Circuit schematic diagram of single tube audio FM transponder

Wireless audio transponders are widely used in wireless headsets, car MP3 transponders, conference broadcasting, building and park broadcasting and other places. The simplest wireless audio transponder can be composed of only a triode, as shown in the figure below:

Figure 1 Single tube audio FM transponder

The above figure uses a triode 9018 to form an oscillator. The audio signal is input from the base of the triode, and the frequency modulation is realized by the characteristic of the varactor diode of the B-E junction. Although the circuit is simple, the sound quality is poor, and the stability of the oscillation circuit is very poor. Changes in voltage and the appearance of human bodies or metal objects near the antenna will change the oscillation frequency and cause instability at the receiving end. Moreover, the circuit can only transmit mono audio, and users cannot listen to high-quality stereo music.

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