[time-nuts] GPSDO warm Up Stability measurement

[Andy Talbot]

Here're a couple of plots of the phase of the  L-B GPSDO at 10MHz compared
with a caesium reference made to investigate how long it really needs to be
left on to stabilise

http://www.g4jnt.com/DropF/gpsdo_cs_10mhz_2hourturnon.bmp
Plot 1   is the period for two hours immediately after turn on, showing it
taking nearly this time to get down to something half respectable.
The 360 deg jumps confuse things a bit, but it's clear there is a complete
mess up to about 10 minutes from turn on, then two complete cycle slips in
the first hour after that (full scale jump up in phase with no
corresponding jump down)

This plot from start up does explain the apparent "error" I thought I saw
on Droitwich the other day.    The GPSDO had only been on for around 20
minutes when that first plot was made and clearly was still stabilising.
 Two cycle clips in an hour at 10MHz correspond to about 14 degrees at
198kHz which is about what that plot showed.

http://www.g4jnt.com/DropF/gpsdo_cs_10mhz_4hour.bmp was started three hours
after the last one completed and shows four hours worth of phase wobble.
The GPSDO had been running for 5 hours by then and 'should' have
stabilised.  There are no complete cycle slips, but there is a period of
instability around 22:50 where the phase shifted nearly a full cycle over
the period of a few minutes.  If we estimate that section as 2 minutes, one
cycle over 120s is 0.8 ppb and is about the most extreme excursion
experienced on this GPSDO, and typical of other observations using higher
frequencies.



[image: image.png]
[image: image.png]

Andy
www.g4jnt.com



On Wed, 21 Oct 2020 at 13:10, Stewart G3YSX <stewart at g3ysx.org.uk> wrote:

> A note on this thread:  plotting phase against time will tell you a lot
> about the effect of propagation on phase, but it is not the technique used
> in the synchronisation world to understand the behaviour of oscillators.
>
> They use Allen variance, TDEV and MTIE plots.
>
> Fundamentally what these measurements do is to look at the error every
> second, and then cumulatively work out what the error is every second,
> every two seconds, every four seconds etc until the limit of the
> measurement, but they do it using a sliding window system so in a one
> hundred second time (say) they have one hundred one second measurements to
> average, 99 two second measurements, 98 three second measurements…. 49 50
> sec measurements etc. Then they plot this on a log scale.
>
> This tells you a lot more about what is going on than the phase error vs
> time measurements and revolutionised understanding of oscillator and hence
> time measurement characteristics. In particular with the more sophisticated
> plots developed after Allen initial work, the slope of the curve at various
> places along the plot tells you the cause of the error in the system.
>
> - Stewart/G3YSX
>
>
>
>
>
>
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