[time-nuts] How close can you trim a Cs?

John Ackermann N8UR jra at febo.com
Wed Mar 9 09:42:44 EST 2005

I've been going nuts trying to trim my 5061A versus GPS.  I have it well 
within 1x10e-12, but trying to get down into the low 13s is proving 
challenging.  Making C fields adjustments of a couple of minor divisions 
that according to the book should be result in a shift of a few parts in 
10e-13 don't seem to have the effect they ought to; in some cases the 
frequency seems to move in the opposite direction.

For example, I did a run that showed about +3x10e-13 and tweaked four 
minor divisions (nominally 3.2x10e-13 according to the book), which 
should have brought it down to nearly flat.  Instead, the offset 
increased to about +6x10e-13.  So, I adjusted the eight minor divisinos 
back (so I ended up the at same shift, but opposite direction from the 
starting point).  Now, I'm reading +7x10e-13!

It may be that I'm being too impatient and not letting the comparison 
(against raw GPS 1pps) run long enough, but when after 15 hours I'm 
seeing a plot that shows a pretty stable slope, it's hard not to assume 
that things need more tweaking!

So maybe the first question is -- at these offsets, how long should I 
track against GPS before making a tweak?  And the second question is, at 
what point am I tweaking in the noise -- how close is it realistic to 
aim for with this class of standard?



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