Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Based on publications in the Radio magazine [1, 2], I developed two VHF converters for a receiver with a range of 88 ... 108 MHz (VHF-2), which provide reception of signals from radio stations in the VHF-1 range. In the proposed designs, it was possible to confine ourselves to one inductor coil, and later to exclude it as well.

A diagram of a VHF converter with one coil is shown in Fig. 1. In a local oscillator made on a transistor VT1, a quartz resonator ZQ1 at a frequency of 8 MHz is excited at the third harmonic. The L1C2 circuit is tuned to a frequency of 24 MHz. The radio signal received by the additional antenna WA1 is fed to the mixer (transistor VT2). Through the capacitance between the crystal of the transistor VT1 and its case, the local oscillator voltage is supplied to the base of the transistor VT2. There it is mixed with the input signal, and the signal selected at the collector load VT2 is fed through the capacitor C4 to the telescopic antenna of the radio receiver.

If the car radio is being finalized, then the converter is included in the gap of the antenna wire.

All used resistors type MLT-0,125. Capacitors - KM or other small ones. Capacitor C1 can have a capacitance of 10004700 pF, C2 - in the range of 47-68 pF, C3 and C4 - from 33 to 56 pF, capacitance C5 - up to 0,1 microfarads.

Diodes - any silicon. Transistors VT1 - series GT322 or GT313, VT2 - KT316 or KT368 with any letter indexes. Coil L1 contains ten turns of PEV-2 0,35 wire, it is wound on a frame with a diameter of 7 mm with a ferrite trimmer with a diameter of 2,8 mm.

The converter setup begins with checking the supply voltage at various points of the device (with the radio turned off). The voltage at C5 should be within 2 ... 2,2 V. When choosing resistor R4, it is necessary to set the voltage at the collector of transistor VT2 within 1 ... 1,1 V.

Next, set the local oscillator transistor mode for direct current. To do this, it is necessary to disrupt the high-frequency oscillations (turn off the capacitor C2 and unscrew the trimmer of the coil L1 or generally close the conclusions of the oscillatory circuit). When choosing resistor R2, it is necessary to set a voltage drop of 3 ... 0,3 V on resistor R0,35. After restoring the oscillatory circuit, you need to make sure that this voltage has not changed, that is, the local oscillator is not self-excited. Then, by installing a trimmer in the coil and slowly introducing it, we will make sure that the voltage increases from 0,3 to 0,8 V. This indicates that the local oscillator is working, and the oscillatory circuit L1C2 is tuned to resonance at the frequency of the third harmonic of the quartz resonator. Rotating the trimmer in the opposite direction, it is desirable to set the voltage slightly less than the maximum. Then, an external antenna is connected to the capacitor C3, and a piece of flexible wire with a crocodile clip at the end is connected to the capacitor C4 and connected to the radio receiver antenna.

Before finalizing the device, you should check which radio stations can be received without a converter. Then, turning on the converter, they check for the presence of signals from new radio stations already in the VHF-1 range (it is desirable to have a complete list of radio stations operating in the area). As a coil L1, you can use a short-circuited (Fig. 2) piece of thin coaxial cable PK50 with a diameter of 4 mm (for example, PK50-4-2). With a capacitance of capacitor C2 of 56 pF, 1,5 m of cable was required.

To operate a converter without coils, a quartz resonator is required, in which the fundamental frequency lies within 24 ... 24,5 MHz [3]. In the device (Fig. 3), the resonator is excited at the fundamental frequency. To do this, capacitor C2 must have a capacitance in the range of 56-75 pF, C3 22-47 pF, and C1 and C2 - from 3300 to 15000pF.

To test imported quartz resonators used in IBM PC XT and AT computers, the instrument described in [4] and a frequency meter were used. By the way, the readings of the frequency meter did not always coincide with the inscriptions on the quartz. It showed 24 MHz for quartz IQG, UNI, KTS, BCG, DMC, KDSI, ETL, TQG, SPK, THS and 8 MHz - ATC, DEL, PINE, KDS, AQUIS, AEC, SAS, MEC, HOORAY, while on quartz had an indication of the frequency "24 MHz".

The disadvantage of the proposed options for converters is a strong capacitive coupling between the antenna and the local oscillator. Because of this, on the VHF band, in some places, the signals of KB band stations are heard.

Literature

1. Ataev D. VHF converter with quartz stabilization. - Radio, 1999, No. 3.
2. Stepanov V. Universal VHF converter. - Radio, 1994, No. 10, p.13.
3. Monakhov M. VHF converter. About the choice of the local oscillator tuning frequency. (Our consultation). - Radio, 1994, No. 7, p. 44.
4. Agafonov Yu. Device for testing quartz resonators. - Radio, 1989, No. 4, p.64.

Author: A.Menshov, Slavutich, Kyiv region, Ukraine

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