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

[Sonja Vrcic]

Ken Sowinski wrote:
> 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?


 From the point of view of CM this should not be difficult to implement.
CM would simply pass instructions to Station Boards. To speed up implementation it is 
important to define requirements.

May I assume that requirements are similar to those specified for the "filter gain panel" 
in the Station Board GUI.

Here is my attempt to capture requirements:
- Target filter gain (and other related parameters) should be specified (and set) for each 
subband independently, i.e. for each subband of each baseband pair (or baseband 
singleton), and applies for all the antennas (stations) in a subarray.
- Alternatively, as for other parameters, user can specify a subset of antennas (stations) 
to be configured.
-




>
> 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.
>
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-- 
Sonja Vrcic
Software Engineer
National Research Council
Herzberg Institute of Astrophysics
Penticton, BC, Canada
Tel: 250-497-2309 / 250-497-2300
Sonja.Vrcic at nrc.gc.ca
http://www.nrc-cnrc.gc.ca/eng/ibp/hia.html