A simple FM transmitter on the CB band. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple FM transmitter on the CB band. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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The transmitter power does not exceed a few milliwatts, and the operating frequency is determined by the frequency of the applied quartz resonator. An electret type microphone (MKE-3 or similar) is used as a microphone.

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Power is supplied to the microphone through a filter consisting of a resistor R1 and a capacitor C1. The audio frequency voltage through the capacitor C2 is fed to the input of the audio frequency amplifier. The amplifier is assembled on transistors VT1 and VT2. The DC operation mode is set and maintained by negative feedback through resistor R3. Capacitor C5 weakens the effect of negative feedback at audio frequencies. The amplified audio signal through the RC low-frequency filter, made on the elements R7, R8 and C4, is fed to the VD1 varicap.

Under the action of an alternating voltage of audio frequency, the capacitance of the varicap connected in series with the quartz resonator Z1 changes. In this case, the frequency of the generator assembled on the transistor VT3 is modulated. The quartz resonator can have a frequency of 26,960 ... 27,280 MHz or 8987 ... 9093 kHz. In the latter case, the generator will be excited at the third mechanical harmonic of the quartz resonator. The circuit in the collector circuit of the transistor VT3 is tuned to an operating frequency of 27 MHz by selecting the capacitor C7 and stretching the turns of the coil L1. Coil L1 is wound with 0,8 mm PEV wire on a mandrel with a diameter of 8 mm and contains 10 turns. The high-frequency signal enters the antenna through the coupling capacitor C9. A piece of wire 100 ... 150 cm is used as an antenna.

Setting up the transmitter comes down to selecting the resistance value of the resistor R3, at which a voltage of about 2 volts is set on the collector of the transistor VT4, as well as to setting the output circuit to the maximum radiation. The circuit is adjusted by compressing and stretching the turns of the L1 coil.

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