[evla-sw-discuss] setting gains on the station board

[Ken Sowinski]

Getting the station board gains, especially the requantizer gain,
right will be important for the three bit samplers.  I talked with
Bruce about this yesterday and summarize are conclusions and questions
here.

Proceeding from the lowest to the highest level.
1.  The full expression of this is to, at each active filter stage,
adjust the filter gain to achieve the desired RMS of samples.  Then
do the same for the requantizer gain.  The measurement for each step
in the chain must wait for settling from adjustment in the previous
stage.  There is sufficient SNR in the power measurement over one
correlator tick, 10 ms, that the measurement is good enough to set
the filter gains.  There will be a systematic offset of a few percent
because of the noise tube contribution, but there is no need to have
the filter gains set with greater precision than that.  Bruce pointed
out that the computation that makes the requantizer power computations
available as "switched power" can be used to provide the current averaged
requantizer power for setting the requantizer gain.  The averaging
interval is long enough that noise tube switching is accounted for and
there is suffici8ent SNR even for the narrowest bandwidth.  Care must
be taken that the averaging interval includes data after the upstream
adjustments have settled.  We discussed that there might be a need for
two varieties of gain adjustment, adjusting the full chain of gain knobs
as described above, and a simpler requantizer gain adjustment assuming
that all the upstream adjustments are correct.

2.  We must decide how to communicate to the statino board when and how
to do this.  The obvious solution is for the executor to send a VCI
'modify' document specifiying when to begin the adjustment, and what
to do.  In addition to an activation time one might want to include
set points, integration time and, perhaps, desired ALC strategy.
this depends on implenting the VCI modify funtion.  Is there any need
for an alternate path which might be available sooner?

3.  We must decide how the executor knows when to tell the correlator
to set gains.  Is it explicitly in the scripts?  Should the executor
understand enough about the meaning of the script to discern when to
send the cause a gain setting?  Getting this right will interact with
setting up the antenna, setting downconverter gains, and antenna motion.