[evlatests] BD T304 power meters and attenuators

[Dan Mertely]

In looking over the shoulders of those in this discussion
I noticed the comment that "the bandpass is different for
different attenuator settings".  That seems unlikely to me.
The attenuators are single, surface mount, solid state
devices.  The spec sheet says that the bandpass flatness
of the device used will only change by 1 dB or less going
from min to max setting.  That change is "in the noise"
compared to the non-flatness caused by the LO settings and
front ends.  If bandpass changes are seen with different
attenuator settings, something else must be going on (non-
linearity in the digitizer perhaps?).

-Mert


Michael Rupen wrote:
> 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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