Encyclopedia of radio electronics and electrical engineering / radio reception

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VHF receivers on the K174XA2 chip, the descriptions of which were published in the Radio magazine in 1997, aroused the interest of readers. However, some of them experienced difficulties when repeating these designs - the receivers were excited. One of the reasons for this phenomenon could be that when self-laying printed circuit boards (they were absent in the description of the receivers), radio amateurs did not take into account some features of the operation of integrated circuits operating at high frequencies. In the design posted here, the PCB was designed with these factors in mind.

A schematic diagram of the VHF receiver is shown in fig. 1. It is designed to operate in the range of 65,8 ... 74 MHz and has an electronic setting for radio stations. The setting is carried out by the VD1 varicap, the control voltage to which is supplied from the variable resistor R10. When the slider of this resistor is moved, the control voltage supplied to the varicap changes, as a result of which its capacitance changes, and, consequently, the local oscillator frequency and the tuning frequency of the receiver. To the output of the detector chip DA1 (pin 1) is connected to the pre-amplifier AF transistor VT1.

The receiver is powered by an AC voltage source of 11 ... 15 V, which is rectified by a VD2 diode bridge. A stabilizer on a DA2 chip is connected to its output.

The receiver is mounted on a printed circuit board made of foil fiberglass with a thickness of 1,2 mm (Fig. 2). During installation, fixed resistors MLT-0,125, variables SP3-30a (R9) and SP3-36 (R10), oxide capacitors K50-35, the rest - KT-2 (C8, C11) and KM5-b were used. Coils L1 and L2 are frameless. The winding of the first contains 12, and the second - 6 turns of PEV-10,5 wire, the diameter of the windings is 4 mm. Winding step - 1 mm.

To reduce parasitic couplings, coils L1 and L2 should be placed perpendicular to each other. In order for the inductance of the leads of capacitors C6, C8, C11 and C13 to be smaller, their length should be as short as possible. In the absence of a varicap, a variable capacitor can be switched on instead, and the tuning resistor R11 and resistors R1, R4 can be excluded.

With proper installation and use of serviceable parts, the receiver starts working immediately after power is supplied and the antenna is connected. All that is required is to check the local oscillator frequency tuning range and adjust the input circuit to the middle of the received frequency range.

The described design has been in operation for a long time. The functions of the AF amplifier can be performed by the K174UN14 chip.

Author: P.Polyatykin, Moscow

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