Abstract: A digital frequency shift keying (FSK) transmitter provides direct frequency modulation in a wide bandwidth loop. In one embodiment, a discriminator circuit recovers from the output signal of the transmitter a base band signal, which is then compared with the input base band signal to provide an error signal for controlling the output oscillator circuit. In another embodiment, the output oscillator is controlled by a phase detector detecting the phase difference between the base band modulating signal and the output signal of the transmitter. An additional narrow bandwidth loop can be added to compensate for frequency drift in the output oscillator due to temperature or other environmental changes.