PURPOSE: To obtain two synchronizing carrier waves having a 90° phase difference from each other, by comparing the phase of output between the two voltage control oscillators and setting this phase difference at 90° via an active filter.

CONSTITUTION: The outputs vO(t) and vQ(t) of the voltage control oscillators 7 and 12 are applied to a phase comparator 9 consisting of a voltage multiplier. The frequency and the phase of the oscillating outputs of the oscillators 7 and 12 are shown by ωO, O(t) and ωQ, Q(t) respectively. The output vd(t) of a comparator 9 is filtered through a low pass filter 10 consisting of an active filter to eliminate the 2ωO component. For the output vc1(t), vc1(t)=0 is satisfied in the case of e(t)=O(t)-Q(t)=±90°. Based on this state, the difference between Q(t) and O(t) is controlled to 90° in case Q(t) has the gains or losses of the phases to O(t).