In this paper, a new Frequency Shift Keying (FSK) modulation scheme is proposed to implement the transmitter section of a transceiver for wireless interface of ultra low-power medical sensor networks. The FSK modulation technique is implemented and simulated using TSMC 90 nm CMOS technology to validate its functionality in terms of power consumption and output signal generation. Using the proposed modulation technique, the circuits except the local oscillator and the power amplifier consume no current in order to save the total power budget. The RF transmitter, a part of the transceiver, operates in North American 902–928 MHz ISM Band while consuming a very low current pulled from a supply voltage of 1.2 V.