I think I understand the general operation of PLLs in the case where a single reference clock generates a single output clock. Once the PLL is locked, there is a deterministic relationship between the input and output clock frequency and phase. I understand the negative feedback loop, VCO, etc. However, I am uncertain about the theory of how PLLs behave when they are separated into physical components on a PCB.

After reading this question, I still am uncertain how fractional PLLs behave with lower output frequency than the reference.


If I configure 2 separate (but identical) PLLs on a PCB such that:

  • I supply the same 10MHz reference clock to both PLLs
  • I control the nets on the PCB to be identical length, width, ground plane coverage, etc.
  • I configure the PLLs "identically"

After both PLLs are locked (Fout1=Fout2), will the signals Fout1 and Fout2:

  • Be identical in phase if Fout > Fin and the relationship is a fractional multiple?
  • Be identical in phase if Fout < Fin and the relationship is a fractional multiple?

In other words, is both the rising and falling edge of Fout1 aligned to the rising and falling edge of Fout2?



1 Answer 1


There is no guaranteed phase relationship, except that it should be stable once locked.

To synchronise the outputs of Fractional-N PLLs, you need to synchronise the sigma-delta modulators. If the reference frequency is low enough, and you power them up and program them at the same time, then they may reliably start in sync, but I don't know how you guarantee it.

Various manufacturers have ways of doing this, here is a random document I found on the ADI site: phase-resync-phase-programmability-and-phase-coherence-between-multiple-fractional-n-plls

  • \$\begingroup\$ Ah yes, phase coherence is the exact term I was looking for! With a purely integer PLL, this is much more straightforward. Thanks, Tesla. This is definitely points me on the right path. \$\endgroup\$
    – cplusruss
    Sep 25, 2021 at 3:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.