A simple FM microphone

The circuit (see figure) uses a capacitive feedback oscillator, whose frequency is stable and adjustable. Its feedback signal is in the form of capacitor partial pressure, and the output signal of the oscillating tube is fed back to the input terminal. Among them, Re is a DC negative feedback resistor, C3 is a DC blocking coupling capacitor, and Ce is an emitter bypass capacitor. L, C1, C2, C form a resonant circuit. Since C2 is equivalent to being connected between the base and the emitter of the transistor BG, it forms a feedback circuit divided by C1 and C2. The feedback signal is taken from the voltage on C2. The oscillation frequency of the circuit is f = 1 / 2Ï€, where C = C1C2 / C1 + C2.

Experimental comments <br> The FM microphone is simple and easy to make, and is more suitable for beginners to imitate. In open areas, the transmission distance of this circuit is 20 to 30 meters. Long-term operating frequency has a large deviation. There are many harmonic content of the signal, which will interfere with the adjacent frequency.
In the specific production, MIC is best not to lead out with flexible wires, but to solder it firmly on the circuit board. The inductance L can be formed by spinning off 5-7 turns on a Φ0.3mm round bar. After adjusting the turn pitch, fix it with high-frequency wax. When judging whether the circuit is oscillating, the following simple method can be used. Use a normal pointer multimeter AC2V block, any test pen is floating, and the other test pen touches the antenna. If the pointer is wobbled, it means that the circuit has started to vibrate, and you can do the span adjustment.

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