[evlatests] L-Band Default Frequency Test

[Rick Perley]

    I have suggested that the default tuning range in L-band be set such 
that the upper edge of the AC and the lower edge of the BD tuning be set 
to 1520 MHz (rather than 1500).  Some concerns were expressed that there 
may be aliased RFI from the 'INMARSAT' satellite (1525 MHz and up) 
showing up in the ~1500 -- 1520 MHz zone.  I ran a short test over the 
weekend to check this. 

    I observed the northern calibrator J1800+7828 for 2.5 minutes, then 
a 'blank' field at J1800+8000 for 2.5 minutes.  A tuning arrangement, 
with the center frequency of 1520 MHz was made, with 1 MHz channel 
width.  Thus, the frequency span went from 1008 through 2038 MHz. 

    The J1800 source was used to calibrate the bandpass and set the flux 
density scale (I know the latter from the recent 'flux densities' 
runs).  I then inspected the calibrated spectra (with Hanning smooth on, 
of course!), and made images of the 'blank' field in both Stokes 'I' and 
Stokes 'RL' for each channel from 1485 MHz through 1519 MHz. 

    The 'blank' field isn't blank of course -- there is about 250 mJy of 
real flux in it, seen equally in all images. 

    Inspection of the 'I' images shows that every frequency channel 
gives an identical image -- none show any of the effects expected with 
strong RFI within the field.  The rms of each image was calculated  in 
both 'I' and 'RL' -- this shows a slow and steady rise from from 1485 
MHz to 1515 MHz (of about 10%) -- this is surely the decreasing L-band 
sensitivity, as shown (for example) in EVLA Memo 152.  The noise rises 
sharply above 1515 MHz - this is as expected from the roll-off of the 
anti-aliasing filter at the upper end of the IF. 
    One channel only within this band shows higher noise than expected:  
corresponding to a frequency of 1504 MHz.  Perhaps this is low-level 
RFI?  Nothing is seen in the individual spectra. 

    I made vector and ampscalar averages over all baselines longer than 
500 wavelengths (see below for explanation of this lower limit), 
excluding ea19 (see below for the reason).  These show that the subband 
from 1456 through 1520 MHz to be as clean as any other (and arguable 
cleaner!).  I've included these four for your perusal.    HiVec => 
Vector averaged from 1520 through 2032 MHz, HiAmp => Ampscalar average 
from 1520 through 2032 MHz.  Loxxx => Corresponding plots for 1008 
through 1520 MHz. 

    It will be noted that the noise in the uppermost subband (1968 to 
2032 MHz) notably rises -- this is due to the receiver rolloff.  But the 
rise is not great (factor ~2), and it is certainly better than in the 20 
MHz 'lost' at the lower edge (988 through 1008 MHz). 

    I recommend adoption of this setup as the default in L-band. 

----------------------------------------------------------------------------------------------------
    Some other details:

    1) ea19 was eliminated due to very low sensitivity.  (Long-standing 
issue, known to FE).
    2) ea23C had a DTS problem, and was flagged out. 
    3) ea25 in LCP (C and D) gave no fringes and was flagged (known 
problem). 

    Inspection of the visibilities as a function of baseline showed that 
all baselines shorter than 500 wavelengths showed much increased noise 
-- for all channels.  That is, it's a continuum issue.  Both RR and LL 
showed this, although 'RR' was more strongly affected in the subband 
that I was looking at (#8).  These short baselines were not utilized in 
the images or the spectra discussed above.   As this test was run at 
about 7:30 AM, local time, I'm blaming the sun for this. 

   

   
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