2.8 A pulse-position-modulation phase-noise-reduction technique for a 2-to-16GHz injection-locked ring oscillator in 20nm CMOS

High-speed transceivers embedded inside FPGAs require software-programmable clocking circuits to cover a wide range of data rates across different channels [1]. These transceivers use high-frequency PLLs with LC oscillators to satisfy stringent jitter requirements at increasing data rates. However, the large area of these oscillators limits the number of independent LC-based clocking sources and reduces the flexibility offered by the FPGA. A ring-based PLL occupies smaller area but produces higher jitter. With injection-locking (IL) techniques [2-3], ring-based oscillators achieve comparable performance with their LC counterparts [4-5] at frequencies below 10GHz. Moreover, addition of a PLL to an injection-locked VCO (IL-PLL) provides injection-timing calibration and frequency tracking against PVT [3,5]. Nevertheless, applying injection-locking techniques to high-speed ring oscillators in deep submicron CMOS processes, with high flicker-noise corner frequencies at tens of MHz, poses a design challenge for low-jitter operation. Shown in Fig. 2.8.1, injection locking can be modeled as a single-pole feedback system that achieves 20dB/dec of in-band noise shaping against intrinsic VCO phase noise over a wide bandwidth [6]. As a consequence, this technique suppresses the 1/f² noise of the VCO but not its 1/f³ noise. Note that the conventional IL-PLL is capable of shaping the VCO in-band noise at 40dB/dec [6]; however, its noise shaping is limited by the narrow PLL bandwidth due to significant attenuation of the loop gain by injection locking. To achieve wideband 2^nd-order noise shaping in 20nm ring oscillators, we present a circuit technique that applies pulse-position-modulated (PPM) injection through feedback control.