Structured tone mitigation in 3rd and 4th order MASH Delta-Sigma Modulators-comparative study

A delta-sigma modulator (DSM) can be thought as a nonlinear chaotic system that may exhibit tonal behaviour in its output spectrum. These tones are sometimes referred to as spurs and they are undesirable. To provide for the mitigation of structured tones, application of dithering, using chaotic modulators, loading irrational initial conditions and maintaining controllable maximum sequence lengths are commonly used and advised methods primarily in Multi-stAge noise SHaping (MASH) DSMs. Higher order MASH-DSMs are less problematic and are commonly used in many high speed and low noise frequency synthesiser circuits. As MASH is composed of cascaded first order digital DSM stages, it is unconditionally stable. In this paper, the tone mitigation techniques for MASH 1-1-1 and MASH 1-1-1-1 modulators are compared and their noise performances presented.