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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Broadcasting VHF receiver with double frequency conversion. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception One of the problems in building a superheterodyne receiver is the elimination of image channel reception. The known method of double conversion with a sufficiently high first intermediate frequency in amplitude modulated receivers has been used for a long time. But for receivers with frequency modulation, this method was not used. Meanwhile, the problem is solved quite simply: you need to choose the right value for the first intermediate frequency. The range of frequencies received by the receiver - 53 ... 108 MHz - is selected with the calculation of reception as the sound accompaniment of television channel programs (from 1st to 5th). and both VHF sub-bands (65.8 ... 74 and 87.5 ... 108 MHz). The sensitivity of the receiver with a signal-to-noise ratio of 40 dB is not worse than 10 μV in television channels, 6 μV - in the range of 65.8 ... 74 MHz and 14 μV - in the range of 100 ... 108 MHz. Selectivity in the image channel, measured at the first intermediate frequency, in the low-frequency section of reception is not worse than 41 dB. in the high-frequency section - no worse than 18 dB. The last of these parameters should not confuse radio amateurs, since with the selected high first intermediate frequency, there are no broadcasting stations at all in the sections of the mirror channel. The monophonic receiver is built using the UPCZ-2 microassembly. previously widely used in the circuits of domestic TVs. It is a 6.5 MHz IF ready amplifier. contains a lumped selection piezoceramic filter with a center frequency of 6,5 MHz, a frequency detector and a pre-amplifier with adjustable gain. The receiver could also be made with a single conversion, but with such a construction with a low intermediate frequency (6,5 MHz), it will either have low selectivity over the image channel, or it will be necessary to use a VHF unit with several tunable selective circuits tuned to the frequency of the input signal . When the local oscillator frequency is higher than the signal frequency (F1 = Fbase + 2Fpm = Fbase + 13 MHz, where Fpm is the frequency of the mirror reception channel, Fbase is the frequency of the main receive channel, Fp is the intermediate frequency), the following disturbances in signal reception are possible:
In addition, in the areas of the frequency range that are not allocated for broadcasting, stations of the mirror reception channel will be received. For example, the range 65.8.-74 MHz will be taken as a mirror when tuning the receiver to frequencies 52.8...61 MHz. If the local oscillator frequency is chosen below the signal frequency, the picture will change, but will not improve, and will create an additional problem - the overlap factor in the frequency of the local oscillator will have to be increased from 1.92 to 2.18. For the above reasons, it was decided to make the receiver using television normalized nodes and double frequency conversion. The receiver circuit is shown in fig. 1.
The first IF is 32 MHz. the second is 6.5 MHz. The first IF roughly corresponds to the first IF of the sound of standard television receivers. She was chosen that way. so that the mirror channel is located between the 5th and 6th television channels (the sound carrier of the 5th channel is 99.75 MHz. The image carrier of the 6th channel is 175.25 MHz). The frequency of the first local oscillator of the receiver is higher than the frequency of the input signal, while the mirror reception channel for the first IF is in the range of 117 ... J72 MHz. The input circuit L2C2VD1 has an inductive connection with the antenna input. To ensure a higher quality factor, the input of the DA1 microcircuit (K174PS1) is connected to part of the turns of the coil 12. The circuit is tuned in the range of 53 ... 108 MHz by the VD1 varicap. The first converter is made on the DA1 chip. its local oscillator with an oscillatory circuit L3C4-C9VD2 has a tuning frequency within 85 ... 140 MHz. Tuning to the received stations is carried out by the SA1 range switch (step change in voltage on varicaps) and variable resistors R8 and R9 (smooth change in voltage) - each subband has its own tuning organ. This construction allows you to save the receiver tuning on a currently unused subband. Switch SA2 performs the role of enabling and disabling the APCG system. The first intermediate frequency (32 MHz) is allocated by the L4C10 circuit and fed through the L5 coupling coil to the input of the second frequency converter assembled on the DA2 chip. The frequency of the second local oscillator (circuit L6C13-C16) is fixed and equal to 38,5 MHz. Therefore, when tuning the receiver to a reception frequency of 77 MHz, an unmodulated carrier will be received - the second harmonic of the second local oscillator signal. It is this single affected point that is chosen as the point of separation into two ranges of frequencies received by the receiver. To convert the symmetrical output signal of the second frequency converter to unbalanced and match the high output impedance DA2 with the low input assembly A1, the L7C17 circuit with the coupling coil L8 is used. tuned to a frequency of 6.5 MHz Capacitor C24 is needed to compensate for frequency pre-distortion introduced on the transmitting side. UMZCH is assembled on a DA3 chip. The receiver is powered by a network unit, the schematic diagram of which is shown in fig. 2. The current consumption in the +9 V circuit in silent mode is approximately 30 mA. It is mainly determined by the current of the UPCHZ-2 module.
