Simple FM transceiver. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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After we were finally allowed to use portable and portable VHF radios, interest in the design of VHF FM transceivers increased markedly.

One of the difficulties that a radio amateur faces in the manufacture of such a radio station. - the need to have matched pairs of quartz resonators (one for TX, the other for RX). Moreover, the spacing of their frequencies, as a rule, is strictly tied to the standard IF values, which are determined by the main selection filters.

There is one ingenious solution to this problem, which was proposed many years ago for the simplest wearable radios designed to work through repeaters. Its essence is the following. For repeaters, the standard frequency spacing between receiving and transmitting is 600 kHz. If a quartz resonator with a frequency corresponding to the input frequency of the repeater (naturally, at some harmonic) is installed in the transmitting path of the transceiver, then the same local oscillator can also be used for the receiver. True, here a restriction is automatically imposed on the intermediate frequency of the receiving path. It must be equal to the frequency spacing between receiving and transmitting the repeater, i.e. 600 kHz.

In industrial production equipment, such a low IF is not used, since in the 144 MHz range, in this case, the input circuits practically do not suppress the receive image channel. However, for an amateur radio station, this is in many cases quite acceptable, since the probability of interference on the image channel at the current very low level of development of VHF communications in ex-U is very small.

A similar solution can be applied to the manufacture of a pair of very simple radio stations designed to organize communication between two correspondents. Moreover, for such a pair of radio stations, only two quartz resonators are required. The limitations on their frequencies are obvious. Since both will be used in the transmit path, their frequencies (taking into account the multiplier to the operating frequency) must be within the amateur band. The second limitation is also not hard. The difference in their frequencies (again, taking into account the multiplication factor) should be no less than, say, 100 kHz and no more than 1 ... 1,5 MHz. It will determine the value of the IF and the receiving path of both radio stations.

The lower bound of this interval is, generally speaking, not critical. In the general case, it can even be 20...30 kHz (i.e., selection in the IF path can also be performed on RC filters), although for design reasons, its value of several hundred kilohertz is preferable. This makes it possible to manufacture filters of the main selection on small-sized magnetic circuits (SB-12a and the like). But at low IF values, it is more difficult to provide the optimal bandwidth (it must be at least 10 kHz), which is necessary when using FM with a modulation index of about 1, adopted on VHF.

The IF cannot be more than 2 MHz (the frequency band reserved for the amateur band is 2 m). Otherwise, the first condition cannot be met, and the frequency of one of the stations will go beyond the amateur range.

There is one more limitation. It is desirable that the passband of the IF path does not include frequencies that are used by local LW or MW radio stations.

A schematic diagram of a variant of the VHF FM radio station, in which the above ideas are implemented, is shown in the figure.

(click to enlarge)

In the master oscillator (made on the transistor VT1), quartz resonators can be used at frequencies of 9000 ... 9110 kHz. The upper frequency of the 2 m range corresponds to a resonator frequency of 9125 kHz, but resonators should not be used at frequencies above 9110 kHz - amateur satellite communications may be interfered with, which, of course, is unacceptable. Resonators from a personal radio station will also work. These resonators are usually 27rd harmonic driven and are labeled accordingly (XNUMX MHz, etc.). However, in this design, such a resonator will be excited at the fundamental frequency.

Bandpass filter L2C6L3C8 selects the RF voltage corresponding to the fourth harmonic of the quartz resonator. The two stages following the master oscillator (VT2, VT3) are frequency doublers. The output stage is assembled on a transistor VT4.

When working on the reception, the cascade on the transistor VT2 (more precisely, its emitter junction, since the power to the transistor will not be supplied in this case) performs the function of a frequency quadrupler. The L12C11 circuit is tuned to the 16th harmonic of the quartz resonator. From this circuit, the RF voltage is supplied to the receiver mixer, which is made on a VT5 field-effect transistor. Although the multiplier uses a passive element (diode) and the transfer coefficient of the multiplier itself is less than one, the gate of the mixer transistor receives a voltage sufficient for its operation (due to the transformation on the L12C11 circuit). The main selection filter is the simplest - it contains only one circuit (L13C20).

The functions of the IF amplifier, demodulator and AF amplifier are performed by the DA1 chip. Variable resistor R14 - volume control (in DA1 there is an electronic level control unit for the output signal).

From reception to transmission, the transceiver is switched by switch SA1, through which power is supplied either to the receiving or to the transmitting path. In the transmission mode, the supply voltage will also be applied to the carbon microphone, the AF voltage from which comes to the varicap. To get a high control slope, the varicap operates at zero bias, which makes it possible to do without an additional microphone amplifier (though, provided that the microphone is carbon, i.e. it develops a relatively high AF voltage).

This transceiver can be reproduced with minimal modifications on the domestic element base. Transistors VT1-VT3 are interchangeable with transistors of the KT342, KT312, KT316 or similar series, VT4 - with KT603, VT5 - with KP350 or KP306. Varicap VD1 can be KV102.

We do not have an analogue of the TBA120S microcircuit, but the K174UR1 microcircuit is very close to it. Judging by the information we have, it differs only in that it does not have additional audio frequency amplification stages. In general, the connection of these microcircuits coincides with the accuracy of the conclusions. However, with a typical inclusion of K174UR1, the C27R15 circuit was not used, pins 3 and 4 are free, and the AF signal with a fraction of a volt level is removed from pin 8. An additional AF amplifier (for connecting a low-resistance speaker) can be made on a KT315 transistor or similar. You can do without the T1 transformer, but then the amplifier must be made on the K174UN7 chip or similar (in a typical inclusion).

Coil L1 can have (depending on the used quartz resonator and varicap) from 1 to 10 turns of wire with a diameter of 0,3 mm on a frame with a diameter of 5 mm. Coil L2 contains 28 turns, and L3 - 25 turns of wire with a diameter of 0,3 mm. Winding ordinary, coil to coil. Frame diameter 3 mm. The tap from the L3 coil is made from the 6th turn, counting from its "cold" end. Coil L4 contains 8 turns of wire with a diameter of 0,8 mm on a frame with a diameter of 6 mm. Winding ordinary, coil to coil. Coil L5 is located at the "cold" end of L4 and has 4 turns of 0,5 mm wire. Coil L6 has 7 turns, L7 - 2. The frame, wire and winding nature are the same as for coils L4, L5. The L8 coil has 6 turns, the L10 has 3 turns of wire with a diameter of 0,8 mm on a frame with a diameter of 6 mm. The L9 inductor contains 5 turns on a miniature ferrite ring with an initial magnetic permeability of at least 400. The L11 coil has 6 turns of wire with a diameter of 0,5 mm on a frame with a diameter of 5 mm. Retraction from 1,5 turns, counting from the "cold" end of the coil. Coil trimmers are made of carbonyl iron.

There is no more detailed information about them (type of material, dimensions) in the source material. Winding data for coils L12 and L13 are not given, since they (like the value of capacitors C20 and C26) are determined by the specific value of the IF.

Literature

1. "PZK Biuletyn"
2. KV magazine №1 1992

Publication: N. Bolshakov, rf.atnn.ru

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