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