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

The practical limits of MASH Delta-Sigma Modulators designed to maintain very long controllable sequence lengths for structured tone mitigation

The delta-sigma modulator (DSM) is an essential block for a Fractional-N (FN) frequency synthesizers and is used for generating the fractional part of the division ratio. Digital DSMs (DDSM) with rational input and rational initial conditions can be thought as Finite State Machines (FSM) and they always produce finite length sequences in accordance with the applied input. To provide smooth quantization noise power distribution (tone free) and to get rid of structured tones, the modulator should complete its cycle and return to initial starting state. This method is called maintaining controllable sequence length. In this paper, the practicality of this method will be investigated for DDSMs composed of up to 5th order MASH 1-1-1-1-1 structures by considering lock time requirements of the synthesizers designed for wireless transceiver applications such as GSM900, DCS-1800, UMTS(WCDMA),WLAN, ZigBee and Bluetooth