the first circuit, filter and {HEAD ~ oscillator} constitute a phase locked loop to synchronize {TAIL ~ the internal clock signal} to the externally provided clock signal 
the first circuit , filter and { HEAD ~ oscillator } constitute a phase locked loop to synchronize { TAIL ~ the internal clock signal } to the externally provided clock signal
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH