Realistic

Realistic (Radio Shack)

43-12 FM Wireless Intercom. Interesting design. Carrier current FM intercom operating at 200 kHz. On receive, it is a TRF receiver with two transistors amplifying the incoming 200 kHz. This amplifier then drives a discriminator which drives the audio amplifier to the speaker. The input to the second RF amplifier also drives a rectifier (D3), which develops a negative voltage when RF is present. This counteracts the positive bias on the base of Q4. Q4 shorts the receive audio to ground when biased on. This is the squelch circuit. When in transmit, Q4 is switched from being the squelch transistor to being the audio preamp. The push pull audio amplifier that drives the speaker in receive instead drives the FM oscillator when in transmit. Transmit audio is applied to varactor diode D1, which frequency modulates oscillator Q1. The oscillator output is then amplified by the two stages of RF amplifier used in receive, then drives the powerline RF interface. Very clever design.

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