A simple VHF FM receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception
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The VHF FM receiver offered to readers (see figure) is made on the basis of a direct conversion radio receiver with a PLL, developed at one time by a radio amateur from Krasnodar A. Zakharov (see "Radio", 1985, No. 12, p. 28-30).
The radio-frequency receiver stage is assembled on a VT1 transistor and is a frequency converter with a combined local oscillator, which simultaneously performs the functions of a synchronous detector. The receiver antenna is a headphone wire. The signal of the broadcasting station received by it is fed to the input circuit L1C2, tuned to the average frequency of the received VHF band (70 MHz) and then to the base of the transistor VT1. As a local oscillator, this transistor is connected according to the OB circuit, and as a frequency converter, according to the OE circuit. The local oscillator is tuned in the frequency range of 32,9 ... 36,5 MHz, so that the frequency of its second harmonic lies within the boundaries of the VHF broadcasting range (65.8 ... 73 MHz). The L2C5 circuit is tuned to a frequency half that of the L1C2 input circuit, and since the conversion occurs at the second harmonic of the local oscillator, the difference frequency is in the audio frequency range. Amplification of the difference frequency signal is provided by the same transistor VT1, which, like a synchronous detector, is connected according to the OB circuit.
Amplifier 3H receiver two-stage. The pre-amplification stage is made on a VT2 transistor, and the power amplification stage is made on a VT3 transistor. Listen to the received transmissions on the head telephone BF1 (TM-4). The output power of the 3H amplifier at a load with a resistance of 8 ohms when powered by one A332 element (1,5 V) is 3 mW, which is quite enough to work on a head phone. The current consumed by the receiver from the power supply does not exceed 10 mA.
The receiver can be assembled in any small-sized case. Hanging installation. Resistors - MLT-0,125, oxide capacitors - K50-6, trimmers - any with an air dielectric, the rest are KM, KLS. Coils L1 and L2 are frameless. Winding inner diameter - 5, step - 2 mm. Coil L1 contains 6 (with a tap from the middle), and L2 - 20 turns of wire PEV-2 0,56. Coils L3, L4 each contain 200 turns of PEL wire 0,06. They are wound on a ferrite (M400NN) rod with a diameter of 2 and a length of 10 mm in two wires. Transistor VT1 can be replaced by KT3102B, while the sensitivity of the receiver will increase.
Setting up the receiver begins with a 3-hour amplifier. The operating mode of transistors VT2, VT3 is set by selecting resistor R5 until the collector quiescent current of transistor VT3 is equal to 6 ... 9 mA. The local oscillator mode is regulated by the selection of resistor R1, the level of the second harmonic of the local oscillator - capacitor C6. The boundaries of the received frequency range are set by changing the inductance of the coil L2. The input circuit is tuned with capacitor C2, focusing on the maximum holding band of the signals of the received radio stations. The receiver is tuned in range by capacitor C7.
Author: D. Alekseev, Tambov; Publication: cxem.net
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