Production of FM wireless microphone

This article introduces the production of a FM wireless microphone, which uses crystal to stabilize the frequency, which can solve the frequency drift phenomenon caused by the three-point oscillator transmitter.

The circuit diagram of the whole machine is shown in the attached figure. The whole circuit is composed of audio amplifier and high frequency oscillation. R2, R3, and V1 in the audio amplifier circuit constitute a negative collector feedback amplifier. The weak signal output by the electret microphone is amplified. V2 and peripheral components form a parallel crystal oscillator, and L2 and C5 resonate at the triple frequency of the crystal. If a 30MHz crystal is used, the center frequency of the emission is 90MHz. L1 is a high-frequency choke coil. On the one hand, in order to prevent the following high-frequency signals from entering the audio amplifier area and causing interference, on the other hand, it is used to provide a static bias voltage for the varactor D1. The signal after the first-level audio amplification is directly added to the two ends of the varactor diode, so that the oscillation frequency changes with the intensity of the audio signal near the center frequency. Of course, since the Q 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 multiplier circuit.

BB910 is used as the varactor in this circuit. I removed it from the FM radio. MIC is an electret microphone. Transistor V1 uses ordinary low-power transistors, such as 9014, BC547, etc. V2 uses 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 of other frequencies whose frequency can fall to 88MHz～108MHz after triple frequency, it is also possible. L1 uses commercially available color code inductors, with inductance ranging from a few microhenries to tens of microhenries. L2 needs to be self-made, and can be formed by tightly winding 4 to 5 turns of Φ0.6mm enameled wire on a Φ4mm cylinder. The antenna uses a half-wavelength flexible wire.

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