being used to switch s1 and s2 with s1 being conductive while s2 is off s1 s2 and vice versa the oscillator also provides a sync signal to a phase modulator 26 the output waveform of the phase modulator is nominally 90 u00b0 displaced as shown in fig3 c from that of the oscillator with zero modulating input however the phase may be deflected i e advanced or retarded from the nominal 90 u00b0 setting by a proper signal to the 34 mod sig 34 input this output waveform from the phase modulator 26 is connected to control the switches s3 and s4 the switches s3 and s4 are connected to be switched so that s3 is conductive while s4 is off s3 s4 and vice versa considering the operation of the block diagram of fig1 the switches s1 and s2 are continually opened and closed with a 100 duty cycle square wave drive 180 u00b0 apart thus each switch is conducting fifty percent of the time whereby a current from the positive source terminal flows successively into primary winding terminals 12 and then 15 and an ac voltage is induced at secondary winding 16 the square wave drive may be minumum two times the maximum modulating signal frequency usually in the 20 khz to 30 khz range up to as high as the speed of the switching devices will allow one or the other of the bidirectional switches s3 and s4 is always conductive as switched by the phase modulator output that is switches s3 and s4 are opened and closed 180 u00b0 apart.