[time-nuts] Trimble thunderbolt accuracy expectations

Forrest Christian (List Account) lists at packetflux.com
Sun Dec 3 08:41:30 EST 2017

A couple of weeks back I caught my thunderbolt way out of lock, and doing
some sort of weird sawtooth oscillation in the DAC and PPS reading causing
the PPS alignment to swing +-50ns or so away from correct alignment (based
on some +-10ns GPS receivers I have here).    After lots of attempts at
various things, I finally issued it a 'factory reset' command and it seems
to be a lot more healthy, but I'm not convinced this is as healthy as it
should be.   In particular, I seem to be getting an ADEV around 4.2e-10 at
1 tau, and 1.5e-12 at 1e5 tau which seems too high, based on various
discussions I've found on the net.

Seems like a good excuse to finally take the time to understand this unit,
and also start down the path of trying to get as accurate of a clock here
as possible within resonable financial constraints.   I'm primarily
interested in getting a highly-accurate UTC-aligned 1PPS source.
 Alignment to UTC is the primary goal, as this is in support of various
projects which rely on UTC aligned pulses.   An accurate 10Mhz frequency
source is only a nice side effect.   I should note that the thunderbolt has
(until now) been more than adequate for past projects, but I'm always
wanting something better... sounds like lots of people on this list
understand that disease.

So...back to the issue at hand:  I'm now trying to finally figure out how
understand all of the parameters and also trying to get my head around the
interrelation between all of the parameters.   I also understand that
there's some tuning possibilities which I'm looking at as well.   I haven't
yet opened it up to see what OCXO is in it.   I'd appreciate some guidance
as to what others look for when ascertaining the health of a thunderbolt,
and what I should be expecting as far as ranges on things that matter like

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?

*Forrest Christian, AC7DE* *CEO**, PacketFlux Technologies, Inc.*
Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602
forrestc at imach.com | http://www.packetflux.com
<http://www.linkedin.com/in/fwchristian>  <http://facebook.com/packetflux>

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