Radio microphone at frequencies 350 ... 450 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio microphone at frequencies 350 ... 450 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The figure shows a circuit diagram of a radio microphone operating at frequencies of 350 ... 450 MHz. The operating frequency is determined by the ZQ1 element, which is a SAW resonator. This element is widely distributed in the amateur radio market and allows you to create miniature generators operating in the VHF range.

(click to enlarge)

Compared to oscillators based on quartz resonators, at frequencies of 350...450 MHz it is much easier to design self-oscillators using a SAW resonator. For example, the ZQ1 resonator of the SRU358N type makes it possible to obtain an oscillating frequency of about 358 MHz. It is very convenient to choose a resonator for a frequency of 430 ... 440 MHz. There are many family radios on this band that can be used as a radio microphone receiver.

A microphone amplifier is assembled on transistors VT1, VT2. The mode of operation of transistors for direct current is set by selecting the resistor R3. The specified mode is maintained automatically with the help of negative feedback, which covers two cascades on VT1 and VT2. The amplified audio frequency signal through the RC low-pass filter R7, C4 is fed to the VD1 varicap type KV109P. With its help, frequency modulation of a high-frequency signal is carried out. The constant voltage taken from the VT2 collector sets the initial offset on the varicap. The master oscillator is made on a transistor VT3 type KT399A. The operating mode of the transistor for direct current is determined by the resistance of the resistor R13. To obtain reliable generation, trimmer capacitors C8, C10 are used.

The installation of the high-frequency part of the radio microphone must be carried out with a minimum shortening of the leads of the parts (the length of the leads should be no more than 1,5 mm). Instead of the types of transistors and other elements indicated in the diagram, small-sized SMD components can be used. This makes the device very compact.

As an antenna WA1, a piece of rigid wire with a diameter of 2 ... 3 mm and a length of about 10 cm is used.

Author: V. Sinitsa, Samara

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