Input circuits for a 27 MHz receiver. Simple FM and AM receivers for radio stations

The easiest way to make a fixed AM signal receiver in the 27 MHz range is to use a ready-made pocket radio with a radio path based on the K174XA2 chip as a basis. One of these receivers is Nevsky 402. The high-frequency section of the circuit is subject to modification. Initially, it has the form shown in Figure 1 (a fragment of the diagram attached to the instructions by the manufacturer).

Then we remove everything related to the CB band, a variable capacitor, a variometer, a magnetic antenna, loop coils, and so on, including the KB band setting elements, a band switch, and on the free printed tracks we mount the elements marked with dashes (for example L1"). Result - shown in Figure 2.

Figure 2

The essence of the modification is clear from the diagram: the input circuit is simplified (tuning capacitors are not needed, since coupling in different parts of the range is not required), the complex tunable heterodyne circuit is replaced by a quartz resonator and a reference circuit on L2. As a result of precise matching of the settings of the input and heterodyne circuits at one point in the range (frequency of the received channel), the sensitivity at this point increases significantly.

New coils L1" and L2" are wound on frames from old coils of the KB range, they are identical and have 8 turns of PEV 0.2 wire.

The tuning is carried out in the traditional way, by tuning L2" stable operation of the local oscillator is achieved, and by tuning L1" maximum sensitivity is achieved (fine tuning of the input circuit).

With the resonator frequency shown in the diagram, the receiver operates on the 26945 kHz channel, intended for security alarms.

The receiver was tested together with a radio microphone with amplitude modulation and together with a radio control system with frequency division of channels; in the second case, a low-frequency signal was supplied to a system of simple loop decoders directly from the telephone jack

It is possible to convert the receiver into a small-sized radio station by adding a simple AM ​​transmitter and a transmit-receive mode switch (just switch the antenna and power supply). The circuit of such a transmitter is shown in Figure 3.

Coil L1 is wound on the same frame as the contour coils of the HF KB range of the receiver, contains 8 turns of PEV 0.2, L2 has the same frame, but contains 12 turns of the same wire. Choke DL1 - wound on a resistor MLT 0.5 100 kom, contains 60 turns of PEV 0.12. Choke DL2 - on a ferrite ring K7X4X3 - 120 turns PEV 0.12.

The radio receiver is based on the imported MC3362 microcircuit. Features of this microcircuit are double frequency conversion and a built-in voltage-controlled first local oscillator. The analogue of MC3362 - AK9401 is described in.

The circuit diagram of the 27 MHz receiver is shown in Fig. 1. The signal received by the antenna is sent through a transmit-receive switch to the RF frequency control unit, assembled on a field-effect transistor VT1 of the KP327 type. The use of this transistor is due to the fact that it contains built-in diodes that protect it from breakdown. This leads to increased reliability both during soldering and when the receiver operates near operating transmitters.

The amplified signal from the RF amplifier is fed through coupling coil 15 to input 1 of the MS3362. After mixing the input signal with the GPA signal, a signal of the first intermediate frequency of 10.7 MHz is isolated at pin 19, which is fed through the piezofilter Z2 to the input of the second converter. To obtain a second intermediate frequency of 465 kHz, quartz ZQ1 is used at a frequency of 10.24 MHz, connected to pin 4. Inductance L6 ensures that the quartz frequency changes within small limits, which allows you to more accurately adjust the second local oscillator. At pin 2 you can control the local oscillator frequency. Resistor R5 is a load.

As a result of the conversion, the second IF signal is isolated at pin 5, which through the piezofilter Z1 (465 kHz) is fed to the input of the limiting amplifier, where the main signal amplification occurs (pin 7). After detection, the audio frequency signal from pin 13 through the low-pass filter R12R14 С21С22 is supplied to the volume control R15. Resistor R8 is used to adjust the threshold for turning on the noise suppressor. The VD1 LED is used to indicate the status of the noise suppression circuit.

When a useful signal (carrier) appears, the LED lights up, transistor VT2 closes and the signal goes to the ULF input. ULF is borrowed from. Moreover, there is no need to use powerful transistors, as in. Transistors KT502, KT503 or KT3107, KTZ102 are quite sufficient. As stated in , the amplifier introduces crossover distortion, but it has little effect on speech intelligibility. The current consumption of the amplifier in the absence of a signal is 0.5.0.6 mA.

