[evlatests] K-Band Tuning Span Restriction

[Rick Perley]

    The upcoming flux density run this weekend has encouraged some 
unusual tests to see what we can get away with. 

    I ran a test last night, utilizing some extreme frequency tuning at 
C and K bands. 

    At C band, I tried tuning the AC and BD IFs to 4.085 and 7.515 GHz.  
This worked very well, with good amplitudes and stable phases on the 5 
working EVLA antennas. 

    At K band, I tried two combinations:
   
          22.485 and 18.935 GHz,      (for AC and BD IFs, respectively)
                       and
          22.435 and 26.015 GHz. 

    This only sort-of worked.  Fringes were obtained on the BD IFs on 
all the EVLA antennas, but they showed very strong phase winding which 
seems proportional to  baseline component 'u'.   Phases on the AC side 
were fine. 

    Ken explains that with the current LO setups for transition, we 
cannot in fact tune the AC and BD IFs apart by more than 2.0 GHz 
(whereas I had thought it was 4.0 GHz).  This restriction only applies 
to K and Q bands.  I leave it to him to give a more complete 
explanation.  He reports that with some coding change, the separation 
can be increased to 4 GHz -- I have encouraged him to consider this.  In 
the meantime, the 'EVLA Returns' page should have this existing 
restriction stated.