[evlatests] C-Band Sensitivity and Frequency

[Rick Perley]

     It was earlier noted that at L-band, the EVLA sensitivity is 
significantly better at 1385 MHz than at 1485 MHz, regardless of IF 
channel.  The difference is significant (~40%), and was similar for all 
four antennas outfitted at that frequency. 

    I also noted that a similar effect has been seen at C-band, on the 
basis of a single frequency setup, where the AC pair is set to 4885 MHz, 
the BD pair to 4835 MHz. 

    Last evening, I ran a short experiment at C-band to check whether 
the sensitivity difference is, like L-band, a function of frequency only. 

    The experiment setup was the same as the L-band experiment:  Four 
different tuning setups, as follows

    Setup   AC      BD
---------------------------
       1        4885   4835
       2        4835   4885
       3        4885   4885
       4        4835   4835
---------------------------

    Each channel (A, B, C, D) is then observed twice at each frequency, 
once with the other pair
at a different frequency, and once with the other pair at the same 
frequency.  (The reason for all
this detail will soon be clear ...)

    Antennas 13, 14 and 16 gave good data.  Antenna 18, although it has 
a c-band receiver, still gives no fringes. 

    As before, the sensitivities were estimated using AIPS weights, and 
confirmed with actual on-sky
noise histograms.  As always, the latter confirms the results of the 
former. 

    Results:

    In short, the frequency effect is confirmed, but there are 
interesting complications.  Antenna 16 behaves exactly the same at 
C-band as at L-band.  For antennas 13 and 14, there is an additional 
effect going on. 
    Read on ...

    Below is a table giving the AIPS weights.  Recall that these are 
proportional to the square of
(efficiency/Tsys), so big numbers are good. 

IF                     4835 MHz                     4885 MHz
          13      14      16      Mode   |      13      14      16      Mode
--------------------------------------------------------------------------------------
A      470      370      420      2     |      210      150      220      1
         560      520      420      4     |      360      320      205   
   3
----------------------------------------------------------------------------------------
B      340      270      420      1     |      350      240      240      2
         570      540      410      4     |      390      340      240   
   3
-------------------------------------------------------------------------------------------
C      390      165      340      2     |      190       84      175       1
         480      290      340      4     |      320      250     180   
    3
---------------------------------------------------------------------------------------------
D      270      245      340      1     |      300      230      200      2
         510      510      340      4     |      345      310      206   
   3
------------------------------------------------------------------------------------------------

    Points of emphasis:

    1) Antenna 16 behaves at C-band like all the EVLA antennas at L-band --
its sensitivity is strongly dependent on the frequency, but there is no 
dependency
on the particular IF utilized, or on what the other IF pair is tuned to. 

    2) Antennas 13 and 14 have quite a different behavior -- there are 
significant
sensitivity differences, ***but the differences depend on what the other 
IF pair
is tuned to!!!***  In particular, the sensitivity is best (and is 
really, really good)
when both IF pairs are tuned to the same frequency.  Also note that when 
both
IF pairs are tuned to the same frequency (modes 3 and 4, in the above 
table), the
sensitivity when tuned to 4835 MHz is much better than when both are tuned
to 4885 MHz.  (!) 

    The immediate suspicion is that certain LO selections are causing 
additional
additive noise to get into the correlated bandpass.  If so, then we 
should be able
to see the effect in the recorded system temperatures.  Below is a table 
of Tsys,
parallel to the sensitivity table shown above.  (The absolute values are 
not as
important as the changes between frequency and observing mode). 

IF                     4835 MHz                     4885 MHz
          13      14      16      Mode   |      13      14      16      Mode
--------------------------------------------------------------------------------------
A      50       41       37        2     |       65       50       42      1
         47       36       36        4     |       54       40       44 
     3
----------------------------------------------------------------------------------------
B      58       46       35        1     |       55       44       43      2
         50       37       35        4     |       53       41       43 
     3
-------------------------------------------------------------------------------------------
C      37       45       23        2     |       42       48       28   
    1
         34       40       22        4     |       37       38      28   
    3
---------------------------------------------------------------------------------------------
D      42       43       22        1     |       41       42       27      2
         35       35       22        4     |       39       38       26 
     3
------------------------------------------------------------------------------------------------

    The correspondance of the Tsys to the sensitivity is excellent.  For
antenna 16, Tsys is lower at 4835 than at 4885 MHz, and there is no
dependence on which mode the frequency was selected. 
    For antennas 13 and 14, Tsys is in general lower at 4835 than at 4885
MHz, *** but in addition, observing with the IF Pairs at two different
frequencies adds 5 to 10 degrees to the Tsys***. 

    I suspect that we're getting significant aliasing still.