An audio amplifier can also be assembled according to the circuit shown in Fig. 2. The current consumption in the absence of a signal in this case is 5.6 mA. Otherwise, the parameters of the amplifiers are the same: current consumption at maximum volume is 25.30 mA, output power is up to 150 mW. Resistor R25 (Fig. 2) sets half the supply voltage at the emitters VT7, VT8, and with the help of R29 the required sensitivity.

Setup. Having disconnected pin 5 of the microcircuit from the piezofilter, an FM signal with an amplitude of 200.300 mV and a frequency of 465 kHz, modulated by a harmonic signal of 1 kHz, is supplied to it from the GSS. Coil L8 is adjusted to the maximum output voltage across resistor R15. The noise reduction circuit must be turned off (the R8 engine is in the top position according to the circuit). It is advisable to use an eight-crystal piezofilter, since the latter, unlike a four-crystal one, has steeper frequency response slopes. Good results are obtained by replacing the L8 coil with a piezoceramic resonator at a frequency of 465 kHz. Resistor R11 sets the output voltage on R15 within 15...20 mV. The resistance of resistor R11 is 3.10 kOhm. Having soldered pin 5 in place, solder pin 19 and apply a frequency-modulated signal of 10.7 MHz with an amplitude of 300.400 μV to the input of the piezofilter Z2. By changing the inductance L6, the frequency of the second local oscillator is adjusted to minimize distortion of the sinusoidal signal taken from R15.

The L7C19 GPA circuit is connected to pins 21 and 22. From pin 20, a signal with a GPA frequency with an amplitude of 200...300 mV is supplied to the input of the synthesizer frequency divider (control point 2 V). Pin 23 receives the control voltage from the output of the frequency-phase detector of the synthesizer (point 4). Having supplied an FM signal with a frequency of 27.2 MHz and an amplitude of 10...15 mV from the GSS, rotate core L8 until the PLL system of the synthesizer is triggered. When switching channels of the frequency synthesizer, control the voltage at pin 23, which should not be less than 1 V. Otherwise, the GPA will not be tuned.

Having reduced the input voltage to 2...3 µV, in the middle of the range the coils L3, L4 are adjusted to the maximum voltage at ULF. Lastly, R9 is selected based on the minimum on/off hysteresis of the noise reduction system.

The microcircuit can also be used with an external local oscillator, supplying a signal with an amplitude of 200...400 mV from it to pins 21 or 22.

During experiments, the MC3362 chip showed performance up to 500 MHz. The GC-158 generator was used as the first local oscillator, and the input signal was supplied from the GC-168 to the SP capacitor. By connecting a piece of wire to the SP as an antenna, and rebuilding the generator used as a local oscillator in the range of 400...450 MHz, you can listen to trunking and cellular radio stations.

The printed circuit board diagram of the receiver with ULF Fig. 2 is shown in Fig. 3 and Fig. 4. Circuit L8C20 is replaced by a piezoresonator at 465 kHz, while the resistance of resistor R11 is 3.10 kOhm. Capacitors C4, C9, C13, C16, C17 are unframed, they are soldered from the conductor side. Resistor R5 and capacitor C22 are not installed on the printed circuit board. Loop capacitors are placed directly on the loop frame and covered with a screen. On the printed circuit board, the foil on the parts side is left as a screen and connected to the body.

Coil parameters: L1-20 turns; L3, L4-12 turns; L2, L5 - 4-6 turns; L7 - 30 turns of wire PEV 0.25; L6 -55 turns of wire PEV 0.15; L8 - 250 turns of wire PEV 0.1.

Literature:

1. Aleksandrov I. Narrowband FM receiver AK9401 for a radio station // Radio amateur. -1995.- No. 1-P.46-47.

2. Belousov O. Economical ultrasonic frequency // Radio amateur. -1997.-No.6.-P.19.

3. Vasiliev V. Modern car radio station 27 MHz//Radio Amateur.-1997.-No.3,-P.36-38.

4. Opanasenko S. Frequency synthesizer 27 MHz // Radioamator.-1998,- No. 7.-P.55-56.

Receive path 27 MHz

The schematic diagram of the receiving path is shown in the figure. The path is built on the basis of the MC3361 microcircuit. When receiving, power and signal from the antenna are supplied to the RF amplifier on transistor VT1. Diodes VD1 and VD2 protect the RF input from static electricity, which may be in the antenna, and from accidental penetration of the signal from the transmitter output, if the path operates as part of a radio station. Next, the signal immediately goes to the base of transistor VT1.

