Production of Crystal Oscillator FM Transmitter

This article introduces the production of an FM wireless microphone, which uses crystal frequency stabilization to effectively solve the frequency drift phenomenon caused by a 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 V1 in the audio amplifier circuit constitute a collector negative feedback amplifier to amplify the weak signal output by the electret microphone. 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. L1 is a high-frequency choke. On the one hand, it is used to prevent the high-frequency signal behind from entering the audio amplifier area to cause interference, and on the other hand, it is used 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, due to the high Q value of the crystal oscillator, 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 variable capacitance tube in this circuit is BB910. I removed it from an FM radio. MIC is an electret microphone. The triode V1 uses an ordinary low-power triode, such as 9014, BC547, etc. V2 uses a high-frequency low-power triode 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 fall to 88MHz~108MHz after tripling, it is also possible. L1 uses a commercially available color-coded inductor, and the inductance can be between a few microhenries and tens of microhenries. L2 needs to be made by yourself, and can be made by tightly winding 4~5 turns of Φ0.6mm enameled wire on a Φ4mm cylinder. The antenna uses a soft wire of half a wavelength.