Feedforward Phase Noise Cancellation Exploiting a Sub-Sampling Phase Detector

Abstract

This paper presents a feedforward phase noise cancellation technique to reduce phase noise of the output clock signal of a phase-locked loop (PLL). It uses a sub-sampling phase detector to measure the phase noise and a variable time delay for cancellation. Both phase noise and spurs are reduced. Analytical expressions have been derived that characterize the performance of this technique and show its fundamental limitations. A subsampling phase-locked loop with the cancellation technique as a built-in feature is described. The feedforward technique has no stability requirements in contrast to conventional PLL architectures. The phase noise reduction bandwidth is increased to almost a third of the reference frequency-3x the maximal bandwidth for 3 rd order type-II PLLs. The proposed analytical model shows a phase noise reduction of 9 dB at a frequency offset of f ref /10. The total rms jitter is improved by 7.2 dB. The analytical results are verified by simulations.

Fractional-N Sub-Sampling PLL Using a Calibrated Delay Line for Phase Noise Cancellation

“…where T F is a fixed delay between the input and output of the VCDL. The [7]. The high-pass filtered RVCO 1 Since a high frequency RVCO output has a very high slew rate (SR SSP D ), a high phase error detection gain is expected.…”

“…The high-pass filtered RVCO 1 Since a high frequency RVCO output has a very high slew rate (SR SSP D ), a high phase error detection gain is expected. phase noise at the SSPLL output Out RV CO undergoes further FPNC-based noise filtering (theoretical filtering upto f ref /2 cannot be reached due to the aliasing from sampling operation and filtering of the hold function in the SSPD [7]). Drawback of the FPNC is that, the reference and DTC noise directly impact the Out V CDL through the feedforward path with minimal filtering (due to sample and hold filtering of the SSPD).…”

“…The principle of FPNC is to measure the phase noise of the PLL by using a sub-sampling phase detector (SSPD) and cancel it by changing the delay of the voltage-controlled delay line (VCDL) [6]. Analysis in [7] shows that the FPNC can cancel the RVCO phase noise up to a bandwidth of f ref /3.6. Since the detected RVCO phase noise by the SSPD is canceled using a feedforward path outside of the PLL loop, the FPNC will not affect the locking conditions of the PLL unlike the IL technique.…”