[time-nuts] Another use for a Trimble Thunderbolt

Arthur Dent golgarfrincham at gmail.com
Wed Dec 24 20:28:30 EST 2014

Those of you who know I had hacked the RFTG-u REF 1 GPS years
ago and had one running for 4 years before other time nuts
discovered these units probably won't be too surprised that
I have tried another hack that may have limited interest but
works for me.

Having owned a large number of Thunderbolts, I ran across a
few that needed repairs of various sorts. One of these had
a defective oscillator so I removed the OXCO and brought the
EFC and 10Mhz connections out through the side of the case with
SMA connectors so I could test various oscillators, as others
have done before. Then I got to thinking that if I connected the
Thunderbolt up to run and output to Lady Heather but connected
a free running oscillator to the 10Mhz input, ignoring the EFC
connection, it might work as a comparator to plot the drift of
the free running oscillator. I have a few Efratom/Datum Rubidium
standards I'm adjusting and I can watch drift on my scope at 5
ns/cm or the 10 Mhz output to the 5th decimal place on my Pendulum
CNT-81 counter and try to determine which way it's drifting but
that gets old pretty fast.

The 10 Mhz output from Lady Heather appears to be an instantaneous
reading so that always looks very good but the PPS output appears
to be the cumulative signed difference between the GPS and the free
running oscillator. The link is to a plot from Lady Heather with
just the 10 Mhz and PPS signals on the screen. The EFC is still
trying to control the oscillator but seeing it isn't connected
the readings could range from a meaningless 0-5 volts and I don't
care about the temp plot either. I also know that there are other
ways of doing this but the definition of "yankee inginuity' is
doing things the hard way. I could also check an RFTG-u REF 1 with
the antenna off to see how well it keeps to the correct frequency
on holdover. I suspect that like the Z3801 it tries to predict and
adjust the output when the GPS signal goes away. Take a look at the
plot where I adjust a rubidium standard and see what you think.



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