[time-nuts] Cheap jitter measurements

Bob kb8tq kb8tq at n1k.org
Thu Apr 5 23:13:58 EDT 2018


Whatever you want to call it (jitter / wander / noise / crud ), an Rb in a stable temperature 
environment ( a few degrees C per hour) will have “stuff” with the dimensions of nanoseconds
when compared to a good GPS.

A “normal” NTP setup with a crystal on the motherboard as it’s main flywheel and the polling 
stretched out will have “stuff” with the dimensions of microseconds when compared to the Rb
or GPS. 

It’s not some defect in NTP, it’s simply the temperature coefficient of the crystal on the motherboard.
If it is in the 0.25 to 1 ppm/C range, 2 C will give you 0.5 to 2 ppm of frequency swing. Move 0.5 ppm
and you are off a microsecond one second later. Since your room temperature is cyclic, it does all 
average out. It’s still a “something” on your timing estimate. 

If you look at Mark’s data, he can get GPS modules to hold < 10 ns of “whatever” for quite a long time. 
A good Rb should be in the < 1x10^-11 /C range. It *might* cycle 2x10^-11 in our hypothetical 2C 
room. After 50 seconds you might pick up a nanosecond. 

The resolution that makes sense for GPS <-> Rb is *very* different than for GPS <-> crystal or 
Rb <-> crystal. 


> On Apr 5, 2018, at 10:09 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>> What do you mean by "jitter" and what do you really want to do?
>> I mean jitter as NTP defines jitter.  Whatever that is.
> I think you need to figure out what you want to do so you don't fool yourself.
> ntpd is a PLL.  There is a low pass filter in the control loop.  It will 
> track the low frequency wander of the source.
> Assume you have a stable Rb reference.
> Assume your GPSDO has a (small) day/night shift.
> If you collect data with a TICC referenced to that Rb, that shift will show 
> up.  If you collect data with ntpd, you will get the filtered version of that 
> shift.  I'd expect small bumps at the day/night changes.
> Ballpark for the time constant on the filter is the polling interval.  You 
> can see the results of the filter by introducing a "bump" with ntpfrob.
> -- 
> These are my opinions.  I hate spam.
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