[evlatests] Setting levels

[Bryan Butler]

all,

i had a chat yesterday with jeff kern about just how they are doing this 
for ALMA.  i've cc:ed him on this so he can tell us if i've translated 
incorrectly.

they are going to do what is essentially solution (e) below - keep track 
of the calibration of the level setting for each downconverter.  they 
will monitor it periodically (period TBD, based on stability of things, 
but it should be more like hours to days or even longer, not seconds) by 
looking at the state counts from the correlator.  so, they don't have to 
actively close the loop from the state counts to the digitizer level 
setting.

	-bryan


On 8/30/05 15:35, Barry Clark wrote:
> I have been operating under the assumption that the way we would run
> the IF system of the EVLA antennas is that we would use the detectors
> in the T304 to set the levels to the digitizers, which, because they
> are multibit devices, have a reasonable latitude for what they are 
> fed.  From the the discussion at Monday's test meeting, it appears 
> that the T304 detectors may not have enough accuracy for the purpose.
> This note is to ask a few questions leading to a redesign of that
> concept.
> 
> In any event, it appears to me that we have had an entirely misplaced
> emphasis on the precision, as opposed to the accuracy, of the T304 
> detectors.
> 
> I ask a few questions below.  I also answer them to some degree, 
> rather tentatively, incompletely, and subject to revision.
> 
> 1.  To what accuracy need the level be set going into the digitizer?
>     By actual measurement on the eight bit sampler transition system
>     (adjusting the level going into the digitizer and making compensating 
>     adjustments on the gain before the intra-FIR resampler in the 
>     deformatter), the answer appears to be that an error of 2db in the 
>     level setting results in a loss of SNR of about 4%.  (The optimum 
>     level is determined by noise in the digitizer on the low end
>     (and possibly some clipping loss), and by aliasing of clipped 
>     highs on the high end.)  One db accuracy would seem to be a reasonable
>     goal.  The corresponding factor is surely known by calculation for 
>     3 bit samplers, but I do not have that number handy.
> 
> 2.  Is there ever a need to have a synchronous detector in the T304?
>     I think that in the usual cases,  synchronous detection should 
>     be done in the T5 for the transition system, and in the WIDAR itself 
>     for the final system.  However, I am very unsure whether such a thing
>     might be needed for solar mode.
> 
> 3.  What is the function of the T304 output attenuator?
>     In three bit mode, it serves to equalize the power levels between
>     the 1 and 2 outputs.  In eight bit mode I do not believe it has
>     a function, and should always be set to zero.  Is the best strategy
>     in three bit mode to always set the output attenuator in the channel
>     with the lower throughput to zero?
> 
> 4.  What is the function of the T304 input detector?
>     None.
> 
> 5.  If, as seem likely, the detectors in the T304, elegant as they are,
>     are inadequate to the task of level setting, should they be replaced
>     by something simpler - a diode, a 1 sec RC, and a SPI A/D?
> 
> 6.  Given that the digitizer itself must serve as the main level detector,
>     what is the best strategy for doing that?
>     Clearly the only acceptable mechanism is through the data capture 
>     buffer in the D30x.  There may need to be a tradeoff between
>     responsiveness (which I believe is important for solar observations),
>     complexity of software, and saturating various system elements with 
>     traffic.
>     Alternative designs are:
>     a)  Moving the T304 output attenuator into the D30x module, under
> 	control of the D30x MIB.
>     b)  MIB to MIB communication.
>     c)  A special ALC program running in the CB that just acts like a 
> 	detector, turning around the messages from the D30x and sending
> 	to the T304.
>     d)  Closing the the ALC loop within the AntennaPhysical software
> 	element.
>     e)  Recording, in the system parameter database, the relation between
> 	the T304 output detector and the actual power level presented to
> 	the digitizer.
>     As a firm believer that hardware problems should be solved as close as
>     possible to the hardware in question, the above is ordered by desirability
>     in my eyes.
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