Converter 144/27 MHz with a smooth local oscillator. Scheme, description

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Converter 144/27 MHz with a smooth local oscillator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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To receive radio stations operating in the VHF band 144 ... 146 MHz on a transceiver or receiver of the "civilian" CB band (27 MHz), a converter is required. If the CB transceiver used is a range one, the converter can be made with a fixed local oscillator. This version of the converter has already been proposed to the readers of the Radio magazine (I. Nechaev. "144 MHz converter for the CB radio station." - Radio, 1999, No. 2, pp. 57, 58). If the transceiver has only one or two fixed operating frequencies (for example, the Ural-R radio station), a converter with a tunable local oscillator is needed.

A diagram of such a converter is shown in Fig. one.

Converter 144/27 MHz with smooth local oscillator

A UHF is assembled on transistor VT1, a local oscillator on transistor VT2, and a mixer on VT3. All of the listed nodes are powered by a voltage regulator on the DA1 chip, so any source with a voltage of 12 ... 30 V can be used to power the converter. The converter works as follows. The received signal from the antenna through the XW1 connector is fed to the input circuit L1C3, where it is pre-selected. The signal amplified by the transistor VT1 is fed to the second RF circuit L3C11 and then to the input of the mixer, the base of the transistor VT3.

The local oscillator signal, through capacitor C14, is also fed to the input of the mixer. Frequency tuning of the local oscillator is carried out by a variable capacitor C2. The difference signal selected by the L4C16 circuit from a part of the turns of the coil is fed to the output of the converter, the XP1 connector. Diodes VD1, VD2 are used to protect the elements of the converter from the signal i of the radio station's own CB transmitter.

The following parts can be used in the converter: transistors VT1, VT2 - KT399A, VT3 - KP307V, diodes - any small silicon ones. Capacitors: variable capacitance C2 - with an air dielectric and, preferably, with a vernier; trimmers - KT4-25; constants - KLS, K10-17, KM. Fixed resistors - MLT, S2-33, R1-4, trimmer - SPZ-19. All converter coils (except L4) are wound turn to turn with PEV-2 0,8 wire on a mandrel with a diameter of 5 mm. L1 - contains 4 turns with taps from 1,5 and 2nd turn; L2 - 6 turns with a tap from the 1,5th turn; L3 - 4 turns with taps from 1,5 and 3rd turn. Coil L4 contains 25 turns of PEV-2 0,2 wire with a tap from the 5th turn.

Most parts of the converter are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in fig. 2. The second side of the board is left metallized and is connected by foil along the edge of the board to the common wire of the first side. The board is placed in a metal case of a suitable size. The input RF connector XW1 is installed on one of the side walls of the case. The output signal of the converter is fed to the input of the CB radio station through an RF cable with an XP1 plug at the end.

Converter 144/27 MHz with smooth local oscillator

Setting up the device begins with setting the frequency and tuning range of the local oscillator. This operation is carried out by selecting capacitors C1, C4 and within a small range, pushing the turns of the coil 12. If, for example, the converter is operated with a radio station operating at a frequency of 27,2 MHz, in order to obtain a received frequency range of 144,5..145,8 MHz , where FM operation is allowed, the local oscillator frequency must be tuned within 117,3 ... 118,6 MHz. Setting the local oscillator frequency should be done with some margin at the edges of the range. Then, with the capacitor C16, the L4C16 circuit is tuned to the operating frequency of the CB radio station, and with the capacitors C11, C3, respectively, the circuits L3C11, L1C3 are tuned to the center frequency of the VHF band. When receiving weak signals from radio stations with a tuning resistor R10, according to the best reception quality, the maximum transmission coefficient of the mixer is set.

After completing the preliminary adjustment, all coils should be lubricated with epoxy glue, and only then carry out the final adjustment of the converter

The layout of the device assembled by the author had a transmission coefficient of 30 dB, which made it possible, together with the Ural-R radio station, which has a sensitivity of 1 ... 2 μV, to obtain a total sensitivity of 0,18 ... 0,2 μV. The current consumed by the converter is 15 mA.

It should be noted that the frequency stability of the local oscillator will be low, but quite satisfactory for receiving FM signals. It can be increased by using a ceramic frame for the local oscillator coil and selecting TKE loop capacitors.

Author: I.Nechaev (UA3WIA)

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