Synchronous heterodyne receiver of VHF FM signals. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Synchronous heterodyne receiver of VHF FM signals. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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The complexity of the design of this receiver compared to simple synchronous VHF FM receivers popular with radio amateurs, according to the author, is justified by the improvement in the main characteristics: sensitivity and stability of operation.

The receiver of VHF FM signals described here for the range of 65,8 ... 73 MHz. differs from those previously published by greater sensitivity and the absence of such inherent disadvantages as tuning instability and spontaneous tuning to a stronger signal of the adjacent channel. The sensitivity of synchronous heterodyne receivers is limited by that. that "zero drift" of the DC amplifier affects the frequency control node of the local oscillator and causes instability in the tuning of the receiver. In the design developed by the author, it is reduced due to the fact that instead of DC voltage amplifiers, AC voltage amplifiers are used, a decrease in "zero drift" is achieved, which made it possible to increase the sensitivity of the receiver, which is now about 8 μV. In addition, the change in voltage at the local oscillator frequency control node is reduced by the ZL1 amplitude limiter, therefore, under the influence of the control voltage, the local oscillator frequency will not change by more than 100 kHz. Thus, spontaneous tuning to a radio station adjacent in frequency is excluded.

The receiver consumes a current of about 34 mA. Its block diagram is shown in Fig. one.

Synchronous heterodyne receiver of VHF FM signals

The received signal from the antenna through a two-section low-pass filter Z1 and a high-frequency amplifier A1 is fed to the signal input of the mixer U1. Its other input receives the local oscillator voltage G2. If the frequencies of the signal and the local oscillator are not equal, then an alternating beat voltage is formed at the output of the mixer, which through the low-pass filter Z2, the low-frequency amplifier A2. the adder A3 and the amplitude limiter ZL1 is fed to the local oscillator frequency control unit U5 and changes the frequency of the local oscillator G2 in such a way that the instantaneous frequency difference between the signal and the local oscillator decreases to approximately 72 Hz. This frequency value is determined by the lower bandwidth limit of the bass amplifier A2.

The signal from the output of the low-pass filter Z1 also goes to the signal input of the modulator U2, the second input of which receives an alternating voltage of a rectangular shape with a frequency of 20 kHz from an auxiliary low-frequency generator G1.

As a result, an amplitude-modulated high-frequency voltage is formed at the output of the modulator, which is fed through the high-frequency amplifier A4 to the signal input of the mixer U3 (fc), the second input of which receives the voltage from the local oscillator G2 (fg). An alternating voltage with a frequency of 20 kHz appears at the output of the mixer. modulated in amplitude by oscillations of the difference frequency (i.e., the beat frequency fb = fc - fg). which through the low-pass filter Z3. low-frequency amplifier A5 is fed to the signal input of the demodulator U4.

The second input of the demodulator receives an alternating voltage with a frequency of 20 kHz from the generator G2. At the output of the demodulator, an alternating voltage is formed, the frequency of which is equal to the instantaneous difference between the frequencies of the signal and the local oscillator, then it is through a two-section low-pass filter Z4. the adder A3 and the amplitude limiter ZL1 enters the local oscillator frequency control unit U5 and changes the frequency of the local oscillator G2 in such a way that the receiver PLL switches from the beat mode to the hold mode. The difference between the frequencies of the signal and the local oscillator, at which the transition to the holdover mode occurs, is determined by the cutoff frequency of the Z2 filter and is 10.6 kHz (at a minimum signal).

Thus, when the PLL system is operating in the hold (synchronization) mode, fast frequency drifts (72 Hz < f < 10,6 kHz) are compensated by a channel consisting of low-pass filter Z1, high-frequency amplifier A1, mixer U1, low-pass filter Z2. low-frequency amplifier A2, adder A3, amplitude limiter ZL1, frequency control unit U5 and local oscillator G2.

Slow frequency drifts (< 330 Hz) are compensated by a channel consisting of modulator U2, high frequency amplifier A4, mixer U3, low pass filter Z3. a low pass amplifier A5, a demodulator U4, a low pass filter Z4 and an oscillator G1. An alternating voltage of audio frequencies (72 Hz < fz < 10.6 kHz), proportional to the deviation of the instantaneous value of the signal frequency at the receiver input, is fed from the low-frequency amplifier A2 to the receiver output.

The dynamic characteristics of the PLL system are determined by the amplitude of the input signal and the shape of the frequency response of the low-pass filter Z2. which is a single link RC circuit. The shape of the frequency response of an open-loop PLL system is close to the shape of the frequency response of the first-order link, so the PLL system operates in synchronization mode with a sufficiently large range of input signal amplitudes. The receiver does not have an AGC system, therefore, at a very large amplitude of the input signal, the PLL system is self-excited (quasi-synchronism mode). But even in this case, the receiver remains operational, since the self-excitation of the PLL system does not affect the quality of the output signal (the frequency of self-oscillations in the PLL system turns out to be higher than 50 kHz).

The selectivity of the receiver in the adjacent channel is determined by the parameters of the low-pass filter Z2, and the selectivity of the spurious reception channels (on the harmonics of the local oscillator) is determined by the parameters of the low-pass filter Z1.

The circuit diagram of the receiver is shown in fig. 2.

