[evlatests] Setting levels

[Barry Clark]

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.