Production of a FM wireless microphone

This FM wireless microphone uses crystal frequency stabilization, which can effectively solve the frequency drift phenomenon caused by the three-point oscillation transmitter.

The circuit diagram of the whole machine is shown in the attached figure. The whole circuit consists of two parts: audio amplifier and high-frequency oscillation. R2, R3, and Vl in the audio amplifier circuit constitute a collector negative feedback amplifier to amplify the weak signal output by the electret. V2 and peripheral components constitute a parallel crystal oscillator, and L2 and C5 resonate at the triple frequency of the crystal. If a 30MHz crystal is selected, the center frequency of the emission is 90MHz. Ll is a high-frequency choke, which is used to prevent the high-frequency signal behind from entering the audio amplifier area and causing interference, and to provide a static bias voltage to the varactor D1. The signal after the first-stage audio amplification is directly added to the two ends of the varactor diode. In this way, the oscillation frequency changes around the center frequency with the intensity of the audio signal. Of course, since the zero value of the crystal oscillator is very high, the frequency deviation obtained by this direct frequency modulation method is very small. If you want to obtain a larger frequency deviation, you can use a crystal with a lower oscillation frequency to cooperate with a frequency doubling circuit to achieve it.

The varactor in this circuit is BB910, which is removed from an FM radio. MIC is an electret microphone, transistor VI uses an ordinary low-power transistor, such as 9014, BC547, etc., and V2 uses a high-frequency low-power transistor 9018. The crystal oscillator uses a crystal with a nominal frequency of 30MHz or 32.768MHz. If you can buy crystals with other frequencies that can drop to 88MHz~108MHz after tripling, it is also acceptable. Ll uses commercially available color-coded inductors, and the inductance can be between a few microhenries and tens of microhenries. L2 needs to be homemade, and can be made by tightly winding 4-5 turns of +0.6mm enameled wire on a 4mm cylinder. The antenna uses a soft wire of half a wavelength.