the device overlaps wide range frequencies, then some nodes in the meter are made according to the principle of frequency conversion - two signals are supplied to the mixer: one from a range generator, the other from a frequency-modulated generator (FMO). At the output of the mixer, low-pass filters select a difference frequency with the same swing as in the MFC. From the switch, the frequency-modulated signal is fed to a wideband amplifier with a system automatic adjustment gain (AGC), where it is amplified to a voltage of 1 V, and then fed through an attenuator to the four-port network under study. From the output of the quadripole the signal goes to the detector head, and after detection - to the vertical deflection amplifier of the CRT. Since the horizontal sweep of the tube is carried out synchronously with the modulation (swing) of the frequency of the self-oscillator, the frequency response of the quadripole under study is reproduced on the screen.

To calibrate the frequency, frequency marks can be formed in the circuit, which are formed in the mark block as a result of zero beats between the frequency range and the harmonics of the calibrated frequencies: 0.1; 0.5; 1; 5 MHz.

2.9. Measurement of parameters of radio circuit elements (R, L, C, tgδ=1/Q)

Voltmeter-ammeter method does not require special devices(Fig. 24).

Rice. 24. Scheme for measuring complex resistance using the voltmeter-ammeter method

When the circuit is powered from an alternating current source with frequency f, the impedance module can be determined:

,

where R U is the internal resistance of the voltmeter. The active part of the resistance is determined by measuring at a constant voltage. After this, the reactive part of the resistance can be calculated. Typically an electronic voltmeter and a thermoelectric ammeter are used. When switched on as a capacitor or inductor, knowing the frequency f of the supply generator, you can determine L and C: 1) X c =1/(ωC)=U/I and C=I/wU, 2) X L =ωL=U/ I and L=U/wI.

Bridge methods are used in the low radio frequency range and allow achieving the highest measurement accuracy total resistances. The equilibrium indicator must have a high resistance to prevent it from affecting the operation of the bridge. Such an indicator can be an electronic oscilloscope or a voltmeter. The equilibrium of the bridge occurs under the condition

Z 1 Z 3 e i(φ1+φ3) = Z 2 Z 4 e i(φ2+φ4) ,

hence Z 1 Z 3 = Z 2 Z 4; φ1+φ3= φ2+φ4. If we take as the measured resistance and as the exemplary resistance, then in the AC bridge to achieve equilibrium there must be two adjustments: the module of the exemplary resistance Z 2 and its argument φ 2. It should be taken into account that these parameters are interconnected during adjustment. It follows that the bridge must be balanced using the method of successive approximation, while simultaneously adjusting the active and reactive components.

Rice. 25. AC bridge circuit

By resonance method you can measure inductance, capacitance, loss resistance in them, as well as the active and reactive components of the complex resistance of any two-terminal network. Since in almost all cases, when determining the above parameters, it is necessary to measure the quality factor of the equivalent circuit, such devices are called quality factor meters or kumeters.

Rice. 26. Schematic diagram kumetra

A certain calibrated voltage U 1 from a generator having a wide frequency range is introduced into a measuring series oscillatory circuit, consisting of a standard (L 0 R 0) or measured (L x R x) inductor and a standard calibrated variable capacitor C 0 . Resistance R 1 of a very small value is set to reduce the source resistance so as not to deteriorate the parameters of the circuit. When connecting the measured inductance coil L x R x the kumeter allows you to directly measure the quality factor of the circuit L x R x C 0: Q = U c / U 1. As a result, a voltmeter measuring U c is usually calibrated in terms of the quality factor. Considering that the model capacitor and resistance R 1 have very small losses, the found quality factor of the circuit will be equal to the quality factor of the coil. With resonance in the circuit, marked at maximum, the readings of the voltmeter U c can be written as

Q=U c /U 1 =ω 0 L x /R x =1/(ω 0 C 0 R x).

From here, knowing C 0, Q and registering the resonant frequency ω 0, we can determine L x and R x. When measuring an unknown capacitance C x, a reference inductance L o R o is included in the circuit and then the capacitance C x = 1/(ω 0 QR 0) is determined based on the resonant frequency and quality factor value.

Using a meter, you can also measure the active and reactive parts of the complex resistance of any two-terminal network. With its inductive nature, a two-terminal network is connected instead of L x R x, with a capacitive nature - instead of C x.

Heterodyne method is based on the dependence of the oscillator frequency on the inductance and capacitance of its oscillatory circuit and a comparison of the frequency of this generator with the frequency of a zero-beat generator tunable using a standard capacitor C0, which makes it possible to obtain high accuracy.

Rice. 27. Scheme of the heterodyne method for measuring capacitance and inductance

Before connecting the measured inductance or capacitance, both generators are tuned to the same frequency using a standard capacitor C 0, which is recorded by zero beats. When C x is connected, the frequency of generator 2 changes and then capacitor C 0 is adjusted so that the frequencies coincide. With the same inductances in the circuits, the measured capacitance will be equal to the change in the capacitance of the reference capacitor. Error 0.2-0.5%.

Discrete counting method (digital) is based on counting frequency-calibrated pulses over a certain time interval. Depending on how this interval is formed, two types of circuits are used: 1) a circuit that uses an aperiodic discharge of a capacitor to a resistor using a time interval equal to the discharge time constant; 2) a scheme that uses the process of damping oscillations in an oscillatory circuit. In the first scheme, depending on what is chosen as a reference (R 0 or C 0), C x and R x can be measured. Before starting measurements, capacitor C x is charged to voltage E (switch in position 1). Then the switch is moved to position 2 and the discharge of the capacitor C x to the resistor R 0 begins according to the exponential law U c = E e - t / τ. At the moment the switch is moved to position 2, a pulse is sent to the digital time interval meter, which opens the time count. From the divider R 1 R 2 voltage E is supplied to the second input of the comparing device. 2 /(R 1 +R 2) = E/2.72. The moment when the voltage on the capacitor during its discharge reaches the value E/2.72 occurs at t = τ = C x R 0. At this time, the comparison device issues a second pulse, which stops counting time. Measurement error ±0.1%.

According to the second scheme, digital meters are built (Fig. 29).

Rice. 28. Scheme for measuring C x R x by time constant τ = C x R x

The principle of operation is based on the following: the ratio of two amplitudes of a damped oscillation, separated by a time interval equal to one period, is equal to Δ = U 1 /U 2 =e δT, where δ=R x /(2L x) is the damping decrement, T is the oscillation period . Hence T=lnΔ/ δ, so the quality factor of the circuit is equal to

Q=(2π L x)/(TR x)= (2L x /R x)(π δ/ lnΔ)=π/ lnΔ.

