A wide spreading range programmable spread spectrum clock generator using a ΣΔ fractional-N PLL

A programmable spread spectrum clock generator (PSSCG) using fractional-N phase-locked loop (PLL) is presented. Spreading is achieved by modulating the feedback divider using a triangular wave through a SigmaDelta modulator. In order to allow wide spreading ratios a new technique is adopted to keep the triangular wave within the input range of the SigmaDelta modulator. The clock frequency is programmable from 8-280 MHz with center and down spreading (0.5%, 1%, 2%, 4%). The RMS period jitter is 8.7 ps at 280 MHz. The electromagnetic interference (EMI) reduction is 18.1 dB at 4% center spreading. The proposed PSSCG is designed using a CMOS 0.18 mum 6-metal technology and has an active area of 0.66 times 0.45 mm².