[evlatests] New RFI Sweep -- interesting new results

[Michael Rupen]

>    4) The 'lumpy-bumpy' phenomenon is present in a major way.  To
> review:  'lumpy-bumpy' means the spectrum in a single subband is
> elevated above the noise by typically 40 dB (this is HUGE) and has a
> 'bumpy' spectral structure with typical scale of ~5 MHz.  In last week's
> final test, 6 baselines showed this problem, all of which were
> associated with antennas 22, 26, 27 and 28.  Yesterday's test shows the
> phenomenon on *most* of the baselines.  The root cause of this problem
> is clear -- the single subband in which this occurs is that containing
> the digital satellite radio signals (Sirius and XM).  This signals are
> *strong*!!!  The total power within the RFI zone (2310 through 2350 MHz)
> is more than 100 times the noise power within that subband.  (This also
> means this power is about 6 times the entire bandwidth noise power!)
> Not all baselines have been affected, but those that are are definitely
> associated with specific antennas.  Although we should probably not be
> surprised that this subband is messed up by these signals, I think we
> need to understand a bit better just what is happening, and learn if
> there is a way to prevent this.  I presume that the internal 7-bit mode
> should help here.

We might also try setting the requantizer gain for this subband.  Pretty
easy to do, and a useful test before 3-bit requantizers arrive...

                Michael