Single tube audio FM transponder circuit schematic

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 repeater can be constructed with only one transistor, as shown in the following figure:

Figure 1 Single tube audio FM transponder

The figure above 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 using the variable capacitance diode characteristic of the BE junction. Although this circuit is simple, the sound quality is very poor, the stability of the oscillation circuit is very bad, the change of voltage, the appearance of the human body or metal objects near the antenna will change the oscillation frequency and cause the instability of the receiving end. And the circuit can only transmit mono audio, users cannot hear high-quality stereo music.