Product detectors can be designed to accept either IF or RF frequency inputs. A product detector which accepts an IF signal would be used as a demodulator block in a superheterodyne receiver, and a detector designed for RF can be combined with an RF amplifier and a low-pass filter into a direct-conversion receiver.
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2 A more sophisticated product detector 3 Advantages and disadvantages |
The simplest form of product detector multiplies an incoming signal by its carrier to produce a copy of the original message and another AM signal at twice the original carrier frequency. This high-frequency component can then be filtered out leaving the original signal.
A simple product detector
Mathematical model of the simple product detector
If m(t) is the original message, the AM signal can be shown by
Multiplying the AM signal x(t) by an oscillator at the same frequency as and in phase with the carrier yields
Which can be re-written as
After filtering out the high-frequency component based around cos(2ωt) and the DC component C, the original message will be recovered.Drawbacks of the simple product detector
Although this simple detector works, it has two major drawbacks:
Frequency of an AM carrier can accurately determined with a phase-locked loop, but for SSB, the only solution is to construct highly stable oscillator.A more sophisticated product detector
A more sophisticated product detector can be constructed in a way much like a single-sideband modulator. Two copies of the modulated input signals are created. The first copy is mixed with a local oscillator and low-pass filtered. The second copy is mixed with a 90° phase-shifted copy of the oscillator and the output of this mixer is also 90° phase-shifted and then low-pass filtered. Both of these copies are then combined to produce the original message.Advantages and disadvantages
The product demodulator has several advantages over an envelope detector.
The only real disadvantage of a product demodulator is the increased complexity.