During steady state conditions when {TAIL ~ the frequency} of the common voltage, as sensed by frequency sensor 80, equals {HEAD ~ the selected frequency} applied to summer 82, the output therefrom is zero  
During steady state conditions when { TAIL ~ the frequency } of the common voltage , as sensed by frequency sensor 80 , equals { HEAD ~ the selected frequency } applied to summer 82 , the output therefrom is zero
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH