Design Considerations for Time-Modulated Injection-Locked Phase Interpolators and Rotators

Abstract

Digitally modulating the injection point in an injection-locked ring oscillator (ILRO) allows it to simultaneously serve as both a multiphase generator and phase interpolator (PI) or phase rotator. The resulting mostly-digital architecture is compact and promises low power in nanoscale CMOS, making it suitable for multi-Gbps dense I/O applications. This paper describes design considerations for such time-modulated ILROs, including the tradeoffs associated with determining their injection strength, modulation frequency, and pattern. These tradeoffs are illustrated in the design of a 14GHz PI in a 16nm FinFET technology. The proposed PI time-modulates between four different injection points within a ring oscillator to generate 96 fine phase settings (6.6 bits resolution). Based on post-extracted layout simulations, the overall PI consumes 24.1mW and occupies an area of 0.0033 $\mathrm{mm}^{2}$. The locking range is 12.4 - 14.3GHz in the typical process corner. The simulated jitter is 592fs-rms (including thermal noise and determinstic jitter) while the PI is rotating at a 200ppm frequency offset from 14GHz. A novel linearity calibration mechanism is used to correct for systematic and random phase imbalances.