What is a product detector?

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A product detector is an electronic circuit that carries the message of 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 a value known as the radio frequency (RF) carrier frequency. The product detector may or may not use a frequency scrambler to retrieve 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 carrier envelope then becomes the attribute of the carrier carrying the original message. To retrieve 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 feed the result into a frequency mixer along with a local oscillator equal to the input frequency. The frequency mixer output will have some 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. In addition, 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 mixer output passes only the message, only the message is retrieved by an envelope detector, which is a simplified product detector.

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A simple product detector can be implemented using four diodes in a circuit similar to a pass-through circuit. When one of the two input signals is at 0 volts (V), there is a lack of forward bias on the diodes to allow the other input to reach the output. The resulting signal is a time domain product, which results in an addition or subtraction in the frequency domain 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 energy and bandwidth is to make use of SSB. On the SSB transmitter, the output has no carrier and one of the sidebands. When the SSB carrier is transmitted to the air and received, it is easy to see when the received SSB audio turns into a ducking sound, while the resulting message tends to change 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 is able to demodulate the message with minimal distortion in terms of phase and frequency.

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