[evlatests] 3-bit tests, Tsys vs. bit levels.

[Vivek Dhawan]

Another note on this: the Tsys is a function of which 3 bits we
select!

I compared corresponding subbands (at the same frequency) from
the 3 paths, A1C1, A2C2, BD.

When using the lowest bits, the antennas/basebands/pol show the
3-bit Tsys to be equal or lower than the 8-bit Tsys, by ~0-10%
across the array.

Using the higher bits usually (but not always) raises the Tsys
of the 3-bit paths, so they spread -10 to +20% on either side
of the 8-bit Tsys.

The explanation must lie with the gain compression/expansion of
Pdif. The scans on/off 3C84 show the Pdif compression, but I have
not tried to fit the numbers.