[time-nuts] Thunderbolt E failing

[Poul-Henning Kamp]

--------
Bob kb8tq writes:

> The point is that there is no need to compensate the part while it is locked.
> It meets all the specs required of it by virtue of the lock to GPS. The big issue
> on these devices ( = cell tower GPSDO’s) is holdover. That’s where all the 
> magic comes in. 
> 
> Indeed, trying to compensate *and* learn at the same time generates even 
> more issues to deal with. The temperature effect is not a simple linear / first 
> order sort of thing ….

Data point:

20 years ago, I needed to buy a batch of a dozen OCXO's for hand-built
NTP servers for the new danish ATC network.

This is where the Soekris 4501 trick comes from, and I wanted a
frequency I could feed directly to the Soekris if at all possible.

I ended up "buying into" an existing production run of IsoTemp
OCXO-0131's at 32.768 MHz spec'ed for "some telco application".

I attach a 16 year long plot of the frequency error for one of
the servers on my side of the firewall-of-doom.

(There were a network misconfiguration, so the data from middle of
2011 to end of 2015 is not valid - long story, mostly about fibers
and backhoes)

It took this one five years to "settle in", but for the last decade
it has consistently drifted 2.2e-9/year = 7e-16/second.

The software PLL in these NTP servers did model drift and we tested
that, unplugging the GPS antenna on one server.

After 18 weeks in hold-over it had still not drifted out of spec.

In the 19th week, they added two 3.5" disks to a server mounted in
the same rack, literally opening the rack-door, checking the dymo
to make sure they had the right server, pulling out the spacers,
stuck in the disks, clsoed the rack-door and left the room.

The plot for that server clearly shows a change in OCXO-behaviour
and 121 hours later, the NTP finally wandered out of spec.

Other tests rules out thermal causes, so our conclusion is that
either the vibration from the intervention or from the rotation
of the two new disks did it.

My personal conclusion is that high-order PLL's are fun, but they
require patience on a timescale of years and you need not bother
unless you have a *very* benign environment.

Poul-Henning


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ocxo.png
Type: image/png
Size: 33325 bytes
Desc: ocxo.png
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20200913/1afc8a32/attachment.png>