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For demodulating AM signals, most radios use diode envelope detectors.
When the amplitude modulated signal passes through the inductor-capacitor
loop, it will produce distortion, and the envelope detector will also produce
distortion. Therefore, one of the best ways to demodulate the AM signal is to
use a synchronous detector.
The synchronous detector reconstructs the unmodulated carrier frequency
based on the input signal, and uses this as a reference to identify noise and
distortion. Synchronous detection technology is often used in FM stereo left
and right channel signal demodulation and color TV chrominance signal
demodulation.
Synchronous detection has a high detection efficiency and can realize small
signal detection. It can eliminate the distortion (transient intermodulation
distortion) generated in the IF filter due to local fading, slight offset, modulation
overshoot, as well as inter-channel interference and cross-talk modulation,
and can also reduce noise interference.
The chart (on the next page) shows some common amplitude modulation
phenomena:
Example 1: It is an undisturbed conventional amplitude modulation signal,
both detectors give distortion-free output.
Example 2: The carrier is reduced because of local fading or the directivity of
the transmitting antenna.
Example 3: Sideband asymmetry is caused by local fading or reception
detuning.
Synchronous Detection: An introduction
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