Hence lnΔ≈π/Q and D≈exp(π/Q). The ratio of the amplitudes of the damped oscillations of the first and nth is equal to Δ n =U 1 /U n =e n / Q. For n=Q we have D n = e π =23.14, whence U n = Q =0.0432.

Rice. 29. Block diagram of a digital camera

From a pulse generator with a high duty cycle, the capacitor of the circuit C 0 is charged to the amplitude U 1, after which a damped oscillatory process begins in the circuit formed by C 0, L x and R x. At the same time, the threshold device 1 opens the time selector and the pulse counter counts the number of periods of pulse oscillations formed in the forming device from damped oscillations in the circuit. When the amplitude of the damped oscillations reaches a value of 0.0432 U 1, at which n=Q, the threshold device 2 closes the selector and the counting of pulses stops. The counter readings are reset after some time, determined by the delay line. The measurement error is 0.1-0.2% and depends only on the accuracy of the threshold devices.

Admitted

Ministry of Communications of the USSRas a textbook for communications technical schoolsspecialties 0701, 0706

MOSCOW "COMMUNICATION" 1980

Kushnir F.V. Radio engineering measurements: Textbook for communication technical schools. Moscow: Communication, 1980. - 176 p.

The basics of radio engineering measurements are outlined. The principles and methods of measuring radio engineering quantities characterizing the parameters of signals, systems and devices of radio communication and radio broadcasting in the entire applicable frequency range are considered. Information is provided on the construction of block diagrams of measuring instruments, errors and methods for taking them into account and reducing their influence. Particular attention is paid to digital devices and those made on microcircuits. Brief background information on many measuring instruments is provided.

Intended for students of communication technical schools studying in the specialties “Radio communications and radio broadcasting”, “Television equipment and radio relay communications”.

Contents of the book Radio engineering measurements
Preface

Introduction
IN 1. Purpose and features of radio engineering measurements
AT 2. Contents and objectives of the subject
AT 3. Basic metrological concepts
AT 4. Measurement errors
AT 5. Classification of radio measuring instruments
Control questions

Chapter 1: Current and Voltage Measurement
1.1. Basic relationships
1.2. Current measurement
General information
Thermal ammeters
Rectifier ammeters
High current measurement
Indirect current measurements
1.3. Voltage measurement
General information
Electronic AC voltmeters
Pulse voltmeters
Electronic DC voltmeters
Digital voltmeters
Measurement error
Control questions

Chapter 2. Measurement signal generators
2.1. Purpose. Classification. Basic technical requirements
2.2. Low frequency signal generators
2.3. High frequency signal generators
2.4. Pulse signal generators
2.5. Noise signal generators
Control questions

Chapter 3. Electronic oscilloscopes
3.1. Purpose. Classification. Basic technical requirements
3.2. Obtaining oscillograms. Image Scan
3.3. Oscilloscope block diagram
3.4. Pulse oscilloscopes
3.5. Measurement of amplitude-frequency characteristics
Control questions

Chapter 4. Measuring parameters of components of lumped constant circuits
4.1. Basic relationships
4.2. Bridge method for measuring parameters
4.3. Resonance measurement method
4.4. Ground resistance measurement
Control questions

Chapter 5. Measuring parameters of elements and paths with distributed constants
5.1. Basic concepts and relationships
5.2. Measuring line
5.3. Voltage Standing Wave Ratio Measurement
5.4. Load resistance measurement
5 5. The concept of automatic measuring instruments for measuring VSWR

Chapter 6. Power Measurement
6.1. Basic relationships and measurement methods
6.2. Absorbed Power Measurement
6.3. Transmitted power measurement
Control questions

Chapter 7. Measuring Frequency and Time Intervals
7.1. General information. Measurement methods
7.2. Comparison method
7.3. Discrete counting method
7.4. Resonance method
7.5. The concept of measures of frequency and time
Control questions

Chapter 8: Measuring Phase Shift
8.1. Basic information. Measurement methods
8.2. Oscillographic method
8.3. Compensation method
8.4. Method for converting phase shift into current pulses
8.5. Phase detector method
8.6. Discrete counting method
8.7. Frequency Conversion Phase Shift Measurement
8.8. Concept of measuring group delay time
8.9. Phase shifters
Control questions

Chapter 9: Harmonic Distortion Measurement
9.1. Definitions. Measurement methods
9.2. Harmonic method
9.3. Combination method
Control questions

Chapter 10. Measuring parameters of modulated signals
10.1. General information
10.2. Measuring amplitude modulated signal parameters
10.3. Measuring the parameters of a frequency modulated signal
10.4. Measuring parameters of a pulse-modulated signal
Control questions

Chapter 11. Measuring Electromagnetic Field Strength and Radio Interference
11.1. Basic relationships
11.2. Measuring receivers and field strength meters
11.3. Radio interference meters
Control questions
Bibliography

INTRODUCTION

B.I. PURPOSE AND FEATURES OF RADIO ENGINEERING MEASUREMENTS
A measurement is a physical experiment that results in finding the numerical value of the physical quantity being measured. Measurements are the most important stage in the activities of workers in all branches of science and technology. Measuring equipment is the main equipment of all research institutes, laboratories, an integral part of the equipment of any technological process, the main payload of artificial Earth satellites and space stations. The level of development of measuring technology is one of the most important indicators scientific and technological progress.

Measurements also play a decisive role in communication technology. The operation of any radio communication, radio broadcasting and television systems is impossible without continuous information about the modes of operating devices, signal parameters and conditions for their transmission or reception. This information is obtained as a result of measurements of the corresponding quantities.

Preventive or emergency repair of radio equipment and troubleshooting are also impossible without measurements. For these purposes, the electrical parameters of elements (capacitors, resistors, etc.) are measured, the modes of blocks, components and the entire installation are checked, and various characteristics are taken. The obtained quantitative values ​​of the measured values ​​are compared with those given in the descriptions, specifications and diagrams, the cause and location of the malfunction is determined and it is eliminated.

The production of radio equipment and especially its development are accompanied by continuous measurements, since the calculated circuit always needs practical verification, and its elements need to be adjusted accordingly. Acceptance tests of various radio engineering objects are basically carefully performed measurements.

Measurements are carried out using special technical means designed for this purpose, which are called measuring instruments.

In radio communications, radio broadcasting and television technology, all types of measurements can be divided into measurements:
- signal parameters - current, voltage, power, frequency, modulation, shape, phase shift, signal-to-noise ratio, electromagnetic field strength; parameters of radio engineering devices - amplification, attenuation, reflection, matching, signal distortion, input (output) resistance;
- characteristics of components and equipment - frequency, amplitude, modulation, time;
- parameters of elements - resistances of resistors, capacitances of capacitors, inductances and mutual inductances of single and coupled inductors and transformers, impedances of two-terminal networks and verification of measuring instruments.

