[evlatests] Frequency offsets between cal & target.

[Vivek Dhawan]

VLA and EVLA now: Each antenna shows a phase change with freq.
offset of (L df), where L is the LO path length to the pad, or
~30 turns per 1MHz offset at the ends of the B array. I can
correct this using known path lengths, but only to the 1%
level. The VLA and EVLA have opposite signs of the correction
from each other. This is true at C, X, K bands (the correction
is RF independent, but the sideband might have made a difference,
it does not.)

The advice to users is as before: to observe in fixed frequency
mode, which avoids the phase jumps between the VLA and EVLA in
addition to eliminating the present effect.

Archival data clearly shows that (as expected from the design),
the OLD VLA was much less sensitive. e.g. AB1219, observed on
13-NOV-2006, C array, 2.1 MHz frequency offset, shows only about
20deg. phase offset (If the old VLA behaved the same as the current
VLA, there would be ~10 turns of phase at the ends ~1.5km away.)

Vivek.