A Soft-Defined Pulse Width Modulation Approach - Part I: Principles

In this work a novel pulse width modulation (PWM) scheme is presented consisting of a mixed digital-analog implementation. The aim of this approach is to obtain maximum flexibility in setting and varying the average switching frequency, as a method to optimally deal with specifications concerning power consumption and bandwidth management. Conventional analog implementations of PWM suffer from inaccuracy due to circuit-level parameters deviation and bandwidth limitations, whereas digital implementations are subject to restrictions imposed by time quantization. Conversely, our mixed approach accurately reproduces the theoretical, natural sampling PWM output without directly comparing the modulating signal to a piecewise-linear carrier, either in the analog or the digital domain. Instead, the proposed design compares suitably pre-distorted versions of both the carrier and the modulating signals, so that their crossing points are not altered. The resulting bandwidths are relatively small to allow a non-aliased discrete-time synthesis. After digital-to-analog conversion, the PWM signal is finally obtained by a simple comparator. We support the proposed design by measuring the PWM spectrum as synthesized by a proof-of-concept prototype.