VHF converter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception
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This is a converter that allows you to listen to imported receivers transmitting VHF radio stations operating in the range of 66 ... 72 MHz. Confident reception is possible at a distance of up to 30 km from the radio station. Schematic diagram of the converter is shown in Fig.1.
Ris.1
The signal of VHF radio stations operating in the range of 66 ... 72 MHz is received by the WA1 antenna, is selected by the L2C1 input circuit tuned to the middle of this range and amplified by an aperiodic RF amplifier made on the VT1 transistor. The functions of the frequency converter are performed by a cascade on the transistor VT2, the base of which receives an input signal from the RF amplifier, and the emitter receives a local oscillator signal through the coupling coil L4.
The local oscillator is assembled on a transistor VT3. The frequency of the oscillations it generates is determined by the L3C5 circuit included in the collector circuit of the transistor VT3. The loop tuning frequency is selected within 22...36 MHz. In this case, the range of the total signal emitted on the collector load of the transistor VT2 (resistor R4) will correspond to the foreign VHF broadcasting standard, i.e. (62 ... 72) + (22 ... 36) \u88d 108 ... 4 MHz. The signal thus converted through the capacitor C22 is fed to the antenna input of the imported receiver. Moreover, the tuning frequency of the local oscillator circuit can be chosen anywhere from 36 ... XNUMX MHz. The only thing that depends on this choice is that at the end or the beginning of the receiver scale, the signals of domestic VHF radio stations will be received.
The board and the power supply of the converter (element 316 for a voltage of 1,5 V) are placed in a shielding case made of tin. The screen is necessary to reduce stray pickups on nearby radio equipment (TV, receiver, etc.). The wires connecting the converter with its own antenna and with the antenna output of the imported receiver are 20 ... 25 cm long. At the end of the wire connecting the converter with the receiver, it is necessary to provide a crocodile clip.
Resistors MLT-0,125 and capacitors KM-4, KM-5 or other suitable sizes were used for installation. Coils L1 and L2 are frameless, their winding diameter is 3 and 6 mm, respectively. The winding of the first coil contains 10 turns of PEV 0,51 wire, and the second coil contains 6 turns of PEV 1,0 wire with a tap from the second from the top (according to the diagram) turn. To tune the L1 coil, a trimmer with a length of 10 and a diameter of 2,8 mm from 100NN ferrite was used. Both coils are located on the board horizontally at an angle of 90° to each other.
Coil L3 is wound on a frame with a diameter of 6 mm and a height of 10 mm. It is equipped with an SCR trimmer with a diameter of 5, a height of 10 mm and contains 12 turns of PEV 0,25 wire. The L4 coil is wound on the same frame with the L3 coil and consists of two turns of PEV 0,25 wire placed below the winding of the L3 coil.
Instead of the transistors indicated in the diagram, you can use GT328A (B) and GT346A (B, C). Transistors must have a current transfer coefficient h21e>=100.
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