A Time-Domain Resolution Improvement of an RF-DAC

We propose a time-domain technique that significantly improves resolution of an RF digital-to-analog converter (RF-DAC). As an alternative to resorting to various resolution improvement attempts in the amplitude domain or through quantization noise shaping, pulsewidth modulation (PWM) of a single least significant bit device is employed with fine timing accuracy easily afforded by advanced CMOS technology. The technique is examined in the context of a commercial Enhanced Data rates for GSM Evolution (EDGE) polar transmitter realized in 65-nm CMOS, which employs an amplitude modulator with basic 10-bit amplitude resolution limited by the RF-DAC switching device mismatches. Behavioral simulations include realistic device mismatches and show worst-case resolution improvement of 2.2 bits assuming 20-ps worst-case time granularity of the PWM signal controls.