Abstract: The present invention provides an FSK modulation circuit and technique that controls a prescaler to generate an FSK output. In particular, a VCO output is sent to a prescaler controlled by a modulus control signal. The modulus control signal contains a series of pulses having a first duty cycle to effect a mark frequency and a second duty cycle to effect a space frequency. During a given cycle, the prescaler divides the VCO output frequency by two different divisors. The resulting frequency is compared with a reference frequency using a phase detector. Changing the duty cycle of the modulus control signal controlling the prescaler causes the VCO to provide a corresponding, synthesized frequency from the VCO for FSK modulation.