[evlatests] Re-formatted version of Rick's message

[Michael Rupen]

Following Ken's lead, here's Rick's message again, with the first
table re-formatted for easy reading:

>From rperley at nrao.edu Fri Sep 10 09:30:56 2010
Date: Fri, 10 Sep 2010 09:29:09 -0600
From: Rick Perley <rperley at nrao.edu>
To: evlatests at aoc.nrao.edu
Subject: [evlatests] Some clarity on 3 bit sampler issues ...

     Michael and Ken ran a test yesterday afternoon in which:

     1) The power levels to the samplers were set at appropriate levels, 
as judged by state counts, and
     2) The stationboard requantizer gains were adjusted to optimum levels.

     This was done, separately, for both 3-bit and 8-bit paths.
     The observations were taken in the 'narrowband' mode, with 1 subband 
at 7056 MHz (thankfully free of RFI).  Observations were made of 3C286 
and a nearby piece of blank sky.

     The results have considerably clarified what is happening.

A)  RMS noise levels.

   I give the two 'noise matrices', with 8-bit on the left, and 3-bit
   on the right, and make some comments below.

                    8-Bit                 3-Bit
   ------------------------------------------------------
        |    12   15   22   28   |   12   15    22    28
   ------------------------------------------------------
   12   |    X   .15  .15  .16   |   X   .22   .45   .25
   15   |   .15   X   .13  .14   |  .17   X    .33   .18
   22   |   .15  .13   X   .15   |  .18  .16    X    .37
   28   |   .16  .14  .14   X    |  .19  .17   .16    X
   ------------------------------------------------------

     Two observations from this:

   1) In the LCP side, we have good uniformity in the noise values for
     both 8-bit and 3-bit paths.  But, the 3-bit path is about 15 to 20%
     noisier.
   2) On the RCP side for 3-bit, we have -- as usual -- values for
     22A which are far too high.  But I argue also that 12A is also
     too high, so there is only one 'representative' value in that
     matrix -- 15 x 28, whose noise value of 0.18 is right in line
     with the LCP correlators.  This interpretation is strongly
     supported by other evidence, given below.  Bottom Line --
     I believe the RCP-LCP asymmetry that we have been focusing
     on for the past week is a *** red herring***.

B) Gains -- as determined from cross-correlation power.

     If all antennas are identical, and all system parameters set
   equally, we should see the same cross-power amplitudes, and hence the
   same calibration gains.  Significant deviations from uniformity then
   give us useful information.
     Below I list the calibration gains (in power, not amplitude,
   units).  Note that a low number means the antenna's correlated power
   (that is, the power from the source which is provided to the correlator)
   is high.

            8-Bit       3-Bit
   -----------------------------
   12R        36         69
   12L        32         44
   15R        24         36
   15L        27         37
   22R        28        144
   22L        27         34
   28R        30         44
   28L        32         37
   ----------------------------

     Note that 3-Bit gains are generally higher -- indicating that the
   power from 3C286 to the corrrelator is lower -- and that 12R and 22R are
   much worse than any others.

C) Autocorrelation Spectra.

     Back in VLA days, autocorrelation spectra were never utilized, as
   they had a 'bad rep' -- they gave weird results.  But in principle,
   these should give us useful information -- when properly calibrated,
   they should tell us the antenna SEFD.
     It appears that this is what the WIDAR correlator's autocorrelation
   spectra are giving us.  Below I give the calibrated autocorrelation
   spectral amplitudes, in Jy.

              8-Bit       3-Bit
   ------------------------------
   12R        280          500
   12L        260          330
   15R        220          270
   15L        220          260
   22R        250         1200
   22L        230          260
   28R        250          350
   28L        250          310
   -----------------------------

      A few spot checks shows a ***perfect*** correlation between the
   geometric means of the products of  autocorrelation amplitudes (above)
   and the noise values in the matrix at the top.  In other words, the
   (easily obtained) calibrated autocorrelation amplitudes are an excellent
   predictor of system sensitivity (as they should be).   We seem to have
   gained an even better way to determine system sensitivity than the old
   (VLA) route of utilizing normalized correlation coefficients.

Bottom line here is:  There in indeed extra noise in the 3-bit 
path, amounting to about 15 to 20% over the 8-bit path. 
There is (I claim) no asymmetry between RCP and LCP -- we have a couple 
of bad antennas/samplers on the RCP side.   The origin of the additional 
noise is yet to be determined.


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