best operation is obtained if the VCO center frequency Foooo is set with dc bias voltage midway in its linear operating range. the amplifier allows this adjustment in dc voltage from that obtained as output of the filter circuit. when the loop is the lock, the two signals to the comparator are of the same frequency, although not necessarily in phase. A fixed phase difference between the two signals to the comparator results in a fixed dc voltage to the VCO . changes in the input signal frequency then results in change in the dc voltage in the dc voltage to the VCO. within a capture-and-lock frequency range, the dc voltage will drive the VCO frequency to match that of the input.