[evlatests] first look at the newly installed three bit samplers

[Ken Sowinski]

The three-bit samplers have been returned to the antennas with FPGA
firmware to allow an arbitrary choice of three bits from the six-bit
sampler output.  All the samplers work about as well as they did before 
being removed last week.

I paraphrase Mike's explanation to me of how it works.  A scale factor
is supplied which is used as a multiplier for the six bit ssample.
After multiplication three bits are chosen from the product with due
care given to sign and over/underflow conditions.  The bits chosen
from the product are those which result in the "traditional"bits
1, 2 and 3 when the scale factor is set to 64.  Other powers of two
can be used to select a diffrent set of three bits:

16 selects bits 3,4,5
32 selects bits 2,3,4
64 selects bits 1,2,3
128 selects bits 0,1,2
255 is meaningless.

Bit 0 is LSB, bit 5 is MSB.  Accepted values for the scale factor are
0 to 255.  Anything over 128 is not useful.  In practice there large
differences in the state count histogram were seen when changing
between a value of 127 and 128.  The scale factor is not restricted to
a power of two; any value between 1 and 127 may be used.

To see that this really works I recorded the state count histograms
for values of the scale factor in the set {16, 32, 64, 127, 128, 255}.
The final value, 255, was used out of exuberance rather than thought
and should be ignored when perusing the attached plots.  At each value
of scale factor the downconverter output attenuator was used to achieve
a sampler RMS of about 1.65 and the state count histogram was recorded.
The equalizers were not adjusted at all, and should not much affect
this result.  Vivek kindly assembled all the plots into one pdf file
which is attached to this note.

The best looking histograms are seen when the scale factor is between
32 and 64.  This corresponds to using some combination of bits one
through four.  A value of 127 yields a histogram which is very centrally
condensed; this changes dramatically for 128 to a broad, lopsided 
distribution.  All the histograms are attached to this email.

Next I think we should come to some consensus as to what scale factor
we provides the best result and then make some SNR measurements.

I note that this provides us with yet another gain knob which, in this
case is equivalent to having adjustable sampler thresholds.  If we choose 
to use it, this could accomodate changes in power to the sampler of at
least 6 dB after the T304 attenuators have been set.
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