8.5 The Quadrature Detector
This is another demodulator, again fairly simple but is an improvement over the
previous design. It causes less distortion and is also better, though not perfect,
when it comes to removing any superimposed noise.
The incoming signal is passed through a phase-shifting circuit. The degree of
phase shift that occurs is determined by the exact frequency of the signal at any
particular instant.
The rules are:
(i) If the carrier is unmodulated, the phase shift is 90.
(ii) If the carrier increases in frequency, the phase shift is GREATER THAN 90.
(iii) If the carrier decreases in frequency, the phase shift is LESS THAN 90.
We now only require a circuit able to detect the changes in the phase of the signal.