VHF receiver with PLL 1. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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VHF receiver with PLL. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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The VHF receiver offered to the attention of readers (Fig. 1) is made according to the direct conversion scheme with phase locked loop (PLL). It is designed to work with any low-frequency amplifier that has a sensitivity of at least 30 mV and an input impedance of at least 50 kOhm.

Main Specifications

Received frequency range, MHz 65,8...73
Sensitivity, µV ..... 100
Supply voltage, V .... 12
Consumed then, mA ..... 5

(click to enlarge)

The input signal highlighted by the contour L1C1C2. tuned to the middle frequency of the range of 69,5 MHz, is amplified by an aperiodic RF amplifier on transistor V1. All other receiver stages are assembled on a single A1 chip, which is a hybrid differential amplifier based on KT307B transistors. Its simplified circuit (without bias and power circuits) is shown in Fig. 1a. The signal is fed to the base of the current-setting (lower according to the scheme) transistor, and a push-pull local oscillator, a balanced mixer and a DC amplifier are made on the differential pair. The functions of the tuning body are performed by the variable resistor R3, which, if desired, can be replaced with a push-button switch (Fig. 2, b). The local oscillator is tuned by the varicap matrix V2. The control signal for the PLL is taken from the collector of one of the transistors of the differential stage of the A1 microcircuit and is fed through the resistor R5 to the varicap matrix.

The receiver uses a PLL with an integrating filter formed by the resistor R5 and the capacitance of the varicap matrix. The filter cutoff frequency is high enough (more than 60 kHz), so there are no problems with ensuring the stability of the PLL loop. Moreover, with strong signals, the local oscillator oscillations are directly captured by the signal, which reduces the phase shift in the PLL at high frequencies and makes the system absolutely stable. To facilitate the direct capture of the resistance of the collector loads of the transistors of the differential stage, they are chosen differently.

VHF receiver with PLL
Ris.2

The receiver is mounted on printed circuit board (Fig. 3) from foil fiberglass with a thickness of 1,5 mm. It uses fixed resistors MLT-0.125, variable resistor SP-1, trimmer capacitor KPK-M (C2), fixed capacitors KT-2 and KLS. Coils L1, L2 are printed. The tracks between their turns are cut with a cutter with a blade thickness of 1 mm. The stray fields of such coils are small. In order for the L2 coil of the local oscillator circuit to be symmetrical, two jumpers had to be installed on its intersecting turns. To improve the shielding, it is desirable to place the receiver board in a closed metal case.

The receiver can be made as an attachment to household equipment with a bass amplifier, or together with a bass amplifier and a power supply unit, it can be mounted in the housing of a conventional broadcast loudspeaker.

To establish the receiver, they begin by checking the operating modes of the transistor and the microcircuit. To pins 13 and 14, where there is a high-frequency voltage, the voltmeter probe (input resistance not lower than 20 kOhm / V) should be connected through a resistor with a resistance of 10 ... 30 kOhm. If the measured voltages differ from those indicated in the diagram by more than 10 ... 15%, resistors R1 and R7 should be selected. Then, having connected the antenna, you need to try to tune the receiver to all VHF radio stations, and then adjust the local oscillator circuit so that these radio stations fall into the receiver tuning range. The circuit is adjusted by soldering the slots (shown by a dashed line in Fig. 3) of the central turn of the coil L2. Entrance-. The circuit is tuned with capacitor C2 for the largest holding band when receiving transmissions from any station. In this case, the signal level at the input should be low, which is achieved by reducing the connection with the antenna.

For the receiver to work well, you should also select the signal level at its input by changing the position of the L1 coil tap (shown in dots in Fig. 3), or, if a wire indoor antenna is used, its position and length. With a weak signal, the holding band is insufficient, and at the peaks of the modulation, distortions in the form of wheezing are heard. With an excessively strong signal, the noise level increases, in free parts of the range the air seems to be “clogged” with noise and interference, and with an input signal above 15 mV, distortions appear due to direct signal detection.

Author: V. Polyakov, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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