means for supplying {HEAD ~ two kinds} of {TAIL ~ clock signals} having the same frequency and a phase difference of 180 degrees; and 
of
means for supplying { HEAD ~ two kinds } of { TAIL ~ clock signals } having the same frequency and a phase difference of 180 degrees ; and
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH