This circuit employs a capacitive feedback oscillator, offering stable and adjustable frequency performance. The feedback mechanism uses a capacitor divider to return the output signal from the oscillating tube back to the input. In this setup, Re serves as a DC negative feedback resistor, C3 acts as a DC blocking capacitor, and Ce is an emitter bypass capacitor. The resonant circuit consists of L, C1, C2, and C. Since C2 is connected between the base and emitter of transistor BG, it forms a feedback network with C1 and C2, where the feedback signal is derived from the voltage across C2. The oscillation frequency is calculated using the formula f = 1/(2π√(LC)), where C is the equivalent capacitance given by C = C1 * C2 / (C1 + C2).
The experimental design of the FM microphone is straightforward and ideal for beginners who want to build their own. In open spaces, the transmission range can reach up to 20–30 meters. However, the long working frequency may result in significant frequency drift, and the signal contains high harmonic content, which could interfere with nearby frequencies. To ensure stability, it's recommended to solder the microphone directly onto the board rather than using flexible wires. The inductor L can be wound around a 5–7 turn coil on a 0.3 mm diameter rod, and after tuning, it should be fixed with high-frequency wax. To check if the circuit is oscillating, you can use a standard analog multimeter set to AC 2V mode. Simply touch the antenna with one probe while keeping the other probe suspended. If the needle moves, it indicates that the circuit is oscillating, and you can then fine-tune the range accordingly.
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