[time-nuts] 5 MHZ PIC PPS Divider?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 12 18:08:52 EDT 2008

John Ackermann N8UR wrote:
> Thanks, Bruce.  I considered the idea of a multiplier but would like to
> use a software-only solution if I can.  That's not only for simplicity,
> but also to eliminate additional sources of jitter; the experiment I
> have in mind is to test the second-to-second noise (and short term noise
> trends) of GPS and other PPS sources,* so I'm looking for as stable a
> reference as I can get.  The additional noise from a mulitplier may be
> small, but I want to minimize the number of unknowns.
> Thanks!
> John
> * I realize that I'll also be limited by counter resolution.
> ----

If minimising the PPS jitter, is important adding a single D flipflop to 
resynchronise the output PPS signal to the 5MHz input will be worthwhile.
A relatively complex chip like a PIC is likely to produce a PPS output 
signal with a jitter much greater than that produced by a single flipflop.

Measuring the PPS output jitter of the PIC will be somewhat challenging 
as It I would expect it to be somewhat less than 100ps.
The corresponding output jitter at the resynchronising flipflop output 
should be significantly less than 10ps even for a 74HC74.
In this case only the flipflop's random jitter is significant as the 
frequency and duty cycle of the PPS input to the flipflop are constant 
apart from the effects of jitter.


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