Measurements of some of the listed quantities are found in the course of electrical measurements, but there they are performed on direct current or power frequency current (50 or 400 Hz). Radio engineering measurements are performed on alternating current over the entire frequency range used in radio engineering, i.e., from fractions of a tertz to tens of gigahertz.

A wide frequency range, large limits of measured values, and a variety of conditions under which measurements are performed are characteristic features of radio engineering measurements. Due to these features, various measurement methods and methods and a significant number of different measuring instruments are used.

Measurements, no matter where and by whom they are performed, must always be reliable, and their results must be comparable. The unity of measurements and uniformity of measuring instruments in the country is ensured by the Metrological Service of the USSR. The USSR Ministry of Communications, like other ministries, has a departmental metrological service. The main tasks of enterprises and organizations in metrological support are determined by orders of the Minister of Communications of the USSR.

The metrological service of the USSR is headed by the USSR State Committee for Standards. Subordinate to him are research institutes and a network of republican and regional state supervision laboratories. The founder of the domestic metrological service was the great Russian scientist Dmitry Ivanovich Mendeleev. In 1893, he headed and until the end of his life led the Main Chamber of Weights and Measures, organized on his initiative - now the scientific and production association “All-Union Scientific Research Institute of Metrology named after. D. I. Mendeleev" (VNIIM), Leningrad.

The industry produces a large number of first-class radio measuring instruments to meet the growing needs of the communications economy and other areas of the national economy in precise measurements. These devices are widely used semiconductor devices, microcircuits and integrated circuits, new design principles. On this basis, the fleet of radio measuring equipment for general use is being intensively updated. However, a large number of discontinued devices are and will be in operation for a long time.

The main directions of development of radio measuring equipment for the Unified Automated Communications Network of the USSR, radio broadcasting and television are currently: automation and acceleration of measurement processes while simultaneously increasing accuracy; performing measurements without interruption of communication or transmission of radio and television programs; improving the technical and operational characteristics of devices through the introduction of new element base and increasing their reliability. The implementation of these areas ensures an increase in the efficiency and quality of measurements, and at the same time, the efficiency and quality of radio communications, radio broadcasting and television.

Kushnir F.V. Radio engineering measurements. Publishing house "Svyaz", Moscow, 1980

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Metrological fundamentals of measurements in radio engineering

1. The concept of measurements. Term O logic and definitions

Measurement is a cognitive process that consists of experimentally comparing a measured quantity with a certain value taken as a unit of measurement. This process can be divided into several stages:

- reproduction of a unit of physical quantity (meter, Hertz, Ohm, etc.);

- transformation of the measured quantity (for quantities for which it is difficult to reproduce the measure, for example, when measuring temperature, the following transformations are possible: temperature - resistance - voltage);

- direct comparison of the measured value with the unit of the reproducible measure;

- recording the measurement result in the form of a number.

Electrical radio measurements, like other types of measurements, are based on metrology - the science of measurements, means of ensuring their unity and methods of achieving the required accuracy. In the Russian Federation, as in other countries, there is a developed metrological service that solves the main problems:

- testing new types of devices,

- supervision over the condition and correct use of measuring equipment in the national economy.

Basic terms and definitions of the theory and practice of measurements are given in GOST 16263-70 "State system for ensuring the uniformity of measurements. Metrology. Terms and definitions."

Metrological characteristics of measuring instruments are characteristics of the properties of measuring instruments that influence the results and errors of measurements. The standardized metrological characteristics of measuring instruments include the instrument error, measurement limits, scale division or least significant unit, input resistance, operating frequency range, etc. Technical measuring instruments that have standardized metrological characteristics are called measuring instruments.

Depending on the purpose, measuring instruments are divided into three types:

Measuring instruments, in the form of a body or a device, designed to store and (or) reproduce a physical quantity of a given size are called a measure (for example, a quartz oscillator is a measure of the frequency of electrical oscillations, a normal element is a voltage measure).

A measuring transducer is a measuring instrument that produces a signal of measuring information in a form convenient for transmission, further conversion or storage, but not directly perceptible by an observer.

A measuring device is a measuring instrument that produces a signal of measuring information in a form accessible to direct observation by the operator.

From these definitions it follows that the main difference between a measuring device and a measuring transducer is the presence of a device for visually displaying information.

It is necessary to distinguish between two concepts “verification” and “verification” of measuring instruments. The first term provides an assessment of devices in terms of their performance (presence of output signals, the possibility of their adjustment, quality of AGC operation, etc.), the second allows one to evaluate the metrological characteristics of devices and compliance with their accompanying technical documentation(accuracy class, measurement errors, adjustment range, input resistance, etc.).

Depending on the metrological functions, measuring instruments can be divided into standards, exemplary measuring instruments and working measuring instruments.

A standard of a physical quantity is a measuring instrument that provides reproduction and storage of a unit for the purpose of transmitting its size to subordinate measuring instruments according to the verification scheme and is officially approved as a standard.

They are distinguished: (primary standard, secondary standard, state standard, witness standard, copy standard, working standard.).

Exemplary measuring instruments are measuring instruments that are used to verify other measuring instruments against them and are approved as exemplary ones.

Working measuring instruments are measuring instruments that are not associated with verification (transferring the size of units). These include all devices used in everyday practice.

A simplified verification scheme is presented in Fig. 1.

Exemplary measures

Standard witness

Reference copy

Working standard

1st category

2nd category

3rd category

4th category

Primary standard

Secondary standard

Highest precision

Highest precision

High precision

Medium accuracy

Low precision

Working measures and devices

As a result of practical work, the following types of measurements are encountered:

Direct measurements in which the desired value of a quantity is found directly from experimental data. For example, measuring voltage or current.

Indirect measurements are measurements in which the quantity being measured is determined as a function of the results of other direct measurements. For example, measurements of gain, power, input resistance, capacitance.

Cumulative measurements - here the measured value is determined by repeated measurements various combinations of the same physical quantity with the solution of a system of equations compiled from particular measurement results. For example, determining the mutual inductance between coils by measuring their total inductance twice.

Joint measurements are measurements of several heterogeneous quantities in order to determine the relationship between them.

For example, the definition temperature coefficients thermistor at

provides resistance and temperature measurement.

It should be noted that in practice the first two types of measurements are most common.

A measurement method is a set of techniques for using principles (physical phenomena on which a given measurement is based) and measuring instruments.

Classification of measurement methods

Direct assessment method - the size of the measured physical quantity is determined by direct comparison with a reproducible measure.

