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