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