[time-nuts] 5 MHZ PIC PPS Divider?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 12 18:25:08 EDT 2008

Bruce Griffiths wrote:
> 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

It is also possible to use a frequency doubler to generate 10MHz from 
the 5MHz input to drive the PIC and resynchronise the resultant PPS 
output from the PIC to the 5MHz input using an external D flipflop.
This will remove the jitter due to both the frequency divider and the PIC.
Using a low phase shift filter (ie none ) at the doubler output will 
ensure a relatively stable delay between the 5MHz zero crossing and the 
10MHz doubler output.


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