[time-nuts] Strange effect with C-Field change

John Ackermann N8UR jra at febo.com
Thu Mar 24 13:44:35 EST 2005

While I was out of town for the last week, I continued an experiment to 
check the C-field sensitivity of my 5061A.  I measured the offset at 
C-field settings of 5.00, 5.50, and 4.50.

The last change was made while I was out of town.  My wife, Jody, made 
the change from 5.50 down to 4.50.  She adjusted the knob fairly slowly 
(over 10 seconds or so) and didn't overshoot or reverse direction.  
Immediately, after she did this, there was a *huge* phase shift (of the 
1pps vs GPS) that lasted for several hours, and then returned to 
virtually the same point as it was before the change.  We're talking 
huge, from an offset of a couple of microseconds up to more than 100 
milliseconds, then returning back to the microsecond range and 
apparently normal operation.  A plot of the phase during this time is 
attached (note that the Y axis is logarithmic).

When I saw this anomaly show up, I assumed that maybe something had 
happened to cause the 5061A to lose lock momentarily.  So, imagine my 
surprise when I came home today and discovered the "continuous 
operation" lamp was still lit and there was no sign of any event that 
would have caused this.  Unfortunately, I wasn't doing any other 
tracking at the time, so I can't tell if it was a GPS anomaly or 
something else -- though the fact that it happened just as the C-field 
was changed sure seems to point to that as the culprit.

Anyway, has anyone seen anything like this, or have any idea what might 
have happened here?

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