The VCO is an oscillator whose frequency fosc is proportional to input voltage vo (=
Vvcoin). The voltage at the input of the VCO determines the frequency fosc of the
periodic signal vosc at the output of the VCO. The output of the VCO, vosc, and a
periodic incoming signal vi are inputs to the phase detector. When the loop is locked
on the incoming signal vi, the frequency fosc of the VCO output vosc is exactly equal
to the frequency fi of the periodic signal vi,