an operational amplifier 12a a pair of resistors 12b and 12c and a capacitor 12d this filter 12 has an input 12e which is connected to the phase detector output 11c and it has an output 12f on which a control voltage v c is generated in part the control voltage v c is generated by filtering the phase error signal s p item 13 is a voltage controlled oscillator hereinafter vco this voltage controlled oscillator 13 has an input 13a which is connected to receive the control voltage v c from the filter output 12f and it has an output 13b on which the signal s o is generated with a frequency f o that is proportional to the magnitude of the control voltage v c item 14 is a sweep voltage generator that has an output 14a on which a sweep voltage v sw is generated this sweep voltage v sw varies with time in a sawtooth shaped fashion and the range of the voltage v sw represents the frequency band u03b4f in which the external input signal s i lies item 15 is a compensation circuit for the sweep voltage generator 14 and the filter 12 this compensation circuit 15 includes an operational amplifier 15a.