A 1-to-2048 fully-integrated cascaded digital frequency synthesizer for low frequency reference clocks using scrambling TDC

Generation of low jitter, high frequency clock from a low frequency reference clock using classical analog phase-locked loops (PLLs) requires large loop filter capacitor and power hungry oscillator. Digital PLLs can help reduce area but their jitter performance is severely degraded by quantization error. Specifically, their deterministic jitter (DJ), which is proportional to the loop update rate becomes prohibitively large at low reference clock frequencies. We propose a scrambling TDC (STDC) to improve DJ performance and a cascaded architecture with digital multiplying delay locked loop as the first stage and hybrid analog/digital PLL as the second stage to achieve low random jitter in a power efficient manner. Fabricated in a 90 nm CMOS process, the prototype frequency synthesizer consumes 4.76 mW power from a 1.0 V supply and generates 160 MHz and 2.56 GHz output clocks from a 1.25 MHz crystal reference frequency. The long-term absolute jitter of the 160 MHz digital MDLL and 2.56 GHz digital PLL outputs are 2.4 ps_rms and 4.18 ps_rms, while the peak-to-peak jitter are 22.1 ps and 35.2 ps, respectively. The proposed frequency synthesizer occupies an active die area of 0.16mm² and achieves power efficiency of 1.86 mW/GHz.