A low distortion wide band CMOS direct digital RF amplitude modulator

This paper describes a low distortion wide band CMOS direct digital RF amplitude modulator, which uses a 10-bit linear interpolation current steering digital to analog converter (DAC) and a Gilbert cell based mixer to generate an amplitude modulated RF signal directly. The linear interpolation increases the attenuation of the DAC´s image components. The low pass filter (LPF) is therefore eliminated, and the RF transmitter structure is simplified. This modulator is suitable for system-on-chip design. The chip has been fabricated in a 0.35 µm, 3.3 V digital CMOS process. When the transmission signal is at 3 MHz, the distortion components are below -54 dBc at 800 MHz and -49 dBc at 1.2 GHz, respectively. The image signals are below -45 dBc when the transmission signal is a 5.2 MHz.