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