Comparison method. This method is implemented by the following systems:

Differential method - the measured value is determined by the difference between the measured value and the measure (unbalanced bridges).

Zero method (compensation method) - the resulting effect of comparison is brought to zero by a corresponding change in the size of the value reproduced by the measure (balanced bridges).

Substitution method - the measured value is replaced by a reproducible measure equal to the measured value, which is determined by maintaining the mode in the measured circuit (measuring the resistance of the magnetic head of the tape recorder).

Coincidence method - the value of the measured quantity is determined by the coincidence of signs related to the measured and known quantities (scale marks, signals and other signs).

2. Units of measurement

A unit of measurement is a value of a physical quantity that is assigned a numerical value equal to 1.

In the USSR since January 1, 1980. ST SEV 1052 - 78 "Metrology. Units of physical quantities" was put into effect, which established the mandatory use of the International System of Units SI (SI - adopted in 1960 by the XI General Assembly on Weights and Measures).

The SI system of units is built on 7 basic units.

		Kilogram
		Second
Current strength
Thermodynamic temperature		Kelvin
The power of light		Candela
Quantity of substance
and 2 additional ones:
Flat angle
Solid angle		Steradian

In radio engineering, non-system dimensionless logarithmic units are also widely used. They serve to evaluate the gain, attenuation, reflection and other characteristics of radio devices.

The unit based on the use of the decimal logarithm (lg) is called the decibel, the unit based on the use of the natural logarithm (ln) is called Neper.

When measuring power

when measuring voltages

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In radio measurements they use the following types signal levels:

Absolutely zero levels are the levels taken as the starting point. Absolutely zero power level is taken to be 1 mW at a resistance of R0 = 600 Ohms. Using the dependence of P on I and U, we can determine absolutely zero current and voltage levels:

Thus, an absolutely zero level at the resistance is ensured at and.

Absolute levels are levels at an arbitrary point in the chain, relative to absolute zero levels.

Relative levels are levels counted from the initial ones, taken as the starting point. For example, the voltage of the amplifier stage is 40 dB, which provides gain

In other words

Measuring levels are the absolute level at any point in the circuit if a zero voltage level is applied to its input.

3. Features of electroradiism e rhenium

The name of electrical and radio measurements (electronic measurements) reflects two circumstances:

Intended purpose - measurements in electronics and other areas using electronic devices and systems:

Performing measurements based on methods electronic technology and radio engineering, construction of measuring instruments based on electronic components.

Measurements during the production and repair of electronic equipment can be divided into the following main groups:

Signal measurements

Measurements of quantities characterizing signal transmission conditions

Measurements of parameters of individual elements of REA

Measuring characteristics that determine the properties of equipment and its paths

Verification of measuring instruments

Determination of the nature and location of damage.

Electrical radio measurements have a number of significant features compared to other types of measurements:

A large number of measured parameters,

Wide range of frequencies used (from 10-3 - geology, medicine to 1010 - satellite television);

Large range of measured values ​​(capacitance 10-12-102F, resistance 10-3-1014Ohm);

High accuracy and speed;

Small power take-off from the measurement object;

Convenient visual reference and relative ease of use computer technology to improve the quality of measurements.

All measurements during the production and repair of electronic equipment can be divided into:

Laboratory measurements (in the development and research of new processes and devices)

Operational and acceptance (in factories) measurements

Measurements during the repair process

Verification of measuring instruments and measures.

Measurement errors

1. Classification of errors

The deviation of a measurement result from the true value is called measurement error.

Measurement errors can be classified according to various criteria.

In accordance with the components of the measurement process, they are distinguished:

- error in reproducing the measure,

- reproduction error,

- comparison error,

- error in recording the result.

Depending on the source of measurement error, it is divided into:

- methodological error - due to the imperfection of the measurement method (measuring resistance using a voltage divider)

- hardware (instrumental) error - caused by the influence of the measuring instruments used. Depends on the connection circuit and the quality of measuring instruments (converters)

- external error - caused by influences external to the device

- subjective error - depends on the characteristics of the experimenter

In accordance with the conditions of use, measuring instruments are divided into:

- the main error that occurs under normal operating conditions specified in GOST or in the technical specifications (TU) for the measuring instrument.

- additional error that appears when the operating conditions of measuring instruments deviate from normal ones corresponding to technical specifications or GOST.

According to the pattern of appearance, they are distinguished:

A systematic error is an error that remains constant (in magnitude and sign) or appears with a certain pattern during repeated measurements of the same quantity. The way to deal with systematic error is to eliminate the source of error, study them first and introduce corrections. The correction is the magnitude of the error with the opposite sign.

- random errors are errors that change randomly during repeated measurements of the same value of a physical quantity. They are characterized by probabilistic characteristics. The method of struggle is statistical processing of measurement results, for example, averaging.

- gross errors (misses) - they are discarded and not taken into account. The way to fight is to use the “law 3u”.

Based on the method of expression, the following types of measurement errors are distinguished:

- absolute measurement error

where is the measured value, is the true value of the measured value.

- relative measurement error

2. Electrical measurement errors And body devices

According to the method of expression in measuring instruments, absolute, relative and reduced errors are distinguished. The first two errors are similar to those discussed above:

- absolute error of the device D=Хп -Х. Here is the instrument reading, X is the true value of the measured value;

The relative error is defined as

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Since the true value is often unknown, a more convenient notation is often used

The reduced error is the ratio of the absolute error to the standard value L, expressed as a percentage (the choice of L is regulated by GOST 13600-68):

For devices with a zero mark at the edge or outside the scale, the normalizing value L is equal to the final value of the measurement range Xk. If the zero mark is in the middle of the scale, then L is equal to the arithmetic sum of the final values ​​of the scale without taking into account the sign.

For real instruments, the dependence of the absolute error on the measured quantity X can be represented by a certain uncertainty band. This band is due to random error and changes in instrument characteristics as a result of influencing quantities and aging processes.

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Therefore, the value of the absolute error is limited to two straight lines, symmetrical relative to the abscissa axis, the distance between which increases with increasing measured value X.

The equation of line 1 can be written as:

where a is the maximum value of the additive error, bx is the maximum value of the multiplicative error.

The absolute values ​​of the additive error do not depend on the measured value X, and the multiplicative ones are directly proportional to the value X.

Sources of additive error are friction in supports, inaccuracy of reading, noise, interference, vibration. The smallest value of the quantity that can be measured by the device depends on this error. Causes of multiplicative bias - impact external factors and aging of elements and instrument components.

The limiting value of the relative error of the device is related to the limiting value of the absolute error by the dependence

According to GOST, in accordance with the value of the reduced error, measuring instruments are assigned accuracy classes.

