[evla-sw-discuss] VCI schema changes

[Ken Sowinski]

Below are proposed changes to support more flexible control of the
summed array AGC circuitry and the mixer in the filter chip.   I
intend to make these change to the VCI scehma in the next week or
so, so speak now if you have any reservations or suggestions.

Ken


This is a proposal for what summed array AGC control for VDIF summed
array output should look like at the VCI level.  The cc element contains
the same attributes, they behave identically and will be ignored here.

The current definition of the vdif element includes three attributes related
to AGC:

    - requantGain    int     0 < requantGain < 255
      To explicitly set the requantizer gain.  Not very useful without
      control of the "window" setting as well.

    - agcRms         float
      Desired setpoint for summed array output power RMS.

    - agcEnabled     boolean
      Chooses between setting requantizer gain explicitly (false), or
      servoing the RMS to the desired setpoint (true).  The default
      is true.

These are all optional with defaults enforced in CM rather than XML.
I propose to deprecate agcEnabled, and to add optional attributes with
default values enforced by CM.

    - agcMode        enumeration
      To control how the AGC behaves.

    - window         int
      Which may be used in the future to specify the "window" setting.
      Currently there is no direct control over this hardware setting.

agcMode may take on several values:

    - dontChange   Do nothing, nothing is sent to the BL boards.
    - setRq        Equivalant to agcEnabled=false.
    - servoRms     Equivalent to agcEnabled=true.
    - setRms       Equivalent to servoRms which freezes when stabilized.
    - table        Calculate RQ gain from window setting chosen by hardware
                   and number of antenna included in the sum.  This is not
                   yet implmented in the BL board software.

If agcMode is not present, agcEnabled is transformed to "setRq" or "servoRms".
agcMode may be an enumeration or string.  I prefer the simplicity and
extendability of a string.  agcMode, like agcEnabled, is modifiable.


Usage of the mixer in the filter FPGA of the station board is controlled
by two attributes of the subband element.

    - useMixer             "yes" to use the mixer to center a subband at
                           an arbitrary non-grid frequency.

    - mixerPhaseErrorCorr  Always ignored by CM, usage controlled internally
                           by the CMIB.

Both are optional with a default of "no" enforced by the XML.  I propose
to deprecate both these attributes, enforce the defaults in CM rather than
the XML, and add two new attributes to describe how to use the mixer.

    - centerFreqInSubband       "yes" to use the mixer to center a subband
 			                                     at an arbitrary
 			       non-grid frequency.  This
 			       is the same as the old "useMixer"

    - fringeRotateInFilterChip  Do fringe rotation (and LO offset removal)
 			                                     in the filter chip
 			       Zeros will be sent to
 			       the correlator chip and RXP summing circuitry.

Both will have default value "no".  The new attributes will be ignored
unless neither of the old attributes is present, so that old scripts
will always work.  The two attributes may be requested independently or
in combination.

Both these changes are designed to allow old scripts to still work without
change.  In both cases if an old attribute is present any new ones will be
ignored.