(click to enlarge)

The signal from the antenna through the coupling capacitor C1 and low-pass filter. formed by capacitors C2 - C4 and coils L1.12. enters the IF, made on the transistor VT1. This amplifier serves to reduce the penetration of local oscillator oscillations into the input circuit, its amplification is small and amounts to Ku < 5. The transistor is connected according to a common base circuit, which ensures high UHF linearity and improves the noise immunity of the receiver (UHF on the VT4 transistor is also made according to a similar scheme) . The characteristic impedance of the Z1 filter is close to 75 ohms. and its cutoff frequency is 75 MHz.

R6 elements. C8. R8. C9 form a phase shifter that shifts the phase of the high-frequency voltage supplied to the mixer, made on the transistor VT2. several tens of degrees. This is necessary to increase the sensitivity of the receiver. The thing is. that in the hold (synchronization) mode, the phase shift of the oscillations of the signal and the local oscillator entering the mixer VT5. close to 90. At the same time, due to the delay of the high-frequency signal in the VT3 modulator, the phase shift between the signal and local oscillator oscillations at the inputs of the VT2 mixer may differ from 90°. When receiving weak frequency-modulated signals with a large frequency deviation, this can lead to short-term synchronization failures at the moments of maximum frequency deviation. A chain consisting of R6 elements. C8. R8. C9. provides an additional delay of the high-frequency signal, which allows you to set a phase shift of oscillations of about 2 ° at the inputs of the VT90 mixer.

The construction of low-pass filters Z2 and Z3 (on elements R10. C12 and R26. C29, respectively) and low-frequency amplifiers A2 and A5 (on DA1 and DA3 microcircuits) of both channels is the same and differs only in the ratings of the elements used. The low-frequency signal is taken from the output DA1. elements R11, C15 are used to correct high-frequency pre-distortion.

The functions of the adder A3 and the amplitude limiter ZL1 are performed by the DA2 chip. The modulator U2 is made on the transistor VT3, and the demodulator U4 - on the transistor VT6. The role of the low-pass filter Z4 is performed by the elements R30, C30. R31. C31. The emitter follower on the transistor VT7 reduces the influence of the adder on the parameters of the low-pass filter. The frequency control unit U5 is made on a varicap VD1, the local oscillator G2 is based on transistors VT8, VT9. and the auxiliary low-frequency generator G1 is on the DD1 chip.

The steepness of the frequency control unit Sγpr - 35 kHz / V. therefore, with a frequency deviation (f \u50d 19 kHz), the audio frequency voltage on the capacitor C1,5 is about 15 V, and at the receiver output (at C0,3) it is about XNUMX V.

The receiver is tuned to the frequency of the radio station by changing the inductance of the local oscillator coil L3.

The receiver is assembled in a case made of sheet duralumin. In its manufacture, a hinged installation was used. The local oscillator is enclosed in a screen, in addition, it is connected to capacitors C19 (control circuit), C41 (power) and to the gates of transistors VT2 and VT5 (local oscillator signal) with segments of a television coaxial cable. Just in case, the wire connecting pin 10 DD1 to the gate of transistor VT3 is shielded, but this is not necessary.

The device can be used fixed resistors MLT-0,125, ceramic capacitors, for example. CT or CM. Capacitors C2 - C4, C37 - C39, C42, C43 must have a small TKE. Oxide capacitors - any type.

As transistors VT1, VT4, VT8 and VT9, in addition to those recommended in the diagram, you can use other microwaves of the appropriate structure and with a cutoff frequency of more than 900 MHz, transition capacitances of not more than 2 pF and a short time constant of the OS circuit (no more than 10 ... 15 ps). For transistors VT1 and VT4, the OS circuit time constant and noise figure are especially important. If it is necessary to replace them, KT368, KT3109, KT325, KT355, KT372 with letter indices corresponding to the above parameters are suitable. As VT6 and VT7, you can use any high-frequency corresponding structures: KT312. KT3102. KT3107 with any letter indices, etc. Instead of K157UL1A (DA1 and DA3), you can use K157UL1B, K157UD2 (DA2) will completely replace any general-purpose operational amplifier that can operate at the supply voltage indicated on the circuit. As VT2, VT3, VT5, KP327 with other letter indices is suitable.

Coils L1 - L3 are wound on frames with an outer diameter of 6 mm with PEL-1 wire 0.45 mm and contain five turns each. Their inductance is adjustable with brass trimmers and with M5 thread.

With proper installation and serviceable radio components, setting up the receiver is extremely simple. It is necessary to set a voltage of +12 V on the capacitor C19 with a variable resistor R4.5. And then, by rotating the coil trimmer L3. tune the receiver to a radio station for the best sound quality. In the presence of interference, it may be necessary to more accurately adjust the low-pass filter boundary with the trimmers of coils L1 and L2. To reduce mutual inductance, these coils should be positioned so. so that the axes are perpendicular.

Receiver parameters can be improved. For example, to increase the suppression of spurious reception channels on the harmonics of the local oscillator by using a three-section low-pass filter at the receiver input. But in this case, it is desirable to shield the filter coils.

By reducing the resistance of the resistor R13, it is possible to increase the capture bandwidth at audio frequencies and thus approximately double the sensitivity of the receiver. But here more accuracy is required in tuning the local oscillator. Unfortunately, this worsens the signal-to-noise ratio at the output of the receiver. You will have to choose what is more important in specific reception conditions.

Author: A. Sergeev, Sasovo, Ryazan region

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