[time-nuts] Cheap jitter measurements

Bob kb8tq kb8tq at n1k.org
Mon Apr 9 08:59:17 EDT 2018


> On Apr 9, 2018, at 2:53 AM, Trevor N. <qb4 at comcast.net> wrote:
> Bob kb8tq wrote:
>> Hi
>> Without the ability to put out a “known good” time pulse there is no quick way to 
>> check NTP. GPS modules suffer a similar issue. They put out a pulse and a 
>> “correction” (sawtooth error) to tell you what they just told you. Doing the same 
>> sort of thing with NTP may be possible. 
>> Indeed the process of correcting this sort of data is open to a bit of debate. It does 
>> give you a way to get around the “hey, all I can do is 300 ns” issue. With GPSDO’s
>> the correction is part of the standard firmware. It would be nice if one of the NTP 
>> guru’s popped up with an equivalent process. 
>> One *could* monitor various bits and pieces of the OS’s timing generation system. 
>> Somehow that does not seem quite as much fun as looking at the whole result all
>> at once. Indeed it might be the only way to get it all worked out.
>> Bob
> Linux has a pps output driver (pps_gen_parport), but  I've never used
> it. A while back  I added an output mode to the Linux pps_parport
> driver: 
> https://github.com/retroman/pps_parport_polled 
> that I will eventually get around to using with the palisade(trimble)
> NTP driver.
> My modified driver's polled input mode has an input-to-echo delay of
> 1.16 to 1.93 microseconds (measured with an old Keithley 775 counter)
> on my machine which has a parallel card attached directly to the pci-e
> port on a sandybridge processor.  Interrupt mode echo delay is 3.8 to
> 4.3 microseconds when the machine is lightly loaded with occasional
> spikes of 5 to 7 us. When the machine is idle delay falls to
> 3.5-4.1us.
> Port read and write delays are equal at about 820ns each. I think that
> pci-express always uses 'split transactions'  so reads can sometimes
> seem to take only half the time depending on the input pulse time
> relative to the start time of a read request.  Delays increase to
> ~1200ns when attached to the chipset pcie ports.

Does NTP generate a “correction” output that tells you when it things the 
pps went out? If the pps is just coming off the clock architecture, it will 
show you part of what’s going on, but not all of it.


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