[time-nuts] Trimble thunderbolt accuracy expectations

Andrew E Mileski andrew.mileski at gmail.com
Mon Dec 4 18:50:18 EST 2017

On Sun, Dec 3, 2017 at 8:41 AM, Forrest Christian (List Account) <
lists at packetflux.com> wrote:

> One parameter in particular that I'm trying to figure out how to determine
is what the expected offset from UTC/GPS is, i.e. +- a certain number of
> ns.   I'd like to be able to look at the unit and determine how healthy it
> is, and what level of uncertainty I should expect.   I know the ADEV is
> based on the deviation of the PPS signal, but it doesn't sound like it's
> related to the deviation from the UTC second.   I also see the PPS
> parameter up at the top in Lady Heather, but this doesn't seem like it's
> what I'm looking for.   Can someone clarify this for me?

Lady Heather can't tell you the quality of what the GPSDO outputs.  Without
a better reference to compare against, the best you can do is infer from
some "normal operation" parameters, that it your unit is likely operating
within spec.

First, some background about my unit:

I'm currently running with a time constant of 1000 seconds, and a dampening
of 1.2, with the DAC at around 2.1062 Volts.  I have the satellite mask set
for 10 degrees and 4 AMU; more restrictive values were found to be unusable
because of the lack of a full sky view.

I strongly recommend using much faster settings initially, like 100 seconds
and 0.7 dampening, and letting it run like that until the DAC trend stays
mostly horizontal over at minimum a week-long 360 minute/div view.  It can
take a while (days, months, or 2 years in my case).

My Thunderbolt E has shown some behaviours that can be exhibited when there
is poor satellite coverage:

1.  There is no window for what is considered a valid offset, so the unit
will chase wildly erroneous values.

2.  Hold-over algorithm seems to fail with multiple short back-to-back
outages, or constellation changes.  This behaviour is probably related to
#1.  A minimum-valid timer would also have helped here.

It typically takes my unit 3 days to recover from a poor-reception event
with wildly erroneous offsets, which would typically last nearly 3 hours.
Though, I've not had such an event since the unit stabilized recently
(perhaps luck).

What I, a total amateur, look for in Lady Heather:

* Low DAC span, and predominantly horizontal trend over an 60 minute/div
view, with some drift trend possibly showing over 360 minute/div or 1440
minute/div views.  Expect some relationship to temperature change, with the
DAC having to compensate for all the electronics located outside the
crystal oven, which includes the DAC itself.

* PPS rms around 3 ns (roughly 1 meter) and under 10 ns.  I think mine is
reporting just under 6 ns, last I looked.  Don't know if this is good or
bad, but it seems reasonable to me.  The relative amount of cloud cover
seems to affect this value slightly (lower during periods of clear sky).

* Temperature can largely be ignored, until you see a trend that doesn't
make sense.

Make sure to properly terminate the outputs, and keep the loads driven
constant, as there is a definite dependence on output loading!

Andrew E. Mileski

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