[time-nuts] TBolt OCXO replacement

John Miles jmiles at pop.net
Tue Feb 16 23:51:22 UTC 2010

> That gets you to the question - just how good do you think the 1
> second AVAR
> is on the existing oscillator, independent of the TBolt environment?
> If for instance you have a 2.5x10^-12 OCXO in yours, you would only see a
> significant improvement with a sub 1.0x10^-12 OCXO. That sort of
> bounds your
> OCXO shopping list right there.

Several 5061As have been appearing on eBay lately and selling for decent
prices in the $400 range, with unknown tube condition.  There's no reason to
think the tubes are any good, but what I did notice in the photos is that
they seem to be late enough models to have the 10811-60109 oscillators
installed.  I have two of those (one in a working 5061A and one from a dead
5061A which is presently installed in my Thunderbolt) and have just recently
been able to start looking at their performance, using a 5065A (which also
has a -60109 OCXO).

So far I'm impressed.  When I disable disciplining in the Thunderbolt, the
oscillator tends to stay well below 1E-12 until t=1000 seconds.  The one in
the 5061A doesn't seem quite as stable, but then it hasn't been enclosed in
an unventilated chassis and left on for a couple of months, the way the one
in the Thunderbolt has been.  I still need to do a couple of overnight runs
on that one to see how it behaves.

Warren has spent a lot of time tinkering with this Thunderbolt remotely via
the Heather server, and has been running it with a 750-second time constant,
damping factor 0.75.  I want to do some further tests to see what effects
the elevation mask and AMU filter have on the actual ADEV performance.  It
is still falling short of Tom's best HP 58503A clock, but perhaps with some
more tweaking we'll get it down below 1E-12 with disciplining turned on.

-- john, KE5FX
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