[evlatests] T304 attenuator changes

[Bob Hayward]

Are any of the LOs (i.e., L301 or L302) happening to change between 
scans? This might not cause T304 to change its attenuator setting but 
even a small LO change could cause the output signal from the various 
mixers to change slightly. Depending on the amount of input gain slope 
or ripple, the resulting bandpass could look quite different.

-Bob


rperley at nrao.edu wrote:
> I reported earlier on the amplitude changes seen in the C-band data, on
> most antennas.  All of these changes occurred between scans, and most
> corresponded to a 1 dB change in power, strongly suggesting that the
> origin is the T304 attenuator chanaging.
> 
> I later reported on apparent unstable C-band bandpass responses -- unlike
> at X-band where perfect stability is seen.  Closer inspection of these
> apparently unstable bandpasses shows that they changed shape between
> scans, and were always stable within any one 5-minute scan.  Indeed, most
> of the bandpass solutions look as good as the X-band ones -- it's a subset
> of the scans which show non-standard shapes.
> 
> Ken suggested that I look for a correlation between the two phenomena
> reported above -- and it is certainly there.  Most (but not all) of the
> scans showing a change in bandpass shape are the ones with a change in
> power level, likely corresponding to a change in T304 attenuator.  For
> each power (attenuator) setting, there is a different bandpass shape.  If
> we take one of these as the 'right one', the other differs by a few
> percent, and has a distinctly sinusoidal shape with a long period -- over
> 100 MHz.  Not all antennas show this -- of the ten, perhaps one or two are
> much less affected.
> 
> 
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