[time-nuts] HP5061A frequency jump

John Ackermann N8UR jra at febo.com
Sat Feb 12 08:18:27 EST 2005

I've been messing around with my 5061A trying to trim for minimum offset 
against GPS.  It's been a bit frustrating as small movements of the C 
field control haven't seemed to have the expected effect.  Now, I have 
another piece of the puzzle.

I've attached a phase plot of the current data run.  This is against a 
UT+ receiver with 600 second averaging.  Note the downward slope of 
about -2x10e-13 that abruptly reverses direction.  The change happened 
almost two days into the run, and there was nothing going on in the lab 
at the time to account for it.  The standard as well as the measurement 
equipment are on a UPS, and the standard has been running continuously 
for at least a few weeks now.

However, this morning when I had a chance to take a look at things, I 
noticed that the continuous operation light was off on the 5061A.  
Hitting the logic reset button restored the light.  Beam current and 2nd 
harmonic meter readings were normal.  The unit was set in LTC (60 second 
time constant) mode, which I know makes it more vulnerable to losing lock.

What's interesting is that the new positive offset is closer to what I 
would have expected after the small C field adjustment (adding about one 
minor division) I made about an hour before the run started.  During the 
first part of the run, the offset had actually become more negative than 
it was prior to the adjustment, which had me scratching my head.

So, for the experts here:

1.  Should I expect to lose lock every now and again if I insist on 
running in LTC mode?

2.  Could a small C field change have effects 48 hours after it was made?

3.  Does this sudden change of offset indicate a problem in the 5061A?



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