The resistance R1 in the base circuit of transistor VT1 is relatively small, so the cascade operates in a barrier mode, characterized by low noise and high RF gain. If it is necessary to reduce the current consumption, the cascade can easily be switched to normal mode by increasing the resistance R1 to 150-250 kOm.

The collector circuit VT1 includes circuit L1-C3-C4, tuned to the frequency of the received channel. Capacitors SZ and C4 are part of the circuit and at the same time constitute a capacitive transformer necessary to match the circuit with the input of the frequency converter of the A1 chip.

The RF-IF path is made on the A1 - MC3361 microcircuit according to an almost standard circuit. The difference is that to improve the startup of the local oscillator of the frequency converter of the microcircuit, an additional serial circuit C5-L2 is included in the local oscillator circuit. By adjusting the L2 coil, you can change the local oscillator frequency within small limits, which may be required when accurately matching the frequencies of the receiver and transmitter, and ensuring minimal distortion during demodulation and maximum reception range.

The local oscillator operates at a frequency of 27.575 MHz, which is higher than the frequency of the received signal (27.12 MHz). It is also possible to operate at a frequency lower than the frequency of the received signal, it depends on what quartz resonators you have at your disposal.

The intermediate frequency signal of 455 kHz is separated by the Q2 piezoceramic filter into a band with a center frequency of 455 kHz. This is a filter from an imported pocket receiver with the AM range. If the resonators you have for the receiver and transmitter give a difference in frequency of 465 kHz, you need to use a domestic filter (at a frequency of 465 kHz) in place of Q2.

The frequency detector uses a T1 circuit, which uses a ready-made IF circuit from an imported pocket receiver with the AM band. The circuit screen is connected not to the common minus, but to the positive pole of the power supply. This is unusual, but not essential - it’s just more convenient from an installation point of view.

The low-frequency signal is isolated at pin 9 of A1 and is supplied in two directions - to the output through C15 and to the noise reduction system of the A1 chip.

The noise reduction system is made according to the scheme recommended by the chip manufacturer MS3361, with the only difference that the threshold is set not by a variable, but by a trimming resistor R5. The noise reduction threshold is set during the installation of this radio path and is not changed during operation. However, you can return to the standard circuit and install a variable resistor together with R5, which will be brought outside the radio housing and used during operation.

The output of the noise reduction system is pin 14 of the A1 microcircuit; there is a key that closes to a common negative when there is no signal reception. It can be used to indicate reception or to block external VLF. In the simplest case, it can be connected to the right-hand pin of C15 in the diagram, so that it shunts the output when there is no reception of a useful signal.

The circuit is assembled on a printed circuit board made of foil fiberglass.

The board can be made either using a personal computer and a laser printer or a “photo positive”, or in the “old-fashioned” way - transfer the location of the holes to the workpiece by punching, drill it out, and draw the printed tracks with nitro paint varnish, but more conveniently - with a permanent marker. Then, etching in a solution of ferric chloride.

Coils L1 and L2 are wound on plastic frames with ferrite cores from color modules of USCT televisions. Now these are the most affordable and practically free frames. You can use other frames with a diameter of 5 mm with tuning cores with a diameter of 2.5 mm made of ferrite. Televisions of this “model range” often end up thrown out in the most unexpected places, and are often dismantled.

Coil L1 contains 6.5 turns of PEV 0.2-0.4 wire. Coil L2 - 8 turns of the same wire.
T1 circuit is a ready-made IF circuit at 455 kHz from an imported pocket receiver. Such contours are often found on sale. If there is no such circuit, you can use a homemade one. To do this, you need to wind a coil of 90 turns of PEV 0.1-0.15 wire on the same frame as L1 and L2 and connect a 620-680 pF capacitor in parallel with it. It is advisable to first adjust this circuit to a frequency of about 455 kHz using a generator, and perform the final adjustment when setting up the receiving path.

