Abstract: This study presents a modulator design and implementation for a wireless sensor transmitter. The transmitter architecture presented combines a Binary Frequency Shift Keying (BFSK) modulator, an up conversion mixer, a power amplifier and an 863-870 MHz bandpass filter. The BFSK modulator uses the Frequency Hopping Spread Spectrum (FHSS) technique operating in the European ISM band 863-870 MHz. This modulator is intended for short-range wireless applications, such as the wireless sensors network. The modulator generates a 7 MHz wide single-sideband frequency hopped spread spectrum waveform. This modulator is designed using the Direct Digital Frequency Synthesizer (DDFS), which enables us to generate BFSK signal with the hopping frequencies. Low power DDFS architecture is presented. It uses a smaller lookup table for sine and cosine functions compared with existing systems using a minimum additional hardware. The evaluated Spurious Free Dynamic Range (SFDR) of the proposed modulator is -88 dBc. A modulator IC has been designed in AMS 0.35 μ m standard CMOS process technology with a layout chip area of 0.16 mm2. A 20-bit frequency control word gives a tuning resolution of 41.29 Hz at 43.4 MHz sampling rate. This modulator consumes 47.7 μW with a 3V supply at 43.4 MHz.