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

Tom Van Baak tvb at leapsecond.com
Wed Mar 9 16:43:22 EST 2005

Hi John,

Good news that you get to 1e-12. A trouble-free
5061A in a home environment should be able to
do that. But it will in fact be challenging getting
down to the low 13's. A couple of thoughts:

1) When you talk about stability you always have
to mention or imply the tau. Are you looking for
stability in low 13's over an hour, a day, a week?

2) There is some magnetic hysteresis on the C-field
adjustment that you have to live with, especially if
you are tweaking by only a few minor divisions.

My hunch is a small, one-way, adjustment of a few
minor divisions would nicely show up in your data
with a week of averaging before, and after, the event.

Also I assume you have a newer 5061A with 5e-14
per minor division, not the older ones with 1e-12?

3) There's a lesson from the timing labs - they do
not typically adjust their C-fields at all. Instead they
adjust phase or frequency using *external* phase
micro-steppers or simply by *post-processing* the
raw phase data from their counters.

I can expand on this more if you want.

Another example is the 5071A where the 1 PPS
and 5 MHz outputs are generated by a DDS and
the user can type in a phase or frequency offset;
the C-field is not used as a way to make frequency

4) If you look at the 5061A manual, the noise floor
for a standard tube is in the mid 13's so that puts a
lower bound on what to expect. Also, you're using
a standard GPS receiver as your reference so you
need to factor in its performance. You can easily
get 1e-12 frequency stability with GPS over a day
but if you're looking for 1e-13 then run times of a
week or more might be more appropriate.

Try reducing your GPS data to one point per day.
Yes, I know that sounds funny and will require a
lot of patience, but the data you then get for your
5061A vs. GPS will be more revealing.

Also, I would not worry about your counter. True,
that at 20 ps, a HP 5370 or SR 620 is 100x better
than a HP 5334. But most of that 20 ps is wasted
when GPS is one of the channels. So your counter
and your reference (GPS) are well matched.


----- Original Message ----- 
From: "John Ackermann N8UR" <jra at febo.com>
To: <time-nuts at febo.com>
Sent: Wednesday, March 09, 2005 06:42
Subject: [time-nuts] How close can you trim a Cs?

> 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?
> Thanks,
> John

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