[evlatests] Suggested new CBand default

[Rick Perley]

    I also suggested that the  default tunings for the upper IF pair at 
C-band be modified, in order to place the two prominent 
telecommunication link frequencies within single subbands.  Currently, 
out default places these two links within two subband, each. 

    I included in the short test described in my previous message a 
tuning arrangement where the upper frequency was set to a center of 5975 
MHz, thus spanning 5463 through 6591 MHz.   The strong RFI links are 
then contained entirely with the 6th (6103 -- 6231 MHz) and 8th subbands 
(6359 -- 6425 MHz). 

    I attach two spectral plots, utilizing *only* the four shortest 
baselines.  One is a vector, the other an ampscalar plot, showing how 
choosing this default prevents these strong RFI signals from 
contaminating the 5th and 7th subbands.  Indeed, the uppermost channel 
of the 7th subband shows increased noise (seen best in the 'vector' 
plot), clearly an edge of the RFI in the 8th subband (hence, starting at 
6358 MHz).  Lowering the center frequency of this IF to 5974 MHz should 
eliminate this edge problem. 

    On the basis of this, I suggest we make the upper tuning default 
center frequency be 5974 MHz. 

    One worrisome note:  The 'spectral sweeps' have shown that the RFI  
in subband #6 (6103 -- 6231 MHz) spanned from about 6103 through 6168 
MHz.  But the attached plots clearly show it extends much higher now -- 
filling the entire subband.  It seems that the span from 6168 through 
6231 MHz -- formerly empty -- is now full of telecommication traffic. 


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