VHF FM receiver at 145 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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VHF FM receiver at 145 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Recently, radio amateurs have shown interest in working on VHF using frequency modulation (FM). This was largely facilitated by the appearance of several publications in the journal "Radio" [1-4]. But so far, there are still few descriptions of the simple designs of VHF radio receivers. This hinders the development and popularization of FM, as well as the organization of VHF FM amateur radio networks.

In developing the receiver described here, the authors pursued several goals. First, I wanted to create an easy-to-replicate design. This would contribute to an increase in the number of observers on the VHF bands and more intensive creation of VHF FM amateur radio networks for local communications. Secondly, it was proposed to use this receiver as a duty and control receiver (including for receiving operational, technical and sports information and monitoring the sporadic passage of radio waves). Thirdly, there was an idea to include it in a simple VHF FM radio station, to use it to work with the Mir space station. In addition, I wanted to use this receiver for the experimental reception of digital information.

In our opinion, the goals set have been achieved. The appearance in the wide sale of the K74 series microcircuits made it possible to create a small-sized, universal, simple and easily repeatable design with sufficiently high characteristics. The use of the UPCZ1M module from TVs in the receiver, which includes the K174UR4 chip and filters, made it possible to reduce the number of winding elements (IF circuits). At the same time, however, the IF path turned out to be relatively broadband (the bandwidth is about three times greater than the optimal one). But it is quite possible to put up with this, since so far the number of working amateur FM stations is small and, as a rule, they all operate on the same frequency.

The receiver is built according to a superheterodyne circuit with one frequency conversion (Fig. 1). It operates in the frequency range 145,4,., 145,7 MHz. Sensitivity - about 5 microvolts. The intermediate frequency is 6,5 MHz. RF bandwidth - 300 kHz, IF - 50 kHz. Receiver input impedance - 75 0m. Output power path 34 - not less than 0,5 watts. The device is powered by a 9 V source and consumes a current (at an average reception volume) of about 50 mA.

(click to enlarge)

The signal from the antenna through the capacitor C1 enters the circuit L1C2, connected completely to the first gate of the field-effect transistor VT1, which acts as an RF amplifier. By changing the bias voltage at the second gate of this transistor with a trimmer resistor R1, you can adjust the gain of the cascade to the required or optimal level. The circuit L2C6, which is the load of the RF amplifier, is partially connected to the drain of the transistor.

From part of the turns of the L2 coil, the RF signal is fed to the mixer, made on the DA1 chip. It also contains a smooth range generator. Its frequency-setting circuit L3C12 is rebuilt with a VD2 varicap within 139,9 ... 139,2 MHz. The 6,5 MHz intermediate frequency oscillations are highlighted on the L4C15 circuit. The selected IF is determined by the used UPCHZ1M module. The module contains a two-crystal band-pass filter, an eight-stage IF limiting amplifier, a detector and an AF preamplifier. The active part of the module is made on a K174UR4 chip.

From the output of the module (pin 6), the voltage of the AF through the volume control (resistor R8) is supplied to the final amplifier 34, assembled on the DA3 chip, which is switched on according to a simpler circuit than the typical circuit.

The output of the DA3 chip (pin 12) is loaded onto the BA1 loudspeaker.

Receiver parts are mostly small. All fixed resistors, except for R11, are OMLT0,125. Resistor R11 can be made independently by winding the required amount of high-resistance wire (nichrome) on the MLT0,25 resistor. As a tuning resistor R1, you can use SPZ38A, SPZ41 and others. Resistors R4 and R8 are almost any that a radio amateur has.

Capacitors of constant capacity can be used any small-sized, for example KM; oxide - K506 or more modern K5016. Capacitors C9-C11, C14 should be, if possible, with a small TKE value. Subscript capacitors C2, C6 - MP, C12-with an air dielectric 1KPVM, which, with the worst result, will be replaced by KPKMN (without changing the printed circuit board).

Instead of the K174PS1 (DA1) chip, you can use the K174PS4 board without modification. It is possible to replace the UPCHZ1M module with UPCHZ2. The K174UN7 microcircuit can be replaced (with a change in the printed circuit board pattern) by the K174UN4, however, the latter, as experience has shown, is unstable.

Transistor VT1 (KP306A) can be replaced by KP306 or KP350 with any letter index. The zener diode VD1 is small-sized with a stabilization voltage of 5,6 ... 8 V. The BA1 loudspeaker can be any with a voice coil resistance in the range of 4 ... 8 ohms and a power of 0,25 ... 1 W.

Coils L1 and L2 are frameless with an outer diameter of 6 mm, wound with a silver-plated wire with a diameter of 0,7 mm. The winding length of the coil L1 is 9 mm, the number of turns is 1+4, the coil L2 is 7 mm, and the number of turns is 1+1+2. In both cases, the turns are counted from the output connected to the power wire. Coil L3 is wound with the same wire as L1, L2, on a ceramic frame with a diameter of 5 mm (wound with tension), followed by impregnation with BF2 glue. Number of turns - 4, winding length - 10 mm. It is very convenient to use ceramic frames from the Mars VHF radio station for the manufacture of this coil. The L4 coil is wound with PELSHO 0,15 wire in the SB9a armored magnetic circuit. It has 20 turns, the tap is made from the middle.