A half-wave rectifier on diodes VD8, VD9 generates a constant voltage on the filter capacitor C34 equal to twice the amplitude of the alternating voltage taken from the winding III T1 (-28 V). For better filtering, an active filter on a VT1 transistor of the KT972 type is used. The output voltage is stabilized by the R21VD10 circuit. The doubling scheme with the subsequent formation of the voltage necessary to power the varicap circuits was chosen from the conditions of better stability when the primary network voltage is reduced by up to 15%. Capacitors C30 and C31 in the primary circuit of the power transformer connect the power cord to the chassis at high frequency, and it serves as a counterweight. The receiver parts are mounted on a universal printed circuit board (Fig. 3) and interconnected by conductors in PTFE insulation.
Coil 12 is frameless, wound with a silver-plated wire with a diameter of 0.6 mm on a mandrel with a diameter of 7 mm (7 turns), when installed on a board, it should be stretched to a length of 10 mm. The taps from the 2nd and 5th turns are soldered directly to the coil wire. The communication coil L1 - one turn of a wire with a diameter of 0,3 mm in the insulation is located on top of the turns 12 in its middle part. Coil L3 - 4 turns. L4 - 15 turns with a tap from the middle. L5 - 3 turns over L4 in its middle part and L6 - 15 turns. All these coils are wound turn by turn with 0.3 mm diameter wire in lacquer insulation on 5 mm diameter frames with ferrite or carbonyl trimmers. For the L7 coil, fittings and ferrite cups from the IF circuits of the Alpinist radio receiver were used; it has 20 turns of wire with a diameter of 0,2 mm with a tap from the middle. Coil L8 - 5 turns of the same wire, located on top of turns L7. The capacitors that are part of the local oscillator circuits (C5 - C8 and C13 - C16) must have a TKE M47 or M75 group to ensure the necessary stability of the receiver tuning frequency. Capacitors C30. C31 - ceramic, with an operating voltage of at least 300 V. Variable resistors R8. R9 and R12 type SP4-1. The UPCHZ-2 module can be replaced with UPCHZ-1M, taking into account the fact that it has a different pin numbering. Transformer T1 is made on the basis of a low-power toroidal power transformer for printed wiring TPP-32. which has only one secondary winding with a voltage of 28 V. In this design, it is used to generate a voltage of +30 V. An additional winding for a voltage of 12 V - 400 turns, wound over the existing wire with a diameter of 0.2 mm and used to generate a voltage of +9 V. The receiver, together with the power supply unit, is housed in a housing made of one-sided foil fiberglass (Fig. 4). The body parts are soldered together. Housing dimensions 53y 170 ^ 36 mm. To connect the antenna, an instrument socket of the type СР-50-73ФВ is provided; The connector for connecting an external speaker is a standard socket for connecting a speaker system.
