[evlatests] P-band (non)-linearities -- Very Good News!

[Rick Perley]

    I reported last Friday that a clear relation between P-band 
non-linearities and power delivered to the quantizers was found, in the 
(unsurprising) sense that the higher the power delivered to the 
quantizers (and re-quantizers), the greater the apparent loss of 
visibility when observing the powerful source Cygnus A. 

    Ken and I discussed the issue.  It seemed clear that the problem 
lies in the requantizers -- these have 8 levels each side of zero.  With 
a normal, cold-sky, setup, the rms level is set to about 2.7 levels.  
Hence, voltages above (or below) 3-sigma will fall off the sample grid, 
and be represented as the maximum (or minimum) levels. rather than their 
true values.  Fine for the cold-sky source.  But when we go to Cygnus A, 
which approximately quadruples the power, the 1-sigma voltage is now at 
3.7 levels, so voltages beyond 2-sigma are now off the grid. 

    To test this, we ran an experiment last night, with three different 
setup strategies:

    1) Set up the power (both the T304 attenuators *and* the 
requantizers, to their normal levels) on Cygnus A.  Then observe the 
calibrator 3C380, and use these gains to calibrate Cygnus A. 

    2) Set up on 3C380, then observe Cygnus A.  Calibrate as above. 

    3) A hybrid arrangement:  Setup the T304 powers on Cygnus A, and the 
requantizers separately on Cygnus A and 3C380.  (Apparently, we can 
remember each ...)  Observe both sources, and calibrate as above. 

    Results:

    The results from setups #1 and #2 are perfectly clear:

    Setup #1 provides exactly correct flux density of Cygnus A (to 1.5% 
of the Baars value) with the assumed value of 3C380 = 40.0 Jy.  (This is 
at 336 MHz).  There are no discrepant antennas. 

    Setup #2 provides Cygnus A fluxes 'all over the map' -- all of them 
too low.  We are clearly in a severe (digital) compression regime. 

    Some sort of error occurred on setup#3 -- we'll investigate this 
further, later. 

    Investigation of the PSum values provides extra, useful information:

    All antennas, except one, had the digital (PSum) power for the 
setup-source (Cyg A for #1, 3C380 for #2) equal to 14.5 to 15 counts.  
This is in stark contrast to the P-band flux density experiment, run 
early last month, where the powers varied from 10 to more than 50 
counts.  The difference appears to be in this new test, the requantizers 
were set separately, following the setting of the T304 attenuators. 
    The one discrepant antenna is ea28, which set itself up (on both 
polarizations) to 28 counts --  two times too high. 

    Ken will probably add some description of the setup regimen, with 
recommendations of how the system setup should be done in the future at 
this band (and perhaps also at L and S bands).