[evla-sw-discuss] Controlling the baseband slope correctors

[Bruce Rowen]

The STBs could create a new data product as described below and let  
Kevin's MCCC data collector scoop it all up and package it into a  
single stream.

-Bruce

On Aug 4, 2010, at 4:56 PM, Barry Clark wrote:

> To set the baseband slope correctly, the least complicated approach
> is to use the Wide Band Correlator on the station board to see the
> overall shape of the spectrum, and have it report the correction
> needed to the Executor, which would then forward it to the slope
> corrector in the T304 modules.
>
> This is because the required slope is a function of antenna, band,
> and location within the band.  To do it by tabulation would require
> maintenance of a very large table which would change from time to
> time as receivers are serviced or replaced.  (The slope required
> may also be a function of which T304s are installed, although these
> are believed to be much more uniform than the receivers.)
>
> The Executor could handle this information in the same way it  
> currently
> handles reference pointing, phasing, and, indeed, setting T304 levels.
>
> What the Executor would like to see is just an rms and slope from
> the WBC, one for each baseband in the array.  I suggest the following
> XML descriptor.
>
>   <xsd:complexType name="WbcDataType">
>     <xsd:sequence>
>       <xsd:element name="rms" type="xsd:float" />
>       <xsd:element name="slope" type="xsd:float" />
>     </xsd:sequence>
>     <xsd:attribute name="antId" type="xsd:string" />
>     <xsd:attribute name="baseBand" type="xsd:string" />
>     <xsd:attribute name="avg" type="xsd:double" />
>     <xsd:attribute name="time" type="xsd:dateTime" />
>   </xsd:complexType>
>
> These would probably come packaged in an envelope, either for a lot
> of antennas if they come from some collector in the correlator, or
> with just a couple if they come directly from the station board.
>
> Worst case is eight basebands from 28 antennas, 224 elements in all.
> The whole works might fit in a 16K datagram.  I think a reasonable
> averaging time (avg above) would be 10 seconds.  (The consideration
> is only to keep the traffic at a low level while still getting answers
> on a script-type timescale.)  I guess CM will need to send this
> averaging time to the StB.
>
> These need only be sent when in three bit mode.
>
> The rms above is just the lag zero autocorrelation, divided by
> averaging time and suitably scaled.
>
> The slope is a little more difficult.  If computation is not a
> consideration, you calculate the spectrum and see which slope
> corrector works best, calculating the goodness of fit of
>    a*10^{0.1*n*(f/bw)}
> for, say, n in the range -10 to 10 and reporting the best n.
> If calculation is a problem, a reasonable guess can be made directly
> from the lags.  (The lag beyond which the correlation is low is
> a measure of the magnitude of the slope; whether there are nulls
> before then gives the sign.)
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