PURPOSE: To obtain a method and a circuit for producing a saw-tooth signal by generating and integrating a square wave having a duty ratio corresponding to the difference of phase/frequency between an input signal and a reference signal.

CONSTITUTION: In a phase locked loop 10 for detecting the phase and frequency between an input signal Fi and a reference signal Fr, phase detectors 12 receive the signals Fi, Fr. These signals are smoothed through a filter 14 and fed, as a control input, to a voltage controlled oscillator 16. The signal Fr is fed back through a programmable frequency divider 18. When a pair of flip-flops are employed as the detectors 12, a square wave having duty ratio corresponding to the difference of phase/frequency between the signals Fi, Fr is generated and integrated to produce a saw-tooth signal. The flip-flop is replaced by an up/down counter 204, a concurrent circuit 206, a shift circuit 208, a supplementary circuit 210, and high and low end circuits 212, 232.