Intermediate frequency filter Q2 from an imported pocket receiver, for an intermediate frequency of 455 kHz. The brand and type of filter is not known (in the store it simply says “455 kHz filter”). Where the entrance and exit are are also not marked. I tried both ways, no difference, it’s probably symmetrical. The middle output is common, and the extreme ones are input and output.

Quartz resonator Q1 must differ in frequency from the frequency of the received signal by an IF value of 455 kHz. If there are no such pairs of resonators, but there are with a difference of 465 kHz, you need to use an IF filter at 465 kHz.

Check the operation of the local oscillator by the presence of RF voltage at pin 2 of A1 (connect an RF voltmeter or oscilloscope to this pin through a capacitor with a capacity of no more than 3 pF). Adjust coil L2 so that there is stable generation.

When setting up, you need to control the output signal by feeding it to some ULF so that you can listen to the quality of reception. In this case, you can use the transmitter of any CB radio station operating at the frequency of this channel with narrow-band frequency modulation as a signal generator. Take the switched on transmitter with one of the tone call buttons pressed and the antenna connected, away. Disconnect pin 16 A1 from capacitors SZ and C4 and connect to it a piece of mounting wire no more than a meter long. Set R5 to the middle position.

A call signal should be heard in the speaker (or headphones) connected to the output of the path. Adjust T1 slightly so that distortion is minimal. Take the transmitter further and repeat the adjustment. Then, disconnect the antenna wire from pin 16 of A1 and connect it to the VT1 base, and connect pin 16 to capacitors SZ and C4. Adjust coil L1 for maximum signal reception range
transmitter.

By experimenting with the reception range, adjust L1, L2 and T1 more precisely.

Agapov V.N.
Literature:
1. Agapov V.N. "CB radio station with individual call." zh.Radioconstructor, No. 8, 2006.

Operating frequency................................................... ...............27140 kHz;

Receiver sensitivity, no worse...................................5 µV;

Ultrasound power................................................................... ....................100 mW;

Call signal frequency................................................................... ......1.25 kHz.

The diagram of the radio station receiver is shown in Fig. 1. It is made on the K174XA10 microcircuit and does not have any special features.

UHF is implemented on transistor VT1. The data of the receiver coils are given in table. 1.

Table 1

Receiver coil winding data

The dynamic head is placed in a separate housing and is connected to the radio station with a flexible shielded wire; the “RX-TX” button is installed in the same housing, which switches the radio station and the “Transmit” mode.

Switching is carried out by small-sized relays of the RES80 type with an operating voltage of 8 V. If you want to increase the output power, you can turn on an additional AF amplifier. The diagram of the radio station transmitter is shown in Fig. 2. The data of the transmitter coils are given in table. 2.

Rice. 1. 27 MHz radio receiver circuit

table 2

Transmitter coil winding data

The switching block diagram is also shown in Fig. 2. The "RX-TX" button is installed either on the front panel of the portable radio station housing, or together with the BA1 loudspeaker in a separate housing. In Fig. 3 shows a circuit for monitoring the supply voltage; it has small dimensions and is assembled by hanging installation; you only need to adjust the threshold of operation of the logic elements of the DD1 microcircuit by adjusting R1 and R2.

Rice. 2. Transmitter circuit for a 27 MHz radio station

Rice. 3. Voltage control circuit for a 27 MHz radio

This unit is especially necessary if the radio is powered by batteries located inside the case.

The receiver coils L4, L5, L6, L7 are placed in aluminum screens. You can use IF circuits from transistor radios.

A detailed description of the radio station and installation are described in the magazine "Radio Amateur", No. 9, 1995.

While repairing a radio-controlled children's toy, I discovered that the entire radio receiver is assembled on one transistor, which serves as a super-regenerative detector. The same simple design of the receiving part is found in the Walkie-Toki children's radio stations. High sensitivity and selectivity are ensured by a detector based on one active element - a transistor, and the peculiarity is that it can detect a signal from both AM (amplitude modulation) and FM (frequency modulation). Such a receiver covers both the amateur band 28 - 29.7 MHz and the C-B band, 27 MHz. In a fit of nostalgia, I decided to assemble such a super-regenerator to use it as part of a super-super-regenerative receiver.

