When is a PLL not an LPF?

A typical PLL has an LPF that rejects high frequency perturbations and “cleans up” the input clock signal. Once locked, a PLL is immune to frequencies above the loop BW. From this perspective, the PLL rejects high frequency jitter.

But what happens to a PLL inside a CDR receiver? The inherent operation of the PLL does not change in a CDR receiver. It will continue to behave like an LPF by tracking the low-frequency components of the input clock. Hence, the recovered clock will not track frequency components above the loop BW of the PLL. This is illustrated by the LPF red trace below.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

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