[evlatests] Some issues arising from tests taken on Monday afternoon

[Rick Perley]

    Michael Rupen spent ~ 30 minutes on Monday afternoon observing a 
strong calibrator at C-band, switching between four different observing 
modes.  A full description of issues  uncovered (most of which deal with 
the 'zero-problem') will be put into the appropriate JIRA location.  
Here I list only those issues of engineering interest. 

    The observational setup alternated between 1 IFPair (AC) and a 2 
IFpair (AC and BD) modes.  Each observation in a given mode was 1 minute 
long.   There were two different subband widths used for each of the 
above modes -- giving a total of four different setups. 

    Overall, the quality of the data was fabulous.  The following issues 
were noted:

    A)  When we were in 2-IFpair mode, with two 64-MHz wide subbands, 
centered at 4.896 and 5.024 GHz, antenna 27, in IFs B and D (5.024 GHz) 
gave no fringes (that is, they apparently failed to tune correctly) on 
two of the three scans.  This problem did not occur when we utlized 
4-MHz-wide subbands, with center frequencies the same as above. 

    B) In both of the combinations noted above, antenna 28D gave no 
fringes, while 28B was fine. 

    C) Antenna 5, in A and C only, shows 60 degree phase changes between 
scans.  This occured for all mode combinations.  Sounds like the problem 
which is corrected by changing a cable length. 

    I could find nothing else negative to report on! 
   
    Perhaps most interesting is the absence of all the diverse phenomena 
reported on last week.  I did not see the 58 second 'square wave' in 
antenna 6B and 6D reported on before -- however, if these transitions 
are triggered by mode or scan changes, then the 1 minute duration of 
this test would effectively guarantee that the 58 second switch would 
not have occurred, since the first 5 seconds of each of these scans were 
flagged.