to the gate circuit 14 whereby the intermediate frequency signal is sampled depending upon the gate pulses input to the gate circuit 14 the gate pulse waveform of the gate pulse generating circuit 13 is shown in fig5 b wherein the quadrature axis is a time base the width of the gate pulse is quite narrow and is about 10 n sec when the width of the first saw tooth wave of the saw tooth wave generating circuit 7 is t 1 sec the repeated period of the first saw tooth wave is t o sec the reference pulses are frequency divided to 1 n by the frequency dividing circuit 10 and the gate pulses of the gate pulse generating circuit 13 are sequentially shifted to t 1 n sec per each generation of gate pulses of the gate pulse generating circuit 13 from the reference point which is the rising point of the second saw tooth wave of the saw tooth wave generating circuit 11 accordingly the nth gate pulse is generated after n t 1 n sec from the reference point at the final when the n gate pulses are generated the reset circuit is actuated to return to the next reference point and the operation is repeated the gate circuit 14 is controlled by the gate pulses generated by the gate pulse generating circuit 13 the intermediate frequency signal is passed only when the gate pulse is applied the.