[evlatests] Spectral line phase jumps - Fluke settings

[Mark Claussen]

I did a short test in spectral line mode to verify that Rick's
phase jumps occurred in the manner that he reported for continuum
observations when the Fluke synthesizers were set in a particular
manner.  First a quick summary of the Fluke settings and the phase
jumps they excited (from Rick):

1. 100 kHz digit set with all others to the right = 0   --- no phase jumps
2.  10 kHz digit set with all others to the right = 0   --- 180 degree jumps
3.   1 kHz digit set with all others to the right = 0   ---  90 degree jumps


The test I did was to observe a strong OH source (W3OH) in spectral line
mode:  bandwidth code 9, normal mode 1A (195 kHz bandwidth, 512 spectral 
channels, 20 sec records).  I set the Fluke to 125.6000, 125.5900, 125.6100,
125.5890, 125.5840 MHz in succession, observing cal/source/cal/source for
2 minutes duration each scan.  After an entire fluke sequence I went back
to 125.6000 for another 8 minute sequence and then to 125.5900 for a 2min
cal, 10min source observation to end.

I examined the phases for W3OH for the peak channel (0.381 kHz resolution) in 
each of the above cases.  For the 125.6000 and 125.5900 settings, I thus
observed over a time range of about 40 - 50 minutes.  For the other settings
the range was only about 5 minutes.

Results:  


Fluke at 125.6000 MHz:  The EVLA antennas have a phase jump ~30 minutes after 
the beginning of the observing.  The magnitude is about 180 degrees.  All 
EVLA antennas jumped at the same time.


Fluke at 125.5900 MHz:  The EVLA antennas have a phase jump ~30 minutes after
the beginning of the observing.  The magnitude is about 180 degrees.  All
EVLA antennas jumped at the same time.


Fluke at 125.6100 MHz:  No phase jumps seen over the ~5 minutes of observing.

Fluke at 125.5890 MHz:  The EVLA antennas have a phase jump ~2 minutes after
the beginning of the observing.  The magnitude is about 90 degrees.  All 
EVLA antennas jumped at the same time.

Fluke at 125.5840 MHz:  No phase jumps seen over the ~5 minutes of observing.


I come to the conclusion that the phase jumps happen randomly in time with the 
Fluke set to a given frequency.  Phase jumps 2 and 3 (from above) happened 
during my observing, but more seriously, a ~180 degree phase jump was also 
noted when the Fluke was set to 125.6000 MHz --- a condition that Rick thought 
might  be "safe".   I suppose it's possible that the phase jumps are dependent on
what the digit is actually set to, but that seems to be speculation at the moment.

Mark