The following is the circuit diagram of FM wireless microphone. High-frequency triode V 1 and capacitors C3, C5 and C6 form a capacitor three-point oscillator. For beginners, we don't need to ponder the specific working principle of three-point capacitor for the time being, as long as we know that this circuit structure is a high-frequency oscillator. Loads C4 and L at the collector of triode form a resonator, and the resonant frequency is the transmitting frequency of FM microphone. According to the parameters of the components in the figure, the transmission frequency can be between 88 ~ 108 MHz, which just covers the receiving frequency of FM radio. By adjusting the value of L (stretching or compressing the coil L), the transmission frequency can be easily changed to avoid FM broadcasting. The transmitted signal is coupled to the antenna through C4 and then transmitted.
R4 is the base bias resistor of V 1, which provides a certain base current for the transistor, so that V 1 works in the amplification region, and R5 is the DC feedback resistor, which plays a role in stabilizing the working point of the transistor.
The frequency modulation principle of this FM microphone is to realize frequency modulation by changing the capacitance between the base and emitter of the transistor. When the acoustic voltage signal is applied to the base of the transistor, the capacitance between the base and emitter of the transistor will change synchronously with the acoustic voltage signal, and the transmission frequency of the transistor will also change to realize frequency modulation.