Economical narrowband FM receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Economical narrowband FM receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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In amateur radio communications and in portable communication radio stations in the range of 27,120 ... .27,250 MHz, it is allowed to use narrow-band FM with a deviation of 4-3 kHz. In this case, it is possible to use simple VXO transmitters with an output stage in class C, having a high and constant efficiency. up to 70%. However, the use of this type of modulation is hampered by the fact that FM receivers made on a traditional element base are more complicated than AM and SSB receivers and consume a rather large current of 20-40 mA, which makes it impossible for long-term battery power, for example, in standby mode.

The following describes an economical narrow-band FM receiver, made on a modern multifunctional IC K174XA26 and having the following characteristics:

Input frequency - up to 30 MHz; FM modulation, deviation - +3 kHz: sensitivity at s/w - 10 dB - 2 μV; consumption current is not more than 12 mA; supply voltage - 2 ... 7 V; output signal amplitude - 6 mV

The circuit diagram of the receiver is shown in Fig.1. The output signal from the antenna is amplified by a resonant amplifier on the VT1 transistor and fed to the input of the UHF IC (pin 18). The frequency-setting circuits of the internal local oscillator are connected to pins 1 and 2 of the IC. In this case, the implementation of a quartz oscillator on the third mechanical harmonic is shown. When performing the generator on the main harmonic, it is advisable to increase C7 to 50 ... 100 pF and connect a 2 ... 30 pF capacitor between pin 60 and the case. In the case of using an external local oscillator, its voltage with an amplitude of 200 ... 400 mV must be applied to pin 1, leaving pin 2 free. It is necessary to choose the local oscillator frequency so that the intermediate frequency fp does not exceed 600 kHz. It is convenient to take fp - 465 kHz, which is done in the above diagram.

From the output of the IC mixer (pin 3), the IF signal through the piezo filter Z1 with a bandwidth of 10 kHz is fed to the input of the IF IC (pin 5). Conclusions 6.7 - UPCH blocking.

(click to enlarge)

If the voltage at pin 14 is greater than the switching threshold (about 0,7 V), then there is no voltage at pin 15, pin 16 (and, consequently, the receiver output) is shorted to the case. In this case, there is no signal at the output of the receiver. In the event that there is a signal greater than 2 μV at the receiver input, then the high-frequency noise at pin 10 is significantly reduced (due to their suppression by the useful signal in the black hole). The DC voltage detected by VD2 at pin 14 also decreases and the key device is triggered. Conclusion 16 is disconnected from the ground and the received low-frequency signal appears at the output of the receiver; a constant voltage of about 15 ... 4 V appears at pin 6. At the same time, the VD1 LED lights up, indicating the operation of the BSHN system, that is, the appearance of a signal at the input of the receiver. The operating threshold of the BSHN system within 2 ... 200 μV is set by a tuned resistor R3. The BSHN time constant is determined by R6 C8. When contacts S1 are closed, the BSHN system is switched off.

Coils L1 and L2, L3 are wound on frames with a diameter of 4 mm with tuning cores made of carbonyl iron with a PEV 0,2 wire. L1 has 20 turns with a tap from the 6th, counting from the bottom. L2 has 20 and L3 has 6 turns. L4, C14 - any 465 kHz IF circuit from a transistor receiver. As Z1, you can use any piezoceramic IF filter with a band of 6 ... 12 kHz.

Set up the receiver as follows. First, the resistor R1 sets the current VT1 to 1 ... 2 mA, and then the circuits L1C2 and L2C5 are tuned to resonance at the frequency of the received signal. Further, by closing switch S1 and applying a signal with an amplitude of 30..50 μV to the input of the FM receiver. tune L4C14 to obtain an undistorted sine wave at the output. If necessary, the width of the linear section of the BH characteristic is set to R10. Then, turning on the BSHN with switch S1 and applying a signal of 2 μV to the input of the receiver, resistor R3 sets the threshold for the operation of the BSHN system.

Author: I. Goncharenko (RC2AV); Publication: N. Bolshakov, rf.atnn.ru

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