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

[Bryan Butler]

barry - are you thinking that the rms and slope will actually change on 
10 second timescales?  or is that just to react to band changes?  i'd 
have thought it'd be stable except for band change, so 10 seconds is too 
often when not changing bands and not often enough when doing so 
(because you want things settled down more quickly than 10 seconds in 
some cases on a band change).  i wonder if we could send a trigger to 
the CMIB to ask for this, rather than having it every 10 seconds, to cut 
down on traffic?

	-bryan


Barry Clark wrote, On 8/4/10 16:56:
> 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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