[evlatests] More on L-Band Phase Stability

[Rick Perley]

    There were two tests done Wednesday afternoon, intended to better 
understand the differences in phase behavior between the VLA and EVLA at 
L-band. 

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    Test 1:  Cyclic observations at 7 different frequencies, of one 
minute each.  IFs 1 and 2 were set to the same frequency.  BW = 12.5 
MHz, spectral line mode '4' .   90 minutes total duration. 

    Result:  The phase was stable within any one observation, but a 
large change in phase was introduced when the observation returned to a 
given frequency.  The phase change was the same on opposite 
polarization, but not the same between the two IFs (despite the RF 
frequency being the same).  All EVLA antennas showed the same behavior, 
no VLA antennas showed this (when a VLA antenna was the reference). 
There is a small residual drift between EVLA antennas.
    The phase 'jump' beween sequential observations of the same 
frequency (7 minutes apart) was about +70 degrees for IF#1, and -40 
degrees for IF#2.  The 'jump' is always the same sign, and always 
occurred. 
    Superposed on top of this accumulating phase jumps were occasional 
(four times for IF#1 and twice for IF#2) 180 degree phase jumps. 

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    Test 2:  Raster scanning in holography mode.  BW = 12.5 MHz, Flukes 
= 118.85 and 218.65 (default frequencies of 1465 and 1385).  No changes 
in frequencies, about 20 scans through the main beam, plus two 
calibration scans.  Continuum, with 0.4 sec. dump.  50 minutes total 
duration. 

    Result:  No phase irregularities at all!!!  A slow drift of EVLA 
antenna phase, for some antennas, w.r.t. a VLA antenna.  All is normal! 
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    A tentative conclusion:

    It appears that changing the L-band frequency is what triggers the 
large phase changes -- both the 180 'flip', and the incremental stepped 
phase -- seen on (either) the EVLA, or the VLA.