It is more convenient to start setting up the receiver with the output UMZCH. By supplying +9 V power to it from a laboratory power supply, and to the input through a separating capacitor (it may be C25) any audio frequency signal, they are convinced of its operability. Then you should check the cascade with the UPCHZ-2 module. Output 3 of the module is disconnected from the L8 coil and touched with a finger - with a working module, you can usually hear the operation of shortwave radio stations (the effect is more evident in the evening). To check the frequency converter at the input of the DA2 microcircuit through an additional coupling coil containing 1 turn and wound next to LA L5 on the same frame, it is necessary to apply a signal from a laboratory high-frequency generator with a frequency of 32 MHz, modulated with an audio frequency of 1 kHz, deviation l 50 kHz. By adjusting coil trimmers L6. and then L7 and L4 and successively reducing the signal level from the high-frequency generator, they achieve the greatest sensitivity from the input of the second converter at a frequency of 32 MHz. In this case, it is necessary to make sure that the frequency of the second local oscillator is higher than the frequency of 32 MHz by 6.5 MHz, and not lower by the same amount. This can be done by applying a frequency of 45 MHz from a high-frequency generator. - a modulating signal should appear at the receiver output, since 45 MHz is a mirror reception channel at the second intermediate frequency. And finally, they check the first frequency converter on DA 1. To adjust it, you need a +30 V voltage source (to change the capacitance of the varicaps). Setting up the input circuit and the oscillatory circuit of the first local oscillator is no different from setting up the interface of the circuits of a conventional superheterodyne receiver with a single frequency conversion. The boundaries of the frequencies received by the receiver are set in the lower part of the range (53 MHz) by the builder of the L3 coil. in the upper part of the range (108 MHz) - by selecting a capacitor Sat. The input circuit is tuned by applying a signal from the noise generator to the receiver input or focusing on the reception of radio stations operating on the air, and even on the own noise of the input stage. C12. doing this several times until an acceptable pairing is obtained over the entire range. It is also necessary to adjust the IF circuit (L2C4) to the maximum sensitivity of the receiver, since after connecting pins 10 and 2 of the DA3 chip to it, its setting may change. With the known skill of a radio amateur, the receiver can be tuned without instruments, if you approach the matter carefully and consciously. You can set the frequency of the second local oscillator, focusing on the reception of its second harmonic by the receiver itself. This signal must have a frequency of 77 MHz (38,5x2). Received as an unmodulated carrier, it should be located between the last received radio station operating in the 65.8 ... 74 MHz band and the audio carrier of the third television channel (83.75 MHz), next to the image carrier of the same channel (77.25 MHz). The circuits with frequencies of 6.5 and 32 MHz, as well as the input circuit, are adjusted to the maximum noise when tuning the receiver to an "unpopulated" section of the air, or they are tuned to the best signal-to-noise ratio when receiving weak station signals (by reducing the size of the antenna or turning it off completely). If desired, you can change the efficiency of the APCG by selecting the resistor R6. With a decrease in the resistance of this resistor, the APCG retention band expands, and with an increase it narrows. True, with the expansion of the holdband, the frequency range accepted by the receiver narrows. In St. Petersburg, for example, there are many radio stations operating in both VHF bands, television broadcasts are conducted on 1.3.6.8 and 11 meter frequency channels. All FM broadcasting stations, as well as the sound accompaniment of programs of the 1st and 3rd television channels, are received on this receiver within the city with a fairly high quality. There are practically no "false" radio stations (from side reception channels). After tuning the receiver to the desired radio station, no adjustment is required during the day, it stably "keeps the frequency". The author's receiver antenna is a mounting wire about 75 cm long (a quarter wave at a frequency of 100 MHz), which is most often twisted into a bay like this. that the length of the antenna does not exceed 30 cm. In conclusion, I want to note that any channel selector from SKM, SKD TVs is quite suitable as the first frequency converter. SLE When using an all-wave selector, it becomes possible to receive the sound accompaniment of programs going on any television channel, and the spacing of the subcarriers of the image and sound does not play a role. This can be useful in areas where foreign television programs are received on domestic televisions, but without sound. In such cases, to obtain sound, it is enough to connect the channel selector instead of the cascade to DA1, winding another communication coil (approximately 4 turns) on the L3 coil, the ends of which are connected to the selector output. Literature
Author: M. Shikin, St. Petersburg
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Comments on the article: Jury Good article. A digital setting would be nice.
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