The accuracy class is a generalized characteristic of a device, determined by the limits of permissible main and additional errors.

For devices whose additive error sharply prevails over the multiplicative one, all error values ​​are within two straight lines parallel to the X axis (straight lines 2) in Fig. 2.

As a result, the permissible absolute and reduced errors of the device are constant at any point on its scale. For such devices, the accuracy class is equal to the maximum value of the given error, expressed as a percentage and rounded to the nearest larger value from a series of numbers: ;; ; ; ; ; , where For example, accuracy classes for ammeters and voltmeters established by GOST 8711-78: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0 and 5.0.

For devices whose accuracy class is expressed in one number, the basic reduced error, expressed in %, does not exceed the value corresponding to the accuracy class.

The accuracy class of devices for which the additive and multiplicative components of the main error are commensurate is designated as two numbers separated by a slash, for example 0.1/0.05. Instruments whose accuracy class is expressed as a fraction include digital instruments, comparison bridges, etc.

The limiting value of the main relative error of the device, expressed as a percentage, in this case can be determined by the formula:

Here Ak is the final value of the measurement range (measurement limit), Ax is the measured value.

3. Random errors

Random errors are errors that change randomly with repeated measurements of the same quantity. They cannot be excluded experimentally, because they come from the simultaneous influence on the measurement result of a number of random quantities (external influences). In addition, random errors also include random errors of measuring instruments.

Reducing the influence of random errors on the measurement result is achieved by averaging multiple measurements of a quantity under the same conditions.

From probability theory it is known that random variables are most fully described by the laws of probability distribution. In the practice of electrical measurements, one of the most common laws is the normal law (Gaussian distribution).

The distribution function for the normal law (Fig. 3) is expressed by the dependence

where is the probability density distribution function of the random error

y - standard deviation,

D=y2 - dispersion characterizing the dispersion of the random error relative to the center of the distribution.

The graph shows that the smaller y, the more often small errors occur (the more accurate the measurements are).

In the general case, the probability of an error with a value from to is determined by the area of ​​the shaded area in Fig. 3 and can be calculated using the formula:

It should be taken into account that this function is normalized, i.e.

therefore, the curves y1 and y2 always have a shape that ensures that the areas under these curves are equal to 1.

The interval from to is called the confidence interval, and the corresponding probability is called the confidence probability. Therefore, a confidence interval is an interval within which the desired value is located with a probability called confidence.

If we introduce a normalized random variable, then the right-hand side is transformed into the Laplace function, often called the probability integral:

It is tabulated and its graph is presented in Fig. 4

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If a certain probability is given, then having found it, you can determine the error using the formula. This error will determine the size of the confidence interval.

The tabulated values ​​of the function show that the probability of error D occurring in the interval from to is 0.9973. The probability of an error occurring is greater than (1 - 0.9973) = 0.0027? 1/370. This means that only one in 370 errors (i.e., approximately 0.3% of their number) will be larger in absolute value.

The error is taken as the maximum error. Errors that are larger are considered misses and are not taken into account (discarded) when processing measurement results. This condition is often called “law 3u”, i.e. if the condition is met

then it is considered that in this case there are no errors in the measurement results (with a probability of 0.3%).

Statistical processing of measurement results

Numerical probability characteristics of errors are determined at infinite number experiments. In measurement practice n is always finite, so they use statistical numerical characteristics, which are called estimates of characteristics. To emphasize the difference between the formulas of probabilistic characteristics and their estimates, the latter are marked with a “?” sign.

To solve many problems, knowledge of the probability distribution function and density is not required, and quite sufficient characteristics of random errors are their simplest numerical characteristics: the mathematical expectation m (true value) and the standard deviation (variance), which characterizes the accuracy of measurements. If it is known that the error distribution is Gaussian, then these quantities are exhaustive characteristics.

Let's consider an algorithm for statistical processing of measurement results of a certain physical quantity (for example, voltage, current, resistance, etc.).

N single, equal-precision measurements are made, as a result of which a series of random values ​​x1, x2,..., xi,.., xn are obtained. It is necessary to determine within what limits the true value of the measured quantity lies.

1. The arithmetic mean is taken as an estimate of the mathematical expectation (true value):

2. Estimation of the standard deviation of the absolute deviations of each measurement is determined by the formula:

where is the absolute deviation (error) of the individual i-th measurement.

In order to make sure there are no mistakes, we use the “law 3y1”. Having chosen the largest value of Di from n, we check the fulfillment of relation (2). If the relationship is not satisfied, then the measurement result(s) corresponding to the selected Di is excluded and steps 1 and 2 are repeated.

3. The error of the average result of n measurements will be lower, because part of the errors Di will be mutually destroyed. It is characterized by estimating the standard deviation of the arithmetic mean

4. Having specified the confidence probability P, we determine the confidence interval within which the true value of the measured value lies. For a normal distribution law, the confidence interval for a given confidence probability (and vice versa) is determined using the probability integral table Ф(Z)=Р. The limits of the confidence interval can be calculated using the formula

In = xsr D = xsr z

In this way, a confidence interval is calculated only when there is a priori information about the Gaussian nature of the distribution of measurement results. With a small number of measurements n? 15, the confidence interval is determined not through, but through tnb - the parameter of the Student distribution. This distribution depends only on the number of measurements n, but not on the values ​​of xcp and.

By setting the confidence probability b and knowing n from the tables, you can determine the coefficient. Next, using the coefficient and value, you can determine the width of the confidence interval D:

The boundaries of the confidence interval are determined by the formula

In = xsr D = xsr

From a comparison of two options for determining the confidence interval, it is clear that with a small number of measurements, the Student distribution somewhat expands the interval within which the true value of x can lie. When n=15 or more, the values ​​of the confidence intervals are compared and calculations can be carried out in any way.

4. Summation of errors

Very often the task is to determine the total error of a device consisting of several blocks.

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Let's consider the most general case, when each of the blocks has both a systematic and random error.

Systematic errors are summed algebraically, taking into account their signs, while the total error is the modulus of the resulting sum

The random error of a measuring device consisting of blocks with independent random errors of each block is found by geometric addition

In the presence of random and systematic errors the total measurement error is found as their geometric sum

It is allowed to exclude from consideration the so-called insignificant error, which is the term(s) with a value less than 30% of the total error.

Current and Voltage Measurement

1. Characteristics of measured quantities. Measurement methods e nia

DC voltage and DC current are characterized by magnitude and polarity.

Alternating current and power frequency voltage have a sinusoidal shape and are characterized by the following values:

Instant value.

Maximum (amplitude, peak) value.

Constant component.

Average rectified value.

Root mean square (current, effective) value, .

