[evlatests] Sensitivity Losses at edge of 1 GHz bandpass

[Ken Sowinski]

> I vaguely remember being told that the subband zero filter rolls off
> harder to zero frequency than other subbands to avoid the poisonous
> effect of digitizer DC offsets on the requantizer.  If so, it is
> overdone - the DC offsets we see are only about 0.1 sigma, which
> shouldn't be much problem; at worst they may force Hanning smoothing.
> We may even be able to do something constructive by removing this
> slope in the shape of subband zero in the digital filter, to boost
> the low end above the requantizer noise (basically, the Dolby
> principle).
>
> Three bit digitizers are quite another game.  We are limited by the
> digitizer clipping noise from the beginning, at about -17 dB.  Things
> will be helped somewhat by the bandpass slope corrector, but it has
> its limitations; it can't do anything about something that affects
> mainly subband 0 as the 8bit path does.  And we may need the extra
> rolloff, because the DC offsets run to at least 0.5 sigma, and WBC
> spectra seem to indicate that the DC spike is slightly resolved.  We
> could attack the latter problem by the rather heroic measure of using
> the input lookup table to subtract a DC offset, in steps of half a bit
> (if I remember how the thing works).

Another way to tackle the DC offset problem is to use the
'DC remover' that Dave put in the filter FPGA.  I believe Bruce
put in some way to use this feature, but even if not we should
be able to set it up by hand for a small number of filters.
Perhaps the way to make this accessible is through an option
in the XML command to set the requantizer gains.