The squarer output is applied to {HEAD ~ a summing amplifier}, which removes the multiplex frequency, and which provides {TAIL ~ a dc signal} which is applied to a comparator 
The squarer output is applied to { HEAD ~ a summing amplifier } , which removes the multiplex frequency , and which provides { TAIL ~ a dc signal } which is applied to a comparator
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH