PLL 20 is thus an electronic circuit that controls {HEAD ~ VCO} 18 so that it maintains {TAIL ~ a constant phase angle} relative to the reference signal 
PLL 20 is thus an electronic circuit that controls { HEAD ~ VCO } 18 so that it maintains { TAIL ~ a constant phase angle } relative to the reference signal
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH