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