[time-nuts] Cheap jitter measurements
Bob kb8tq
kb8tq at n1k.org
Tue Apr 10 14:11:46 EDT 2018
Hi
> On Apr 10, 2018, at 12:23 PM, David J Taylor via time-nuts <time-nuts at febo.com> wrote:
>
> Hi,
>
> Yes, but isn't generating pulses OUT of a PC with low latency/jitter one
> of the difficult issues?
>
> If we (somebody smarter than me...) flip this around and modify a copy
> NTP to grab the QueryPerformanceCounter value when it gets a PPS pulse
> and log that count, don't we now have a way to compare the high
> resolution uncorrected counter to NTP and the external PPS?
We would still be relying on NTP to report on it’s self. I can feed the PC a PPS from my GPSDO
and let it report on that. Tell it to simply monitor the input, but not use it for disciplining. I get information,
but it’s what NTP “thinks of it’s self”. Ideally one would like to get an output that can be independently
measured.
Bob
>
> Agreed, not trivial. But maybe fun anyway!
>
> Dan
> ================================
>
> Dan,
>
> Take a look at Dave Hart's patches to the Microsoft serial port driver, that does something similar. The source may be in the NTP distribution, or Meinberg may have a copy. At one time using the QueryPerfomanceCounter call was an option (look for "QPC"). With Windows-8 and later there is a much more precise GetSystemTime call which NTP uses.
>
> I played a little with those calls (in a non-serious way) and wrote some code:
>
> http://www.satsignal.eu/ntp/TSCtime.html
>
> based on:
>
> http://www.lochan.org/2005/keith-cl/useful/win32time.html
>
> and just messing around! Perhaps it's of interest or use?
>
> Cheers,
> David
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
> Twitter: @gm8arv
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