This paper presents a 2.2-GHz low jitter sub-sampling based PLL. It uses a phase-detector/charge-pump (PD/CP) that sub-samples the VCO output with the reference clock. In contrast to what happens in a classical PLL, the PD/CP noise is not multiplied by 2 in this sub-sampling PLL, resulting in a low noise contribution from the PD/CP. Moreover, no frequency divider is needed in the locked state and hence divider noise and power can be eliminated. An added frequency locked loop guarantees correct frequency locking without degenerating jitter performance when in lock. The PLL is implemented in a standard 0.18-m CMOS process. It consumes 4.2 mA from a 1.8 V supply and occupies an active area of 0.4 0.45 mm 2. With a frequency division ratio of 40, the in-band phase noise at 200 kHz offset is measured to be 126 dBc/Hz. The rms PLL output jitter integrated from 10 kHz to 40 MHz is 0.15 ps.
This paper presents phase-locked loop (PLL) reference-spur reduction design techniques exploiting a sub-sampling phase detector (SSPD) (which is also referred to as a sampling phase detector). The VCO is sampled by the reference clock without using a frequency divider and an amplitude controlled charge pump is used which is inherently insensitive to mismatch. The main remaining source of the VCO reference spur is the periodic disturbance of the VCO by the sampling at the reference frequency. The underlying VCO sampling spur mechanisms are analyzed and their effect is minimized by using dummy samplers and isolation buffers. A duty-cycle-controlled reference buffer and delay-locked loop (DLL) tuning are proposed to further reduce the worst case spur level. To demonstrate the effectiveness of the proposed spur reduction techniques, a 2.21 GHz PLL is designed and fabricated in 0.18 m CMOS technology. While using a high loop-bandwidth-to-reference-frequency ratio of 1/20, the reference spur measured from 20 chips is 80 dBc. The PLL consumes 3.8 mW while the in-band phase noise is 121 dBc/Hz at 200 kHz and the output jitter integrated from 10 kHz to 100 MHz is 0.3 ps rms .
A divider-less PLL exploits a phase detector that directly samples the VCO with a reference clock. No VCO sampling buffer is used while dummy samplers keep the VCO spur <-56dBc. A modified inverter with low short-circuit current acts as a power efficient reference clock buffer. The 2.2GHz PLL in 0.18μm CMOS achieves -125dBc/Hz in-band phase noise with only 700μW loop-components power.
In a classical PLL, the phase detector (PD) and charge pump (CP) noise is multiplied by N 2 , when referred to the VCO output, due to the divide-by-N in the feedback path. It often dominates the in-band phase noise and limits the achievable PLL jitter•power Figure-Of-Merit (FOM). A subsampling PLL uses a PD that sub-samples the high frequency VCO output with the reference clock. The PD and CP noise in this PLL is shown to be not multiplied by N 2 , and greatly attenuated by the high phase detection gain, leading to lower in-band phase noise and better PLL FOM. This article reviews the development of the PLL FOM, the sub-sampling PLL techniques and their applications in recent PLL architectures.
Abstract-This paper shows that, for a given power budget, a shift register based multi-phase clock generator (MPCG) generates less jitter than a delay-locked loop (DLL) equivalent when both are realized with current mode logic (CML) circuits and white noise is assumed. This is due to the factor that the shift register MPCG has no jitter accumulation from one clock phase to the other as in the DLL based MPCG. For N-phase clock generation, the shift register MPCG needs a reference clock with N times higher frequency and thus requires a VCO with higher frequency than the DLL counterpart. However, we can show that this does not lead to additional power consumption.
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