[evlatests] [Fwd: 3-bit test: A1 fed to A1 and C1]

[Rick Perley]

    Doug and Rob arranged that the output from the RCP T304 (IF 'A') was 
fed into both RCP and LCP samplers.    If the asymmetry seen in our 
'noise matrices' were due to something amiss in the T304s, we should see 
identical noise matrices.  If the asymmetries are due to differences in 
the samplers/demux/DTS, then the asymmetries should remain. 

They remain.  The only cross-products giving the same (or close to same) 
rms values are for baseline 15 X 28. 

Here is the noise matrix, values are in Jy.  Note that the 'right' 
answer is 0.14, or thereabouts. 

       12         15          22          28
---------------------------------------------
12 |  X         .20         .43         .22
15 | .16         X          .32         .16
22 | .17        .15           X         .34
28 | .17        .16         .17           X
_______________________________

    The gain values used to calibrate the data (using cross-correlation 
amplitudes), in power units, are

12R           61
12L           42
15R           34
15L           32
22R          146
22L           37
28R           37
28L           34

    Bottom Line:

    The result is as expected.  It explains the asymmetry (something 
wrong in the samplers/demux/DTS for antennas 12 and 22), but does not 
identify the origin of the additional noise. 

    As I write, the opposite experiment (LCP output from T304 replicated 
into RCP and LCP inputs to samplers) is being completed. 

    After lunch, Ken/Michael plan a more detailed (and lengthy) test to 
re-do the '3dB' stepped power test, but with optimization of the levels 
for the samplers and requantizers. 

-------- Original Message --------
Subject: 	3-bit test: A1 fed to A1 and C1
Date: 	Fri, 10 Sep 2010 10:54:11 -0600 (MDT)
From: 	Michael Rupen <mrupen at aoc.nrao.edu>
To: 	Rick Perley <rperley at aoc.nrao.edu>, Vivek Dhawan 
<vdhawan at aoc.nrao.edu>
CC: 	Michael Rupen <mrupen at aoc.nrao.edu>, Ken Sowinski 
<ksowinsk at aoc.nrao.edu>, Robert Long <rlong at aoc.nrao.edu>, Doug Gerrard 
<dgerrard at aoc.nrao.edu>



Hi folks --

  Doug & Rob put in the splitter, such that the signal from A1 is fed to A1
and C1.  Ken adjusted the attenuators at each antenna to get the optimal rms 
for one of the samplers, without over-driving the other sampler: i.e., in
each case he set the highest-rms sampler to give an rms of 1.7 (the
theoretically desired value for a Gaussian distribution of state counts).

  The measured power levels at the output of the downconverters and
corresponding rms values of the samplers were as follows:

ea12  initial   -29.2  0.99  1.34
      final     -27.9  1.09  1.45   best we can do!

ea15  initial   -29.5  1.25  1.25
      final     -25.3  1.59  1.66

ea22  initial   -31.0  1.08  1.36
      intermed. -23.8  1.66  1.93
      final     -28.0  1.35  1.63

ea28  initial   -30.8  1.00  1.24
      intermed. -22.6  1.65  1.91
      final     -25.9  1.40  1.65

  "initial"   --> as set automatically ("set-and-remember")
  "intermed." --> what happens if we set attenuators to get the weaker sampler
                  to have rms 1.7
  "final"     --> final setting used for the rest of the observation
                  (stronger sampler has rms 1.7)

After setting the attenuators to the "final" state given above, Ken
ajdusted the filter scalings & requantizer gains to give an rms of 2.7
after re-quantization to 4-bits.

The final attenautor settings & filter scalings & gains were all in place by
1556 (scan 13), and remained undisturbed for the following ~20 minutes
(through the end of the observation [scan 32]).

The attached figure shows the distribution of state counts as seen at the
Input FPGAs on the Station Boards (i.e., before subband filtering &
requantization) for the 3-bit antennas, after converging on the final
attenuator settings.  The RED (top, Path 0) subplots are for A1; the BLUE
(bottom, Path 1) subplots are for C1; and the four plots are laid out as:

        ea12    ea15
        ea22    ea28

The observing script is our standard: one subband pair (128 MHz) at C band,
centered at 7056 MHz. The first scan is 5 minutes on 3C147, to let the
samplers stabilize.  After that we have pairs of 1-minute scans {3C147,
off-source}.

Please DISCARD all data before the attenuators & filters are set up (at
1556, or scan 13).

The data are available at
  /home/scamper2/WIDAR0/C3tuned_3bit128MHz_000.55449.65253503472

Enjoy --

       Michael