[evlatests] Serious C-Band RFI

[Rick Perley]

    The new 'spectral sweep' of our entire 1 -- 50 GHz has emphasized 
the challenge we have before us to operate in S and C bands. 

    In S-band, the problems have not changed:  Two extremely strong 
interferors, spanning 2180 -- 2200 MHz, and 2310 -- 2360 (this is 
Digital Satellite Radio).  The spectral densities (in 125 kHz 
resolution) are typically 40 dB above the noise floor.   The power from 
these signals completely  dominates the total system power.   Rob Long 
has today inserted stop band filters into two antennas to test if these 
give sufficient isolation while preserving the rest of the band for 
science. 

    In C-band, a new RFI signal has appeared, and it is strong.  Between 
6.0 and 6.7 there are three quite strong interfering signals -- I 
believe all three are microwave links running overtop the VLA):  6.11 -- 
6.17 GHz, 6.36 -- 6.42 GHz, and 6.612 -- 6.618 GHz.  The spectral 
densities within these are 15, 25, and 25 dB above the noise floor.  
Integrated together, these three contribute a power about 12 times (11 
dB) above the noise power within the 2 GHz IF..  The middle one of these 
(6.36 -- 6.42) is the new one -- it was absent last spring, but present 
last September. 
    We were pointed at the NCP during this test -- perhaps the signals 
are weaker in other directions, but nonetheless, we have a serious 
problem for wideband observing in C-band.  The 3-bit samplers are not up 
to the task of handling such strong signals (2 bits are needed just to 
span the RFI power).   We're going to have to think seriously about 
using more than 3 bits if we are to cover the entire span of C-band.