[time-nuts] Another use for a Trimble Thunderbolt
kb8tq at n1k.org
Wed Dec 24 23:23:13 EST 2014
> On Dec 24, 2014, at 8:28 PM, Arthur Dent <golgarfrincham at gmail.com> wrote:
> 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.
’d say that the plot is telling the truth. It also seems to be giving you information fast enough that thermal drift and barometric pressure is not to big an issue. If you had to wait a day or three for the same data, drift would be a much bigger issue. Yes, when you get to the “close enough” trace, drift may be an issue. (yes close enough is indeed close enough …).
I suspect that if you try the trick with something way far off frequency (many 10’s of ppm), the GPS may not play nice. At any normal tune range on an Rb, it should be fine.
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