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22
Example 4: The carrier phase shift is caused by the asymmetry of the
skywave transmission or radio intermediate frequency phase.
The chart shows the demodulation output waveforms of the envelope detector
and synchronous detector. In each situation, the synchronous detector gives
an undistorted output (the frequency response may be uneven, but there is
no distortion). This is not the case with the envelope detector. The envelope
detector can only work correctly when the carrier is large enough and the
upper and lower sidebands are complete mirror images of each other in
amplitude and phase.
Synchronous detectors do not have the limitations of envelope detectors and
can demodulate a wide range of AM signals, for example double-sideband,
double-sideband with a suppressed carrier (DSB-SC), single sideband
without a carrier or with a suppressed carrier, vestigial sideband (VSB),
quadrature AM (QAM) and other signals.
Spectrum
Phasor Diagram
Signal
Envelope
Detector Output
Synchronous
Detector Output
LSB USB
Carrier
Carrier
USB
LSB
LSB
USB
Carrier
USB
LSB
Carrier
LSB USB
Carrier
Carrier
USB
LSB
LSB USB
Carrier
Carrier
USB
LSB
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