A 1.5 GHz All-Digital Spread-Spectrum Clock Generator

An all-digital spread-spectrum clock generator (SSCG) has been fabricated in a 0.18 mum CMOS process. The analysis and design of this all-digital SSCG is presented. A mixed-signal phase and frequency detector is adopted to reduce the jitter, eliminate a digital adder, and also reduce latency. A Vernier time-to-digital converter (TDC) with time amplifiers is realized to enhance the timing resolution of the TDC and to track the frequency modulation in the SSCG. A digitally controlled oscillator with a resolution enhancement circuit is also presented. The measured electromagnetic interference reduction is 10.48 dB. The measured peak-to-peak jitter and rms jitter are 28.4 ps and 4 ps, respectively, at 1.5 GHz.