Radio transmitter with high carrier frequency stability. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio transmitter with high carrier frequency stability. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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When using a quartz resonator with a high frequency, it becomes possible to create a simple radio microphone with a high stability of the carrier frequency. Below is a description of such a device. The radio microphone operates in the range of 61-74 MHz with frequency modulation.

(click to enlarge)

The signal from the M1 microphone of the MKE-3 type is amplified by a two-stage amplifier based on transistors VT1, VT2 of the KT315 type. The master oscillator is made on a transistor VT3 type KT368. Frequency modulation of the carrier frequency is provided by the VD2 varicap. Resistors K6 and R7 in the base circuit of the transistor VT3 determine its DC mode. Capacitor C9 sets the required generation mode, providing positive feedback.

The frequency stability of the generator depends mainly on the supply voltage. To increase it, you need to use a 6-9 V stabilizer, which will complicate the circuit. You can also stabilize the frequency in another way. To be precise, the cause of the instability of the carrier frequency is determined mainly by fluctuations in the operating point of the transistor VT2 of the audio amplifier when the supply voltage changes. The position of this operating point determines the reverse bias voltage on the VD2 varicap, and hence its initial capacitance. To stabilize the operating point of the amplifier on transistor VT2, a resistor R4 is included in its base circuit, the voltage to which is supplied from a parametric stabilizer assembled on resistor R2, LED VD1 and capacitor C1.

The device uses fixed resistors MLT-0,125, capacitors of types K50-16 and KM. Inductors Dr1, Dr2 can be used standard, for example, type D-0,1, with an inductance of 15-30 μH, or made independently. The inductors are wound on MLT-0,25 resistors with a resistance of more than 100 kOhm and contain 50-60 turns of 0,1 mm PEV wire. The L1 loop coil is wound on a frame with a diameter of 8 mm and contains 6 turns of 0,8 mm PEV wire. Coil L2 is wound on the same frame and with the same wire as coil L1. Coil L2 contains 3 turns placed at a distance of 1 mm from the turns of coil L1. The antenna is made as follows: a segment of a 50-ohm cable 10-12 cm long is stripped of insulation and the central core is removed. A PEV-0,6 wire is wound round the entire length of the cable segment - the antenna is ready. In extreme cases, a wire 30-50 cm long can be used as an antenna.

Tuning begins with an audio frequency amplifier. By changing the resistance of the resistor R4, the voltage on the collector of the transistor VT2 is set equal to half the voltage of the power source. The capacitance of the capacitor C9 must be selected according to the maximum current consumed by the generator, and then, with the resistor R6, set this current to about 10 mA.

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Comments on the article:

Ivan
In this scheme, even the varicap somehow strangely stands in the forward direction. How can this even be? All the voltage is shunted to the case, or I don’t even understand ...?

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