RF Transmitter Powered by Voice Signal

The project aims to create an interesting double-sideband (DSB) radiotelephone transmitter that is powered by the operator’s voice due to the instantaneous energy produced.

The prototype contains the RF output power that falls within the range of 5 to 15mW since the circuit is essentially a high-level DSB modulator or crystal-controlled RF oscillator. An ordinary 8 Ohm speaker can be used to provide DSB modulation and power. It can produce a peak DSB output power of 100mW by replacing the voice-driven loudspeaker with an electrets microphone that directly drives an LM386 audio frequency amplifier.

Based on the schematic, an audio frequency alternating current will appear at the secondary winding of transformer by speaking into the permanent magnet loudspeaker. The diode D1 will become reverse-biased when the signal on the upper end of this winding turns positive. The diode D2 will become forward-biased at the same time and both the collector of Q2 and the lower end of the primary transformer will be clamped to ground. The Colpitts crystal oscillator will begin to oscillate. To ensure proper DSB modulation, an identical 180° phase-shifted amplitude RF signal will appear on the secondary of transformer.