[evlatests] BD T304 power meters and attenuators

[Michael Rupen]

One comment here: we really need some focused testing of the effects of 
changing attenuator levels from scan to scan.  Obviously the amplitude
changes; that ought to come out when we apply switched power (which makes
switched power calibration much more important for the 3-bit samplers!),
but we have to show this.  I have heard anecdotal evidence of occasional
phase jumps but it's been a while since we've checked this in any
detail.  Finally, we have clear evidence from a few WIDAR-0 tests
that (1) the bandpass is different for different attenuator settings,
and (2) the bandpass for a given attenuator setting is different
depending on the history of prior attenautor settings (there is
hysteresis).  There are also indications that the bandpass for a given
attenuator setting may "settle down" if the attenuators are left alone
for a while.

All of this could use some dedicated testing, and could benefit even
from OSRO low-frequency data.

            Michael

> Rob has told me that power meters are now available in all T304s.
> This raises the question of what to make the default attenuator
> setting algorithm for IFs B and D.  I would like to make the
> ALC at scan change the default and no longer maintain the BD
> attenuator settings in parminator.
>
> Some considerations.
>
> OPT generated script would have to set the BD T304s to use
> set-and-remember whenever it does so for AC.
>
> Both IF pairs will now be reasonably set regardless of tuning
> within a band.
>
> We will have to use this, probably without set-and-remember,
> for the three bit samplers.
>
> Use of ALC modes will force us to understand how the attenuators
> modify bandpasses.  If set-and-rememeber is used too avoid
> attenuator changes, the details when multiple tunings within a band
> are used can be messy.
>
> When eight bit samplers are used it can be argues that the best
> approach might be to set all the attenuators from a table of
> values.  There might be some merit to this at L band where
> interfence can make ALC operation unreliable.
>
>
> I argue we make ALC the default and allow the ovserver to choose
> different strategies in the OPT based on the particular scenario
> for that observation.  Someone will have to invent a few probable
> and useful scenarios.
>
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