A tone switching {HEAD ~ logic circuit} normally maintains {TAIL ~ the reference frequency} at a value substantially equal to the frequency f1 
normally maintains
A tone switching { HEAD ~ logic circuit } normally maintains { TAIL ~ the reference frequency } at a value substantially equal to the frequency f1
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH