This circuit compares the phase of the original input signal with the output of the
phase shifting circuit. It then produces a DC voltage level that depends on the
result of the comparison according to the following rules:
(I) It provides no change in output voltage if the signal phase has been shifted by
90.
(ii) Phases over 90 result in an increased DC voltage level.
(iii) Phases less than 90 result in a decreased DC voltage level.