The instantaneous value of current (voltage) is the value of the signal at a given moment in time. It can be observed on an oscilloscope and can be calculated from the oscillogram for each moment in time.

The maximum voltage (current) value is the highest instantaneous voltage value during the period T.

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Peak deviation “up” and “down” are, respectively, the largest and smallest instantaneous values ​​of the variable component of the signal over a given period T.

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The difference between the maximum and minimum signal values ​​over a given period is called the voltage "peak-to-peak"

The constant component (average value) of voltage (current) is the arithmetic mean of instantaneous values ​​over the period T.

The magnitude of the constant component of the signal over a period can also be found graphically. To do this, it is necessary to subtract the area under the abscissa from the area above the abscissa axis and divide the resulting difference by the period. Otherwise: the time axis must be moved so that the areas occupied by the voltage curve above and below the abscissa axis are equal.

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It follows that all electrical signals that are symmetrical about the abscissa axis (for example, a sinusoidal signal) have a constant component equal to 0.

Example 1. Determine the DC component of the signal (voltage) shown in the figure

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a) we use the graphical method: the signal amplitude range will be. Considering that for the "sine" the range, we get,

Therefore, the constant component of the signal is equal, and the function has the form

b) determine by calculation:

because the integral of the sine of any angle over a period is equal to zero, we get

Average rectified value - defined as the arithmetic mean of the module of instantaneous values

At unipolar voltages, the constant component is equal to the average rectified value (see formulas 3 and 4). For opposite-polar voltages, these two parameters are different. It is known that for harmonic voltage. Let's calculate for this signal:

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Therefore, for a harmonic signal with full-wave rectification

The root mean square (rms) voltage value is the square root of the average value of the square of the instantaneous values

Substituting into formula (5) and using the substitution, we can obtain for a harmonic signal

The relationship between the amplitude (maximum value) and the root mean square value for any form of change in instantaneous values ​​is determined by the formula

where is the amplitude factor. For sinusoidal voltage.

There is a relationship between the root mean square and average rectified voltage values

Form factor. For sinusoidal voltage one can obtain

Substituting formula (6) into formula (7) we obtain the relationship between the amplitude and average rectified values ​​of the harmonic signal

When determining the root-mean-square voltage for non-sinusoidal signals, use the same formula (5) by substituting the given voltage shape as the integrand.

However, to determine the root-mean-square value, the given voltage can be expanded into a Fourier series, determining the root-mean-square value of each harmonic Ui and the constant component U0. Then the root mean square value of the non-sinusoidal voltage Uсk will be

The average rectified value is found using formula (4), and the maximum value using formulas (6) and (8).

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For some frequently occurring voltage forms, their values ​​and values ​​are known and tabulated. For example, for a sawtooth voltage one can obtain by substituting u(t)=t:

Example 2. Consider determining the values ​​of Usk for pulse voltages:

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where is the duty cycle of the pulses.

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substituting Um=Uск, we get

Therefore, the constant component is equal to or

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For pulsed unipolar signals

2. General properties of electromechanics And logical devices

Any electromechanical device consists of 2 components - a measuring transducer and a measuring mechanism.

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The measuring transducer converts the measured quantity X into some intermediate electrical quantity Y associated with X by a known functional dependence.

The measuring mechanism is a converter of electrical energy supplied to it into mechanical energy, which is necessary to move its moving part relative to the stationary one.

Depending on the type of converter, devices are distinguished, which are conventionally designated as follows:

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- magnetoelectric system

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- electromagnetic system

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- electrodynamic system

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- electrostatic system.

Any mechanism of a measuring system consists of a moving and a stationary part, which are acted upon by mechanical forces proportional to the value being measured. These forces create a torque M, which turns the moving system in the direction of increasing indications of the pointer (arrow).

where We is the total energy concentrated in the measuring mechanism,

- angle of deflection of the arrow.

In general

Under the influence of torque M, the needle is deflected. In order for each value of the measured quantity to correspond to only one pointer value, a counteracting moment is created in the measuring mechanism, directed towards the rotational moment. The counteracting moment can be obtained due to mechanical (usually special spiral springs that also serve as a current supply) or electrical forces.

The mechanical counter moment is equal to

where W is the specific counteracting moment, depending on the properties of the elastic element.

The instrument needle stops moving when the moments become equal. In some devices it is created due to forces of electrical origin; such devices are called logometers.

Any measuring instrument also contains a device designed to speed up the process of damping vibrations of the moving part of the device. This device creates an acceleration moment:

where p is the damping coefficient, depending on the type and design of the damper,

- angular speed of movement of the moving part.

The most common in practice are air, liquid and induction dampers.

To assess the qualities of electrical measuring instruments, the following parameters are introduced:

The sensitivity of a device is the ability of the device to respond to changes in the measured value. It is estimated by the ratio of the change in the value at the output of the device to the change in the value of X at the input

If the scale is uniform, then

There are current, voltage and power sensitivity. The reciprocal of the sensitivity of the device is called the device constant C.

where n is the number of divisions of the instrument scale.

The accuracy of the device is characterized by the following values: (absolute error), (relative error), (reduced error), K (accuracy class).

Internal energy consumption is a parameter characterizing the ability of a device to consume power from the source of the measured signal. In practice, this power ranges from 10-11 to 10-5 W.

The resting time is the time from the moment the measured value is turned on until the moment when the oscillations of the pointer needle do not exceed the absolute error value. For all devices.

Samples of the magnetoelectric system

Magnetoelectric (ME) system devices are based on the interaction of the field of a permanent magnet with the field of a current-carrying circuit.

They can be of two types:

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- with movable frame

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- with moving magnet

The former have better accuracy and greater sensitivity. The second ones are simpler, more reliable and cheaper. In practice, ME system devices with a moving frame have become more widespread (Fig. 5).

The total energy concentrated in the measuring mechanism consists of the energy of the permanent magnet field, the energy of the current-carrying coil, and the interaction energy of the magnet field with the current-carrying coil, where is the flux linkage equal to the product of the number power lines, crossed by both sides of the coil when it is rotated through an angle, by the number of its turns n:

where B is magnetic induction (T), S is the area of ​​​​both sides of the coil (m2). Thus, the total energy of the mechanism will be equal to

It was previously shown that the torque is equal. Differentiating (9), we obtain М=В·s·n·I. It has also already been noted that the moving system rotates until the torque and counteracting moments are equal. Let's consider three cases.

A direct current flows through the device.

Considering that the counteracting moment, we get. By solving this equality with respect to the angle of rotation of the arrow b, we can determine the equation for the scale of the ME system device

where is the current sensitivity of the device

This shows that the scale of the device is uniform, and the deviation of the pointer depends on the direction of current flow.

