[evlatests] BD T304 power meters and attenuators

[Rick Perley]

I am one of those who believe that we should fix (as in set and don't 
change) the attenuators when in 8-bit sampler mode.  The dynamic range 
offered by the samplers is far larger than the range of input powers for 
all conceivable observations (excluding solar).  Even Cyg A and Cas A 
multiply the total powers by factors of less than ten at L-band (the 
worst case) -- that's less than 2 bits out of the eight.  The only 
question remaining is:  what level should the system be set at?  Ken is 
quite right that using a dynamic system to find and set the level could 
be dangerous at L-band (and probably S-band as well), so a table value 
strikes me as the best approach.  The variation in total system 
temperature at these frequencies is very small -- virtually all the 
power comes from the receivers themselves.  So a table of attenuator 
values sounds good to me.

With 3-bit sampling, we have much less headroom, so we'll likely have to 
adopt a dynamic approach, particularly at high frequencies where 
atmospheric emission will greatly increase system power depending on 
elevation and weather...  Rather than opine in advance of what we should 
do, I'd prefer some careful tests to determine the best level to set, 
and to determine by how much the sensitivity changes as the power changes. 



Ken Sowinski wrote:
> 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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