[evlatests] Level setting problems

[Bryan Butler]

seems like waiting longer for the invoking the ALC is the right solution 
to me.

	-bryan


Ken Sowinski wrote:
> Hichem, Keithm Jim and I were able to track down why the fast
> downconverter ALC did not always work.  The problem is that
> the input power to the downconverter is still changing long
> after the ALC loop is started for many band changes.  The
> cause of the power change is subreflector rotation which can
> change noise power by 2 or 3 dB between the wrong and right
> rotation setting.  This was very obvious when switching between
> X and K bands which are on opposite sides of the feed ring.
> How important it is depends on how long the move in rotation
> is and, to some degree, the system temperature at the band
> we are switching to.
> 
> The slow ALC loop was a little more acommodating to this change,
> but it does not always work correctly either for the same reason.
> If we want to do a better job, there are only two choices, to
> pre-move the subreflector which breaks the old scan, or wait
> a little longer before invoking the ALC.
> 
> In principle, this could contribute to phase changes between
> successive scans at the same band if the first one mis-sets the
> attenuators at a band change, and the second sets them correctly.
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