[evlatests] 3-bit status

[Michael Rupen]

The new 3-bit samplers were installed on three antennas today:
ea15, ea22, and ea28. There were some problems with the 3-bit sampler
for ea12 so Mike brought it back to the lab to work on (see his last
e-mail).

Some top-level results from a couple hours of Vivek & I:

* The basics work for most of the samplers (see ea22-DP0 discussion below):
   - changing input attenuation changes the state counts in the sampler
   - gain-slope equalizer seems to work
   - fringes seen on all baselines & polarizations, except ea22-DP0 --
     see attached figure 3bit_fringes.pdf
   - state counts are not beautiful but not ridiculous (after manually
     setting the attenuators) -- see attached figures ea*_3bit_6apr11.gif

* IF A as seen on Ken's 3-bit signal summary screen corresponds to
   Data Path 1 (LCP) on the Station Board (and hence in the resulting data
   files).  IF C is sent to DP0 and will be labeled RCP.  I suspect this
   is a labeling error in Ken's code but will track that down tomorrow.

* ea22 has some odd issues:
   - 3-bit DP0 (RCP as seen in the data)...
     (1) shows very odd state counts (overpopulates "edge" states)
     (2) produces no fringes
   - 3-bit DP1 seems fine
   - 8-bit IF A has the unique behavior (amongst the 3-bit antennas
     at least) that the auto-level setting of the attenuators leads to
     an rms of 15-16 (consistently) rather than the expected rms of 10.
     All the other 8-bit samplers I checked, including those on ea15 and
     ea28, were correctly coming out with an rms of 10.
   - 8-bit IF A also shows an oscillating amplitude and phase across
     128 MHz (see attached figure ea28_8bit_128MHz.gif).  Maybe a
     reflection in a cable?

* One technical detail:
   - We did remember to turn off the 8-bit sampler, and to issue a "reset"
     after switching to 3-bit samplers.  This helped some odd state
     counts for other antennas/IFs, but *not* ea22-DP0.  We'll try this
     again tomorrow to be sure.

All in all, we did better than I expected.  Lots more to do, but it's
looking hopeful for getting at least one useful measurement this week.

          -- Michael
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