A product detector recovers the message from an amplitude-modulated or single sideband carrier. In AM, a diode detector rectifies the carrier to produce a DC proportional to the message. In SSB, the product detector reconstructs the message. A simple product detector can be made with four diodes. SSB saves power and bandwidth but requires an accurate carrier frequency reference for minimal distortion.
A product detector is an electronic circuit that elicits the message from an amplitude-modulated carrier or single sideband (SSB) carrier. It is based on the mathematical model of the message translated into the frequency domain by a quantity 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 message-modulated or modified carrier.
In communication circuits, the message, which can be vocal, is combined or used to modulate an RF carrier. The carrier envelope then becomes the attribute of the carrier carrying the original message. An envelope detector or demodulator is used to retrieve the message, called modulation. In amplitude modulation (AM), the simple diode detector rectifies the carrier to produce a direct current (DC) with an average level proportional to the original message. In SSB, the product detector, together 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 incoming RF once, then send the result into a frequency mixer along with a local oscillator equal to the incoming frequency. The frequency mixer output will have some significantly loud outputs. There will be a signal in the message frequency range that will be available at the mixer’s output. 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 will be 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 control loop. When either input signal is at 0 volts (V), there is no 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 message bandwidth or baseband. This is the original message.
In AM, the RF carrier and both sidebands are available in the carrier. One technique used to save power and bandwidth is to use SSB. In SSB transmitter, the output does not have a carrier and one of the sidebands. When the SSB carrier is transmitted over the air and received, it is easy to notice when the received SSB audio changes 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 is able to demodulate the message with minimal phase and frequency distortion.
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