To regulate the angle of deflection of the mechanism, a magnetic shunt is used.

The plate through which part of the magnetic flux passes is made of soft magnetic material. By moving it, you can regulate the flow branching into the magnetic shunt and thereby change the induction in the air gap of the device.

Calming of the mobile system of ME devices is magnetic induction, without the use of special devices. The moment of magnetic induction calming occurs as a result of the interaction of the magnetic flux with the Foucault currents arising in the aluminum frame of the coil.

Let's consider the second case, when the measured current has a sinusoidal shape

In this case, the instantaneous torque value

The average value of the torque for the period is equal to

Consequently, when connected to a sinusoidal current circuit, ME system devices show zero.

The case when a complex alternating signal containing a constant component is supplied to the device

When an AC signal is supplied to the ME system device, the device measures the DC component of this signal (or the average value).

ME system devices are integrators, because perform the averaging operation

Advantages of ME system devices:

High sensitivity (up to 3?10-11A).

High accuracy (up to accuracy class 0.05).

Good protection from external magnetic fields, because the self-inductance between closely spaced poles of a permanent magnet is large and amounts to 0.15 - 0.3 Tesla.

Low power consumption from the measured circuit (10-5-10-6 W).

Small dimensions.

The disadvantages of ME system devices include:

The device is not protected from overloads.

Measures only the DC component of the signal (average value), and does not allow the measurement of alternating signals.

Increased sensitivity to ambient temperature.

Application area.

Ammeters and voltmeters for measuring current and voltage in DC circuits. In combination with various converters they can also operate in alternating current circuits. On the basis of the ME system, ohmmeters, exemplary laboratory and working measuring instruments are created. highly sensitive galvanometers.

Electromagnetic devices With Topics

Electromagnetic (EM) system devices are based on the interaction of the magnetic field of a solenoid or coil with a moving ferromagnetic core serial.

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Fig.6

I. Theory of work - fulfillment of the equilibrium condition

where is the counteracting moment.

Energy of the mechanism

As in the previous case, let's consider. Several variants.

Measurement of direct current I0. In this case, we have a torque equal to

and the opposing one

.

The scale of the device is quadratic, and the direction of the current does not matter.

The input current is sinusoidal.

In this case, the moving part of the device, due to its inertia, will respond to the average value. Then:

where I is the rms value of the current.

EM system instruments respond to the rms value and are also calibrated in rms values. Therefore, the readings of such devices do not depend on the shape of the measured signals.

Advantages.

Simplicity of design and reliability.

The readings are independent of the signal shape.

Resistance to current overloads.

Suitable for operation on direct and alternating currents.

Flaws.

Unevenness of the scale (at the beginning it is compressed, at the end it is stretched).

Low sensitivity.

Large power consumption from the measured circuit (up to 1W).

Low accuracy (variation of readings, influence of temperature, frequency of the measured current).

Poor immunity from external magnetic fields due to a weak internal magnetic field. To protect against external fields, two methods are used:

Shielding with soft magnetic iron (reduces the influence of external magnetic field).

Astating. The idea of ​​the method is to use 2 identical units that create torque. The coils of the nodes are connected in series, so they magnetic fields opposite. The external magnetic flux F is added to the magnetic flux F1 of the first coil and subtracted from the flux F2 of the second coil. As a result, the total torque remains unchanged.

Application area

Due to their simplicity and low cost, they are widely used for measuring currents and voltages of industrial frequency (50 and 400 Hz) with an accuracy class of 1.5-2.5. The highest accuracy class achieved in laboratory samples is 0.5.

Electrodynamic system devices

The principle of operation is based on the interaction of magnetic fields of stationary and moving coils through which measured currents flow.

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Fig.7

The scale equation is derived in a similar way from the equilibrium condition

where is the mutual inductance between the coils. Let's consider several cases.

Both currents flowing are constant, i.e. and - const.

Then

, A

From here you can get the equation of the instrument scale

Thus, the nature of the scale of the electrodynamic system device is uneven at. When the nature of the scale is quadratic.

When measuring in AC circuits, the moving part of the device will respond to the average torque value

.

It follows from the formulas that the readings of the ED system devices are proportional to the product of currents, and the scale calibration is valid for both constant values ​​and variables.

Advantages

They can have a high accuracy class (up to 0.2).

They provide multiplication of measured quantities, i.e. When connected in series-parallel, power can be measured.

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Flaws

Low sensitivity.

Nonlinearity of the scale.

Large dimensions and complexity of design.

Poor protection from the influence of external magnetic fields, temperature, frequency.

Inadmissibility of overloads.

Low frequency range (1.5-3kHz).

Application area

Used as ammeters (up to 200A), voltmeters (up to 600V), wattmeters (up to 1.5 kW). They can serve as exemplary instruments for calibrating working instruments. To increase sensitivity, the fixed coil is enclosed in a soft magnetic core. Such a device is called a ferrodynamic system device and is designated.

Electrostatic system devices

Operating principle of ES system devices is based on the interaction of two electrically charged bodies, which are movable and stationary plates, to which the measured voltage is applied.

In practice, two types of mechanisms have become widespread.

The change in capacitance is carried out by changing the active area of ​​the electrodes (Fig. 8).

Electrical capacitance changes due to changes in the distance between the electrodes. Energy concentrated in the moving part of the device

Then the rotating and counteracting moments are respectively equal

Equating these values, we obtain the equation for the scale of the ES system device

Capacitor plates

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It follows that the ES system devices are voltmeters suitable for measuring both direct and alternating current voltage. When measuring sinusoidal voltages, they respond to the RMS value of the signal.

Advantages

When measuring DC voltage, voltmeters are ideal (no power is consumed from the signal source).

When measuring a sinusoidal signal, they have capacitive reactance (very large), therefore they operate at frequencies up to 10-30 MHz.

Can be made to the highest accuracy class.

Since air serves as an insulator in the devices, the devices can be used to change voltage up to (102 - 103) kV.

Flaws

Low sensitivity (Umin about 10V).

Instability (change in capacitance, influence of temperature and external electric fields). Shielding is used for protection.

Nonlinearity of characteristics.

Low reliability.

Application

Used for measurements in DC and AC circuits with a frequency from 20Hz to 30MHz. Can be used as standard voltmeters for measuring high voltages (accuracy class up to 0.5).

In conclusion, we present a summary table of equations for the scales of measuring mechanisms of various systems.

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	RMS

3. Measurement of current and voltage in DC and industrial current circuits w linear frequency

Post circuit measurements O yang current

Measurement I and U in DC circuits are most often performed by devices of a magnetoelectric system with a total deviation current of (20-50) mA. The internal resistance of such devices is usually = 1000 2000 Ohms.

