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

Brian Kirby kirbybq at bellsouth.net
Wed Mar 9 11:39:16 EST 2005

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.

John Ackermann N8UR wrote:

> Chuck Harris wrote:
>> Hi John,
>> 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
>> agreement.
>> 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.
>> -Chuck
> Hi Chuck --
> Thanks for the response!  I'm using the raw 1pps output from a 
> Motorola UT+ receiver as the "stop" input to a time interval counter 
> (with 2ns resolution), and the 1pps from the 5061A as the "start" 
> input.  I then log the time interval at 100 second averaging and plot 
> that.  So, my situation is a bit different than using the Loran 
> receiver (which I agree takes a long time to stabilize).  The real 
> issue is the noise on the GPS signal, which with 100 second averaging 
> is around 10 nanoseconds with occasional spikes of 20ns or so.  But 
> over several hours you can get a pretty good view of what the slope is 
> -- the question is whether it's good enough to measure parts in the 
> low 13s or even better, high 14s.
> John
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