[time-nuts] FTS4060 C Field Time Constant

Brooke Clarke brooke at pacific.net
Thu Feb 17 17:06:58 EST 2005

Hi John:

The only reason I saw this was because the plot had a steep positive 
slope, then 24 hours later appeared to be perfect, now a few days after 
that obviously has a negative slope.  My initial estimate of the time 
constant, based on the perfect performance, of 1 tau = 3 hours, was way 
off.  The real tau is more like 1 day.  So I'll wait another day before 
making the next adjustment.

Do you have plots that cover longer time periods?

Have Fun,


John Ackermann N8UR wrote:

> Brooke, I've been seeing something similar myself.  Last weekend I 
> posted a message with an attached plot vs GPS that showed similar 
> behaviour with a significant change in slope a couple of days after 
> changing the C field.  And, I lost the green "continuous operation" 
> light at the time of the change (I was running with the long 60 second 
> time constant, which will knock out of lock very easily).
> A new plot (check 
> http://www.febo.com/time-freq/plots/hp5061a-n8ur_1.html) against the 
> quieter of my two Z3801As is showing better behaviour now that the 
> last C field adjustment is over a week ago, but even there it appears 
> there's an initial downward slope that levels out.
> John
> ----
> Brooke Clarke wrote:
>> Hi:
>> In still in the process of setting the C field on my FTS-4060 and 
>> have an interesting observation.  After a cockpit error where I made 
>> a change in the wrong direction the time interval plot was going up 
>> steeply.  In line with my latest theory of how to make the adjustment 
>> (which is to do a binary search rather than try and compute the 
>> required adjustment) I then set in a new value.  I thought I saw a 
>> time constant of about 3 hours (i.e. it took about 3 hours to get to 
>> 63% of what I thought was the final value). The next day (24 hours 
>> later) for about 8 hours the time interval was constant to within 10 
>> nano seconds, phenomenally stable operation.
>> Now 3 days after that I can see that the time interval has a down 
>> slope.    So the very stable operation 24 hours after the C field 
>> change was during the time when the frequency was changing and was at 
>> the level part of the curve.
>> This indicates that for a C field change on the FTS4060 you need to 
>> wait at least 2 or 3 days before making any measurements! This also 
>> may explain why trying to compute the amount of a change does not 
>> seem to work.  This would be the case if the effect of the prior 
>> change had not yet settled in.
>> Have Fun,
>> Brooke Clarke, N6GCE
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