Sigma-delta modulation techniques for wide spreading range programmable spread spectrum clock generators

Spread Spectrum Clock Generators (SSCG) have recently become a popular method adopted to solve electromagnetic interference (EMI) issues. In this paper, different sigma delta (ΣΔ) modulators for fractional-N phase-locked loop (PLL) are investigated. Two ΣΔ modulators are implemented, one is a 3
^rd
order single loop modulator and the other is a MASH 1-1-1. A comprehensive comparison between the two modulators designed for SSCG is presented. Spreading is achieved by modulating the feedback divider using a triangular wave through the ΣΔ modulator. In order to allow wide spreading ratios a new technique is adopted to keep the triangular wave within the input range of the ΣΔ modulator.