A product detector is an electronic circuit that brings out the message from an amplitude-modulated carrier or a single-sideband (SSB) carrier. It is based on the mathematical model of the message translated in the frequency domain by an amount known as the radio frequency (RF) carrier frequency. The product detector may or may not use a frequency mixer to recover the message from the modulated or message-modified carrier.
In communication circuits, the message, which can be voice, is combined or used to modulate an RF carrier. The envelope of the carrier then becomes the carrier attribute that carries the original message. To recover the message, called modulation, an envelope detector or demodulator is used. In amplitude modulation (AM), the simple diode detector rectifies the carrier to produce a direct current (DC) with an average level that is proportional to the original message. In SSB, the product detector, along with additional circuits, will reconstruct the message even if only one of the two sidebands is available.
It is possible in an AM radio receiver to amplify the incoming RF once, and then feed the result into a frequency mixer together with a local oscillator equal to the incoming frequency. The output of the frequency mixer will have a few significantly strong outputs. There will be a signal in the frequency range of the message that will be available at the output of the mixer. Moreover, there will be other products like a signal with a frequency equal to the sum of the incoming RF and the local oscillator. If the filter at the mixer output passes only the message, only the message will be recovered by an envelope detector, which is a simplified product detector.
A simple product detector may be implemented using four diodes in a circuit similar to a gating circuit. When one of the two input signals is at 0 volts (V), there is lack of forward bias in the diodes to allow the other input to reach the output. The resulting signal is a time-domain product, which results in a frequency-domain addition or subtraction of the message band or the base band. This is the original message.
In AM, the RF carrier and both sidebands are available in the carrier. One technique used in saving power and bandwidth is to make use of SSB. In the SSB transmitter, the output does not have a carrier and one of the sidebands. When the SSB carrier is transmitted into the air and received, it is easy to notice when the SSB audio received turns into a duck-like sound, while the resulting message will tend to shift in audio frequency. One option is to equip an SSB receiver with a very accurate and stable carrier frequency reference so that the product detector will be able to demodulate the message with minimal distortion in terms of phase and frequency.