Frequency-Domain Analysis of Digital PWM-Based RF Modulators for Flexible Wireless Transmitters

Abstract

This paper presents a frequency-domain analysis of the noise and distortion terms produced by a digital RF modulator that uses pulse width modulation (PWM) for the amplitude modulation and a square wave as RF carrier. Insight in these terms is important as they limit the error vector magnitude (EVM) the modulator can achieve. For each of the terms, frequency-domain expressions are derived which are valid as long as the quantization noise is small and the digital PWM is sufficiently close to natural-sampling PWM. The dependency of the terms on the different system parameters is estimated, and the calculations are supported and complemented with simulation results. The presented analysis improves the understanding of the dominant noise and distortion sources, which can significantly speed up the design of PWM-based transmitters.