[evlatests] Sampler histograms

[Walter Brisken]

I have modified one of Mike Revnell's test programs to request a series of 
data chunks from the evla deformatters (D351s).  From these data I've 
created histograms showing the frequency of occurance of the various 
sampler levels.  I took no real care to make sure attenuator levels were 
set to be sane, but in all cases I got reasonable looking RMS values.

The resultant histograms are not nice smooth histograms but rather show 
that level frequency alternates with bin, indicating that the least 
significant bit (and to a much less severe level the second least 
significant bit) seems biased.  In the case of IFs 16c and 16d the effect 
is fairly strong: adjacent bins with ratios of nearly 2:1.  Similar 
phenomena were seen as early as June 6, 2005, but this is the first time 
I've systematicly looked at all the IFs.

I have two plots, source code, and this email message stored in my new 
EVLA web repository :

http://www.aoc.nrao.edu/~wbrisken/evla/

I'll keep things organized in directories named after the date of the 
corresponding evlatests report.

-W