How is a product detector used?

A product detector is a type of demodulator used in Single Sideband (SSB) and Continuous Wave (CW) receivers. In an SSB transmission, the original carrier wave was suppressed at the transmitter and is missing from the received signal. To recover the original audio, the receiver's product detector mixes the incoming SSB signal with a locally generated unmodulated carrier provided by the Beat Frequency Oscillator (BFO). By mathematically multiplying (or finding the "product" of) these two signals, the detector extracts the original audio frequencies so you can hear the voice transmission. A product detector is strictly a receiver component used for demodulation. It is not used for frequency multiplication in transmitters, nor is it a filter for FM receivers.