PURPOSE: To compress the extent of variance for the modulation characteristics, by almost making the tuning frequency of a tank circuit conicide with the selected carrier frequency, through the interlocking between a channel change-over switch and a DC bias selector switch.

CONSTITUTION: When quartz oscillators X1∼X5 are connected alternatively to a quartz oscillator 1 with a channel change-over switch S0, the carrier is selectively led to a phase modulation circuit MOD. Simultaneously, the DC bias is switched with a switch S2 and the tuning frequency of tank circuits T1, T2 is reset to an optimum value every time when the carrier frequency of the phase modulation circuit MOD changes, allowing to compress the extent of variance of the modulation characteristics.