[evlatests] EVLA Phase Troubles

[Rick Perley]

    A two-hour test was run last evening, the purpose of which was to 
check L-band polarization characteristics in spectral line mode -- but 
which of course has revealed some other issues as well. 

    This note addresses EVLA phase. 

    Observations were alternated between L-band and X-band, 5 minutes 
for each, over two hours.  The correlator mode was 'PA', so only two IFs 
(A and C) were utilized.  The BW was 25 MHz, and we have 8 frequency 
channels for each of the four correlations (RR, RL, LR, LL).    There 
was no change in Fluke synthesizer commands between the two bands. 

    Issue #1:  Jumps

    Phase jumps were found in four EVLA antennas, at both frequencies.  
These are not 'global' -- each jump is particular to one antenna at a 
time.  For all jumps, the A and C IFs behaved identically. 

    At L-Band, antenna 18 jumped by 143 degrees *** in the middle of a 
scan ***.  The change occured in less than the integration time (3 
seconds).  The new phase was stable, and there was no return to the 
'old' phase value for the rest of the run (40 minutes).   The X-band 
phase for this antenna also changed, but by only ~40 degrees. 

    Also at L-band, antenna 25 jumped by 100 degrees, between scans.  In 
other words, when we returned to L-band after having spent 5 minutes at 
X-band, the phase of this antenna had changed by 100 degrees.  During 
this scan, the phase jumped  back to the original value *** in the 
middle of the scan ***.   No change in X-band phase was noted (which is 
consistent with the changes noted above if the jump actually took place 
between the X and L-band observations). 

    At X-band, antenna 23 had its phase change by 55 degrees, following 
an L-band observation.  In the middle of this scan, its phase returned 
to the previous value.  At precisely the same times, antenna 25 also 
jumped -- but the value was by 122 degrees.  No change in L-band phase 
was seen for either antenna. 

    Issue #2:  Oscillatory Phase

    Antenna 18 is showing a sinusoidal phase variation, identical in IFs 
A and C, at both X and L bands.  The amplitude of the oscillation is 
closely proportional to frequency -- about 25 degrees at X-band, and 
about 4 degrees at L-band.  The period of the oscillations is of the 
order of 1 minute -- but variable from scan to scan!  At X-band (where 
the effect is most easily seen), successive 5-minute scans show a period 
of 3.3 minutes, 2.0 minutes, and 1.25 minutes -- but this trend is not 
continued in successive scans.  At L-band, during the same time period, 
the cyclical period seemed to be increasing (it's harder to determine 
this when the amplitude is much lower). 

    Any ideas?