[evlatests] Sensitivity Losses at edge of 1 GHz bandpass

[Bruce Rowen]

These control points are already included under the 'stage1' element  
of the filters. Provided are attributes to enable/disable correction,  
read the current offset and applied correction, plus a parameter to  
set the integration/normalization period (in ticks) for auto-setting.

-Bruce

On Sep 26, 2010, at 3:09 PM, Ken Sowinski wrote:

>> 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.
>
>
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