man with drill
A product detector is an electronic circuit that carries the message of an amplitude modulated carrier or single sideband (SSB) carrier. It is based on the mathematical model of the message translated in the frequency domain by a value known as the radio frequency (RF) carrier frequency. The product detector may or may not use a frequency encoder to recover the message from the carrier modulated or modified by the message.
In communication circuits, the message, which may be voice, is combined or used to modulate an RF carrier. The carrier envelope then becomes the attribute of the carrier carrying the original message. To recover the message, called modulation, an envelope detector or demodulator is used. In amplitude modulation (AM), the single 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 circuitry, 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 input RF once and then send the result to a frequency mixer along with a local oscillator equal to the input frequency. The output of the frequency mixer will have some significantly loud outputs. There will be a signal in the frequency range of the message that will be available at the output of the mixer. Also, there will be other products such as a signal with a frequency equal to the sum of the input RF and the local oscillator. If the filter at the output of the mixer passes only the message, only the message is retrieved by an envelope detector, which is a simplified product detector.
A simple product detector can be implemented using four diodes in a circuit similar to a bypass circuit. When one of the two input signals is at 0 volts (V), there is a lack of forward bias in the diodes to allow the other input to reach the output. The resulting signal is a time-domain product, resulting in a frequency-domain addition or subtraction of the message band or baseband. This is the original message.
In AM, the RF carrier and both sidebands are available on the carrier. One technique used to save power and bandwidth is to make use of SSB. In the SSB transmitter, the output has no carrier and one of the sidebands. When the SSB carrier is transmitted over the air and received, it is easy to see when the received SSB audio becomes a fading sound, while the resulting message tends to change in audio frequency. One option is to equip an SSB receiver with a very precise and stable carrier frequency reference so that the product detector can demodulate the message with minimal distortion in terms of phase and frequency.