[time-nuts] How close can you trim a Cs?
cfharris at erols.com
Wed Mar 9 10:08:33 EST 2005
The manual for my old Sulzer 2.5A crystal oscillator says to not make
frequency adjustments more often than every 72 hours.
I never understood exactly why, but that is what they said.
I know that with my Austron 2100F/T loran receivers, the displayed
frequency offset reading takes about 12 hours before it is strictly
correct. Last night I had just finished surgery on my Sulzer 2.5A
and was tracking it with two 2100's, one set for the Master, and one
set for a Secondary transmitter. I started the two receivers tracking
about 30 minutes apart. At the end of 3 hours, one 2100 said the
standard was 10E -2.8, and the other 2100 said it was 10E +3.2.
By morning, they both agreed that it was 10E +5.8. If I had started both
receivers tracking at the same time, they would have both been in exact
My point is the software that calculates the frequency offset in these
receivers (and probably your GPS receiver too) is kind of funny. Because
the tracking loops have very long time constants, you need to give the
receivers plenty of time to settle down.
John Ackermann N8UR wrote:
> 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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