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