[time-nuts] 5 MHZ PIC PPS Divider?
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.
> * 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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