[Gb-ccb] Minutes from 07jul03 videocon

[Roger Norrod]

At 06:38 PM 7/9/2003 -0700, Martin Shepherd wrote:
>> backends which I didn't quite get down clearly (the signal is down by
>> 15 dB at the nyquist frequency, which results in aliased signal being
>> 30 dB down?)
>
>Curious. How do you get from 15dB aliased signal strength to 30dB
>down? I would have thought that it would simply be 15dB down. If I am
>wrong, please tell me, since a doubling of the filter dB stop-band
>attenuation would be great :-).
>

The comment in the meeting referred to specific filters used in a
spectrometer situation, and really does not apply in this case.  The
spectrometer users throw away spectral channels above about the -1dB point
in the band-defining filters.  For the GBT spectrometer, we set the filter
rejection at the Nyquist frequency to -15dB, and for the 8 or 9 pole
filters used, the folded over Nyquist spectrum is down about -30dB at the
-1dB point.  The extra noise between the 1dB point and the Nyquist
frequency ramps up from -30dB to -15dB, but is thrown away.  In the
continuum backend of course, the detectors will respond to the integrated
noise spectrum, so there is no opportunity to throw away portions.  To get
the true foldover noise contribution, I think one has to integrate the
receiver noise spectrum with a perfect filter and with a physical filter,
and compare the results, but basically Martin is correct.

Martin, I think we need to digest the meat of your message for a few days
(at least I do), before commenting.

Thanks,
Roger