Super regenerative 28 MHz receiver.

This is where we had to start in order to completely assemble the entire circuit of a super-super-regenerative receiver for the FM range (87.5 - 108) MHz.

The frequency of 28 MHz turned out to be optimal, since neither the third 84 MHz nor the fourth 112 MHz harmonics of the super-regenerator fall on the input of the range 87.5 - 108 MHz, VHF FM (FM) receiver, which I decided to make. It turns out that the radiation from the super-regenerator will not clog the reception of FM radio broadcasting stations with interference. At this frequency (28 MHz) I tried to optimize the detector, thus ensuring acceptable nonlinear distortions and noise levels, sensitivity, and stable generation accompanied by blanking bursts at a frequency of 70 kHz. It is much easier to do this at a fixed frequency than to tune a super-regenerative detector over the entire 20 MHz FM band.

Rice. 1. Super regenerative receiver for frequencies 27 and 28 MHz.

The superregenerator circuit itself (transistor T2) does not differ from traditional circuits of similar detectors that are used to this day.

The selective stage (transistor T1) has a bandpass filter (L 1 - L 3) at the input, and its output is loaded onto a filter (L 4 - L 6) on connected circuits, which prevents the passage of radiation into the antenna, and further increases the sensitivity of the receiver. Thanks to this cascade, there is no influence of the antenna on the detector, which further stabilizes its parameters.

Photo 1 shows the spectrum of a high-frequency signal from a super-regenerative detector. The high-frequency amplifier cascade on transistor T1 prevents such interference from passing into the antenna.

Photo 3. ULF.

All that remained was to install the selective amplifier and the detector itself. It is better to use a printed circuit board for SMD parts no thinner than 1 mm, otherwise its slight deformation will lead to failure (delamination) of the chip components.

You can use resistors and capacitors of any size for SMD mounting, for example, size 0805, the value in inches is (2 by 1.5) mm, well commensurate with the dimensions of the inductors. Capacitors over 1 µF - electrolytic CE or tantalum, of any convenient size. Capacitors less than 1 µF are ceramic. The size of the printed circuit board itself will depend on the size of the radio components.

A properly assembled receiver does not need tuning, because for convenience I used all commercial inductors with a nominal value of 1.5 µH. The circuit used a Fixed inductor (Chip Inductors) from the manufacturer Panasonic, size 2520 (dimensions in mm) or 1008 (dimensions in inches), inductance 1.5 μH, designation ELJFC 1R 5 F, which has a quality factor of 25. You can use coils from another manufacturer, for example, Murata LQH 4N 1R 5MO 4, (SMD) chip inductance 1210, 10% with a quality factor of 20 or coils similar in inductance and quality factor. It should be noted that coils from another manufacturer may have a different intrinsic capacitance and possibly a better quality factor, which will only improve the sensitivity and selectivity of the receiver, but then additional tuning is necessary. But mainly this will concern the super-regenerator circuit, the L 8 coil. The range adjustment can be done with a trimming capacitor or using a varicap.

Photo 5.

Photo 6.

Photo 5 shows a demodulated signal at the output of an audio amplifier. Receiving signal parameters: carrier frequency 28 MHz, frequency deviation 50 kHz, modulation frequency 1 kHz. Nonlinear distortions are not noticeable when compared with a reference signal from an audio generator.

Photo 6 shows the reading of an oscilloscope connected to the emitter of transistor T2. The frequency of the ultrasonic flash suppression generator is 70 kHz.

Sensitivity at a signal/noise ratio of 10 dB - 3 µV.

Radiation into the antenna – 60 dB.

This receiver came in handy for me. With its help, it was possible to identify a malfunction in a 27 MHz radio-controlled toy. It turned out that one command was not heard from the remote control; the switch was not wired. And at this frequency it picks up conversations between truckers within a radius of 2 kilometers.

Photo 7. Layout of a radio receiver for 27 - 28 MHz.

But this design has other tasks. In fact, I made an intermediate frequency path at 28 MHz with a detector and ULF. Now it’s enough to connect another transistor as a mixer, connect an RF generator and you’ll get super super regenerative receiver, which will have all the ranges that the generator produces, but with a difference of 28 MHz. But more on that in the next post.