Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Using a simple converter, you can receive broadcast programs in the VHF-1 band (65,8 ... 73 MHz) and on a radio receiver that has only the VHF-2 band (88 ... 108 MHz).

In the version of the converter proposed by the author, the local oscillator frequency is stabilized by a quartz resonator, which ensures high stability in receiving radio stations.

In those cities of Russia where there is broadcasting in the range of 88 ... 108 MHz, the frequency band from 92 ... 100 MHz is practically not occupied.

In order to transfer the frequencies of the broadcasting range of 65,8 ... 73 MHz to this section in the converter, the local oscillator can have a frequency in the range of 26,2 ... 27 MHz. A frequency was chosen in the middle of this interval - 26,6 MHz. The crystal oscillator of the converter operates at the third harmonic, so it should be selected with a nominal frequency of 8,86 MHz. If in the VHF range of 88 ... 108 MHz there are other frequency sections free from radio stations, then quartz resonators with a different nominal frequency (ranging from 7,5 to 11,7 MHz) can be used.

Schematic diagram of the converter is shown in Fig.1.

The radio station signal received by the WA1 antenna and selected by the L1 L2 C1C2 circuit with a tuning frequency of 69,4 MHz is fed to a mixer made on a VT1 transistor. The local oscillator is made on the transistor VT2. The generation frequency of its oscillations is stabilized by a ZQ1 quartz resonator. Through the coupling coil L5, the local oscillator voltage is transmitted to the emitter circuit of the mixer transistor VT1. The load of the mixer is the L3 coil, on which a signal with the total frequency of the input and heterodyne signals is selected. Through the L4 communication coil, it goes to the radio receiver with a range of 88 ... 108 MHz or to the second (radiating) antenna.

On fig. 2 shows the printed circuit board of the converter, made of foil fiberglass.

All used resistors type MLT-0,125. Capacitors, except C8, tubular or disk. Capacitor C8 - any oxide. KT315 transistors with any letter index, they can be replaced with KT312 or KT361 (in the latter case, you need to change the polarity of connecting the power supply and capacitor C8). Capacitor C5 should have a capacitance of 33 - 68 pF at a frequency of a quartz resonator in the range of 7,5 ... 9,5 MHz and 18 - 33 pF - at high frequencies. Its capacity is established empirically when adjusting the device.

Coils L1 and L2 are wound with PEV-1 0,4 wire on a 100NN grade ferrite rod. Its length is 14, diameter is 2,8 mm. The number of turns L1 - 4, L2 - 6. Coils L3 and L4 are frameless with an inner diameter of 5 mm, each has five turns of wire PEV-2 0,35. Coils L5 and L6 are wound on a frame with a diameter of 7 mm with a carbonyl iron trimmer from the IF filters of the image or sound boards of any television receivers. The number of turns L6 is 14, the winding is turn to turn, in the immediate vicinity on the frame there is an L5 coil having two turns. For their winding, wire PEV-2 0,35 was used.

The converter consumes current no more than 10 mA. Its power supply must have good filtering and output voltage stabilization.

The adjustment of the converter begins with checking the operation of the local oscillator. It is easiest to control its excitation with a probe, the circuit of which is shown in Fig. 3. It should be connected in parallel with the L5 coil. Connection of outputs is arbitrary. The PA1 measuring head will fit any with a full deviation current from 50 to 200 μA. In the author's version of the probe, a small-sized pointer indicator of the M476 type was used without any alterations, which is used in reel-to-reel and cassette tape recorders and radio tape recorders.

The generator is tuned by rotating the trimmer of coils L5, L6 until the probe indicator arrow deviates by the maximum possible value. If the deviation is small, you need to reduce the resistance of the resistor R1 (Fig. 3) to 7,5 kOhm.

For stable excitation of the local oscillator, it is recommended to set the trimmer to the middle position of the zone in which generation is maintained. This zone is determined by slowly rotating the trimmer of coils L5 and L6 from the beginning of generation to the stall according to the probe.

If the indicated method fails to achieve the local oscillator generation mode, you should check the correct installation of this cascade, the serviceability of the elements and try to select the capacitance of the capacitor C5. It should be noted that there are quartz resonators that are difficult to excite on the third harmonic, even if they work well on the fundamental. Such a quartz resonator must be replaced.

After that, one of the known (for the given region) radio stations is received on the receiver with a converter within the frequency range of the scale 92 ... 100 MHz and the input oscillatory circuit is adjusted by shifting or stretching the turns of the L2 coil until a good sound quality of the received program is obtained.

In some regions where television broadcasting is organized using the 4th and 5th television channels, there may be problems with the compatibility of the converter and the TV (especially if an indoor antenna or its surrogate varieties are used for the TV). In this case, it is best to connect the converter directly to the antenna input of the radio receiver with a wire of minimum length, and place the converter itself in a metal screen. If the signal is transmitted to the receiver by an additional antenna connected to the L4 coil, then it must have a minimum length (about 20 mm).

Author: D.Ataev, Sterlitamak, Bashkortostan

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