The design of the receiver can be any. One of the possible design options for the device is shown at the beginning of the article. It is very convenient, for example, to assemble the receiver in the housing of a household subscriber loudspeaker using any power source with a voltage of 8 ... 12 V.

Most of the radio elements of the receiver are installed on printed circuit board, made of one-sided foil fiberglass with a thickness of 1,5 ... 2 mm. The placement of parts is shown in fig. 2, photomask - in fig. 3.

According to the size of the board, a base is made from a duralumin alloy, attached to it from below by means of M3 screws and metal bushings 5 mm long, which plays the role of a screen (Fig. 4). Holes should be drilled in the board and base for access to trimmers (C12, L4) and for fasteners.

VHF FM receiver at 145 MHz

The DA3 chip is attached to the printed circuit board with M2,5 screws and bushings. You can not put a heat sink on the microcircuit.

To communicate with external elements, mounting studs (or pieces of wire 10 ... 15 mm long) should be pressed into the printed circuit board. Resistor R4 ("Setting") is provided with the simplest scale with divisions every 25 kHz.

On the board, from the side of the parts, the area where the DAI chip is located, the L3C12, L4C15 circuits and some other parts are enclosed by a copper foil screen 0,15 ... 0,5 mm thick (see Fig. 2). Screen height 30 mm. Holes are provided in the board for its fastening and soldering.

With serviceable parts, the adjustment of the receiver consists in tuning the oscillatory circuits to the appropriate frequency. To establish, you need a signal generator, a VHF generator, a frequency meter operating at frequencies up to 150 MHz, and an AF generator.

The audio frequency path is checked by applying a signal with a frequency of 34 Hz and an amplitude of 1000 ... 50 mV from the generator 100 to the upper output of the volume control according to the circuit. The IF path - 34, with a working module and the DA3 chip, as a rule, works immediately. When a small piece of wire is connected to terminal 1 of the UPCZ1M module, broadcasting stations operating at frequencies near 6,5 MHz are heard.

When establishing the FC-34 path using a signal generator, a frequency-modulated signal with an amplitude of 1,.,8 mV and a frequency of 5 MHz is fed to the DA10 input (pin 6,5), By changing the position of the L4 coil trimmer, the maximum output signal volume is achieved receiver. If there is no frequency modulation in the device, then the L4C15 circuit is tuned until the hiss disappears in the loudspeaker.

Next, the L3C12 circuit in the GPA is tuned to a frequency in the range of 138,9 ... 139,2 MHz. The frequency meter is connected to pin 13 of the DA1 microcircuit through the smallest possible capacitance of the capacitor (1 ... 2 pF). In the presence of oscillations in the circuit, the capacitor C12 "drives" the GPA into the desired frequency range with the average position of the variable resistor R4. After that, the frequency overlap of the local oscillator is checked, it should be 300 ... 500 kHz. If necessary, the tuning interval can be changed by selecting capacitor C14.

The RF amplifier is adjusted by applying an operating frequency signal with an amplitude of about 100 μV to the receiver input. The slider of the resistor R1 should be in the middle position. First, the L1C2 circuit is tuned to the maximum output signal, and then, by reducing the signal level from the VHF generator to 10 μV, the L2C6 circuit. By the level of the output signal, the position of the taps of the coils LI, L2 and the position of the slider of the resistor R1 are specified.

The receiver with an external antenna (with an input impedance of 75 0m) is finally tuned while amateur radio stations are operating. Using a room antenna in the form of a vertical pin about 0,5 m long, the authors of the article observed the work of many amateur stations of the VHF FM radio network in Tver using a receiver.

Literature

1. Polyakov V. UK8 FM radio station. - Radio, 1989, No. 10, p. 30-34.
2. Allika M. FM transceiver at 144 MHz.-Radio, 1988, No. 3, p. 19-21, No. 4, p. 15-17.
3. Mikhelsoi A. FM receiver for the range of 430 MHz, - Radio, 1989, No. 11, p. 29-31.
4. Zakharov A. VHF FM receiver with PLL.-Radio, 1985, No. 12, p. 28-30.
5. Bondarev V. Rukavishnikov A. Application of the microcircuit K 174PS 1.-Radio. 1989, No. 2. p. 55-56.
6. Polyakov V. Radio communication with FM, - Radio, 1985, N9 1, p. 24-26.
7. Gorshkov B. Elements of radio electronic devices. Handbook. - M .: Radio and communication, 1988, p. 77, 78, 83.

Authors: E. Frolov (UA3ICO), V. Dolomanov (UA3IBT), N. Berezkin (UA3JD), Tver; Publication: N. Bolshakov, rf.atnn.ru

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