[evlatests] 3-bit update 8apr11

[Michael Rupen]

Rick & Keith got a fair bit done today at the site:

1- Checked that the input signals to ea12's samplers are OK,
   suggesting that the problem is with the sampler rather than (e.g.)
   a T304 module.

   - They brought the offending sampler back to the AOC for Mike to
     play with on Monday.

2- Switched input cables for the other samplers, so RCP at the
   antenna is now labeled RCP at the correlator (and in the resulting
   archive data sets).

3- Put in 15 dB attenuators between the T304s and the samplers,
   so we can simultaneously feed both the T304s and the samplers the
   right power levels.

I did a couple quick checks to show that #2 and #3 indeed work as expected,
confirming that:

   - RCP is RCP on all 3 antennas
   - I can pick attenuator settings that make both the T304s and the
     samplers happy for all 3-bit samplers
   - We still have fringes, at roughly the right strength, on all
     expected baselines and correlator products.

I'm hoping for 3-bit noise tests on Monday.

         -- Michael