[time-nuts] How close can you trim a Cs?
brooke at pacific.net
Wed Mar 9 17:37:54 EST 2005
When talking to the local Middletown LORAN-C folks they mentioned that
they have 3 HP Cesium standards and use a phase micro stepper.
I know that Austron used to make a micro stepper. Can you say some more
PS the 2100T has been reporting -1.2E-13 for some time.
Tom Van Baak wrote:
>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?
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