To expand the measurement range of ammeters, shunts are used.

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Rpr Ix

Here n = Ix\Ipr is the shunt coefficient.

Shunts are divided into internal (allowing to measure currents up to 30 A) and external (for measuring currents more than 30 A). They can also be divided into individual (used only with the mechanism with which they are calibrated) and calibrated (calculated for rated currents and suitable for any measuring system).

To expand the limits of voltmeters, additional resistances are used.

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Here is the limit expansion coefficient.

The calculation was carried out using a similar algorithm. Considering what we get

Additional resistances are divided into types, similar to shunts.

Measurement of voltage and current in power frequency circuits

For these purposes, electromagnetic, electrodynamic and electrical devices are used. crostatic systems.

when used to measure current, the coil of the EM system device is connected in series to the circuit;

ED system devices as an ammeter are switched on in series at currents up to 0.5A; at high currents, the coils are switched on in parallel. For voltmeters, the coils and additional resistances are connected in series;

ES system devices are used only to measure voltage. On direct current, the limits are expanded with the help of additional resistances, on alternating current - with the help of capacitors.

From the material discussed above it follows that ME system devices have the highest metrological and operational characteristics. This led to their dominant use in the field of electrical (radio) measurements. However, it should be noted once again that their main drawback is the inadmissibility of even short-term overloads (current-carrying springs, tension threads and suspensions burn out and become deformed).

4. Measurement of current and voltage with devices from converters A telami

Measuring alternating current with ME system devices requires a special operation - converting alternating voltage into direct voltage with its further measurement with a magnetoelectric system device.

If semiconductor elements are used as converters, then in this case the device is called a rectifier. Thermal converters can also be used as a converter - in this case we have a thermoelectric voltmeter. Thermoelectric devices are used in the low and high frequency range.

Rectifier voltmeters

According to the rectifier circuit, they are divided into half-wave and full-wave. Half-wave circuit option rectifier is shown in Fig.9.

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In this case, when a harmonic voltage is applied to the input, a pulsating current i(t) will pass through the device. Considering that the reading of the ME system device is proportional to the average value, we obtain

Rectifier devices are calibrated in rms values ​​of sinusoidal current. The calibration factor C, which relates the response of the device to its readings, is the form factor Kf (C=Kf), which can be expressed through the average rectified and root-mean-square current values

The instrument reading or scale reading is

where is determined by the well-known formula

With half-wave rectification, K1f = 2.22, and with full-wave rectification, Kf2 = 1.11.

Thus, in rectifier devices, the response and calibration do not coincide, so their readings are valid only for a sinusoidal signal. If the shape of the measured current (voltage) curve is different from sinusoidal, then a measurement error appears.

Let a non-sinusoidal voltage be measured and the reading of the rectifier device is equal. Then the average rectified value of the measured voltage can be calculated using the formula

If the shape of the measured voltage curve or its Kfh is known, then the root-mean-square value of the measured voltage can be determined as follows:

As you can see, the values ​​of Usk and Ushk for a non-sinusoidal voltage do not coincide. The relative error between the desired value of the non-sinusoidal current voltage and the reading on the scale Ushk is equal to

To determine the voltmeter readings for a given current (voltage) curve, you must do the following:

Knowing the shape of the measured voltage, determine the shape of the current flowing through the measuring mechanism.

Determine the value of the average rectified voltage using the formula

electronic voltmeter oscilloscope instrument

Calculate instrument readings using formulas

Half-wave rectification,

Full-wave rectification.

Example 3. Determine the current through the meter when a sawtooth voltage is applied

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We determine at the output of the rectifier

Indications on the instrument scale taking into account the graduation.

The RMS voltage value for a given signal can be calculated using the formula

Then the measurement error will be

Advantages

Simplicity of circuit implementation.

High reliability.

Ability to work with high-frequency signals.

Flaws

Nonlinear current-voltage characteristics of diodes and their spread.

Influence of ambient temperature.

Frequency errors due to availability containers р-n diode transition.

To eliminate the last two shortcomings, frequency and thermal compensation circuits are used.

Application area

They are used in combined instruments for measuring current and voltage in combination with ohmmeters, for example in instruments of the Ts series (Ts20, Ts4117, Ts4353).

Thermoelectric voltmeters

This combination milli- or microammeter ME system with one or more thermocouples (thermal converters).

The flow of the measured current Ix through the heater (nichrome or constantan wire) leads to its heating. A thermocouple contact is connected to the heater (gold - palladium, platinum - platinum-rhodium, chromel - drops, etc.).

Under the influence of heat, a thermal current arises in the thermocouple, which deflects the device pointer. In steady state, due to thermal inertia, the temperature of the heater is constant and is determined by the power dissipated on it.

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Ix

The devices respond to the root mean square value and are calibrated in the same values, so the readings do not depend on the shape of the measured signal.

Advantages

Independence of readings from signal shape

High measurement accuracy

Possibility of measuring at high frequencies.

Flaws

Low sensitivity

Influence of ambient temperature

Short service life even under normal operating conditions

Low overload capacity.

High power consumption.

5. Electronic voltmeters

Most often in radio electronics, voltage is measured with analog (arrow) or digital electronic voltmeters.

Electronic is a device whose readings are caused by the current of electronic components, i.e. energy of the power source. Such devices have a number of advantages compared to rectifier and thermoelectric devices.

Advantages

High sensitivity.

High input impedance and low input capacitance.

Wide frequency range.

Ability to withstand overloads.

The disadvantages include more complex circuit and the need for power sources.

In accordance with GOST, electronic voltmeters are designated by the letter B and a number from 1 to 9 (for example, B7-27A). The first number indicates the purpose of the voltmeter, the others indicate the design option (model).

B1 - voltmeter for performing verification measurements

B2 - voltmeters for measuring DC voltage

B3 - voltmeters for measuring alternating voltages

B4 - peak voltmeters

B5 - phase-sensitive voltmeters

B6 - selective voltmeters

V7 - universal voltmeters

V8 - voltmeters for measuring voltage ratios

B9 - voltage converters

Group B2 - voltmeters for measuring constant voltage A yarns

The structural diagrams of such voltmeters largely depend on the range and measurable quantities and therefore they are conventionally divided into two groups.

Voltmeters for measuring high voltages

Such voltmeters allow you to measure a minimum voltage of 1 V and have the following circuit:

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Main elements block diagram are the input device VU, the DC amplifier UPT and the indicator device IU of the magnetoelectric system.

The VU input device is designed to expand the limits of the measured voltages, filter the input signal and provide a high input resistance. Typically it includes input terminals, a voltage divider, preamplifier and various filters.

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