Electrical and radio engineering measurements. Measurement of electrical and radio technical parameters. where We is the total energy concentrated in the measuring mechanism
ANALOG ELECTROMECHANICAL INSTRUMENTS
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 electrical converter(measuring circuit), electromechanical transducer (measuring mechanism), reading device (Fig. 5.1).
Rice. 5.1. Block diagram of analog EIP
Measuring chain. 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 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 electro magnetic 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). Numeric 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 switched 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 dampers are most often used, less often liquid dampers (when very high damping 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 logometer
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 possible if the basic 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 using direct current and alternating current. wide range frequency Described measuring circuits, their principles of construction and are given 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) are called physical quantities. In electrical and radio engineering, physical quantities are electrical voltage, current, power, energy, and electrical resistance, electrical capacitance, inductance, frequency.
A physical quantity can have different meanings. Defined value accepted 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. Measurement DC voltage magnetoelectric device
§13. DC voltage measurement electronic devices
Chapter Four. AC Current and Voltage Measurement
§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 parameters of elements of electrical and radio engineering 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. Parameter measurement field effect transistors
§29. Vacuum tube testing
Chapter seven. Measuring generators
§thirty. General information
§31. Signal generators low frequencies
§32. Signal generators high frequency
§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.
Ministry of Education and Science of the Russian Federation
Federal State Budgetary Educational Institution
higher professional education
Chuvash State University named after I.N. Ulyanova
Faculty of Radio Engineering and Electronics
Department of PC and C
Laboratory work No. 2, 3
Measurement of electrical and radio parameters
CIRCUITS BY BRIDGE METHOD
Completed by: student of group RTE-11-10
Ivanov A.O.
Checked by: Kazakov V.D.
Cheboksary 2012
Lab 2
MEASUREMENT OF ELECTRICAL AND RADIO ENGINEERING PARAMETERS
CIRCUITS BY BRIDGE METHOD
Goal of the work: introduction to the bridge method of measuring active resistance ,
inductance L, containers WITH, quality factor of the coil and oscillatory circuits Q
and dielectric loss tangent 
, studying the principle of operation of devices based on bridge circuits and acquiring skills in operating these devices.
Brief theoretical information
Electrical and radio circuits consist of resistors, inductors, capacitors and connecting wires. To select or test these components, the active resistance must be measured R,
inductance 
, capacity WITH. In addition, capacitor losses, coil quality factors and oscillatory circuits. Losses in capacitors are determined by the dielectric loss tangent 
.
Comparison of the measured quantity (resistance, capacitance, inductance) with a reference standard using a bridge during the measurement process can be carried out manually or automatically using direct or alternating current. Bridge circuits have high accuracy, high sensitivity, and a wide range of measured parameter values. On the basis of bridge methods, measuring instruments designed to measure any one value, and universal analog and digital instruments are built.
DC measuring bridge
DC Bridge(Fig. 6) contains four resistors connected in a closed circuit. Resistors 
,
,
,
of this contour are called the arms of the bridge, and the connecting points of adjacent arms are called vertices. The chains connecting opposite vertices are called diagonals. Diagonal ab contains a power source and is called power supply diagonal. Diagonal Withd, in which the G indicator (galvanometer) is included, is called measuring diagonal.
|
|
|
Fig.6. Bridge diagram |
.
(balance), which looks like: 
The equilibrium condition of a DC bridge is formulated as follows: for the bridge to be balanced, the products of the resistances of the opposite arms of the bridge must be equal. If the resistance of one of the bridge arms (for example 
,
) is unknown, then, having balanced the bridge by selecting the resistance of the bridge arms 
And 
.
, we find it from the equilibrium condition In the equilibrium state of the bridge, the current through the galvanometer equal to zero
and, therefore, fluctuations in the supply voltage and resistance of the galvanometer do not affect the measurement result. Therefore, the main error of a balanced bridge is determined by the sensitivity of the galvanometer and circuit, the error of the arm resistances, as well as the resistance of the wires and contacts.
Radio engineering measurements are also used very widely in various sectors of the national economy. Non-electrical quantities, such as pressure, humidity, temperature, linear elongations, mechanical vibrations, speed and others, can be converted into electrical ones using special sensors and assessed using methods and instruments of electrical and radio engineering measurements.
Radio engineering measurements cover the area electrical measurements and, in addition, include all types of special radio measurements.
Radio engineering measurements are also used to estimate non-electrical quantities. Such quantities as pressure, temperature, humidity, mechanical vibrations, linear elongations when heated, etc. can be converted using special sensors into electrical ones and assessed using instruments and methods of electrical and radio engineering measurements. The purpose of measurements is to obtain the numerical value of the measured quantity.
The subject of radio engineering measurements, in accordance with the program, includes the following sections: basic metrological concepts; brief information about measurement errors, ways to take them into account and reduce their influence on measurement results; measurement of current, voltage and power over a wide frequency range; studying generators measuring signals; electronic oscilloscopes; measurement of phase shift, frequency and time intervals; measurement of modulation parameters, nonlinear distortions; measurements in radio circuits with concentrated and distributed parameters; measuring electromagnetic field strength and radio interference.
Features of radio engineering measurements of voltages and currents.
In radio engineering measurements, systematic errors that vary over time are often encountered. Thus, highly sensitive devices are characterized by systematic error, caused by regular interference in the form of a pulsed or quasi-harmonic signal induced into the input circuits of the device. To reduce the level of interference, constructive measures are taken: input circuits are shielded, and the grounding point is rationally chosen. General method reducing the influence of periodic interference consists in averaging the measurement results over a certain time interval. Averaging is achieved in two ways, often used together: pre-filtering input signal and carrying out multiple measurements with subsequent calculation of the arithmetic mean.
For radio engineering measurements in the audio, low and very low frequency ranges, C-generators are mainly used, which at these frequencies have significant advantages compared to LC generators. This is explained by the fact that the elements of the oscillatory circuits of LC generators for audio frequencies are too bulky (primarily inductors), and their parameters are unstable when temperature changes, which determines the low stability of the frequency of the generated signals. In addition, it is difficult to tune the frequency of LC oscillators in the audio range.
In ordinary radio engineering measurements carried out in laboratory conditions, Tm is assumed to be 292 K (approximately room temperature 19 C), and the ratio Tsh in / 292 is called the noise number.
When performing electrical and radio engineering measurements, it is customary to indicate on instruments the sign of an ungrounded wire in relation to the ground; thus, the opposite rule of signs applies here.
The introduction of radio measurement technology coincided with the beginning of the development of radio communication systems and radio electronics.
Wide use radio engineering measurements in various areas radio engineering entails the emergence of new measurement methods and special measuring instruments. The most specific measurements are at ultrahigh frequencies, which is explained by design features oscillatory systems and energy transmission lines in this range.
The degree of accuracy of radio engineering measurements, as well as electrical ones, is determined by the error, or measurement error.
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. Provides information about the construction block diagrams measuring instruments, errors and ways to take them into account and reduce their influence. Special attention devoted to digital devices and those made on microcircuits. Brief background information on many measuring instruments is provided.
