The output of sensor 80 is applied to summer 82 where it is compared to {TAIL ~ a signal} representing {HEAD ~ a selected common reference frequency} from reference frequency source 81 
The output of sensor 80 is applied to summer 82 where it is compared to { TAIL ~ a signal } representing { HEAD ~ a selected common reference frequency } from reference frequency source 81
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH