Spectral characteristics of the double-loop sigma-delta modulator

An important factor affecting the performance of ΣΔ modulators is their tone behavior. A recent approach to alleviate the tone problem consists of moving the open-loop poles outside the unit circle. In this paper, we determine the frequency location of two dominant tones as a function of pole-location, and discuss the effect of pole location on the relative amplitude of low-frequency tones. Audio testing along with a series of simulations are then performed to compare the efficacy of pole placement with that of the more traditional tone removal technique of dithering