[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.
> ----
>   
John

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.

Bruce




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