[time-nuts] How close can you trim a Cs?
John Ackermann N8UR
jra at febo.com
Wed Mar 9 12:40:58 EST 2005
Brian Kirby wrote:
> John, when I was in the Air Force, we had to make fine adjustments to
> the cesiums, per USNO's orders. It took several days for the effects to
> start to show up, and sometimes a week or two, to really see them.
> I would also say you need to increase your averaging time. If I read
> you right, you said you can resolve 2 nS in 100 sec, thats only 2x10-11,
> you need to be a magnitude lower.
Hi Brian --
Thanks, it's really helpful to know about the long time for effects to
show. That could explain a lot. Apparently I'll have to be much more
patient and let things run for a couple days between tweaks.
Re the averaging, my earlier response may have been confusing as Chuck
raised a similar point. I'm doing 1pps comparisons and doing a time
interval average (using the counter's math) to get a reading every 100
seconds. But I'm logging that to a data file and plotting the results
over much longer times; I'm not basing my results on individual samples.
The main reason for the averaging isn't to get additional resolution
(though it does effectively increase the resolution from 2ns to 200ps),
but to average the GPS noise down enough to be able to discern the
slope. The raw GPS data would show noise of probably 100ns peak-peak,
where the averaging brings that down to 10ns or so. I have a 5370B
counter that can measure 20ps single shot, but the GPS noise is so
dominant that the higher resolution doesn't help, so I'm using an HP
5334A for this exercise.
Here's the current plot that I'm running, automagically updated every 15
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