[evlatests] Phased VLA fringes!

[Vivek Dhawan]

Thanks to Walter, and  "real time"  DIFX, (2 hours/GByte transfer), the VLA
clock was found at +1704 usec on April 5th.    Backtracking to projects in
March,  Paul Dyer has in short order found all the missing fringes.

expt       Date        usec.     BW

ty028a   Apr 05    1704     1 MHz
bb321N  Mar14    1667     128
bb321L  Mar13     1667     128
bk178c  Mar01     1670     16

The offsets may depend more on bandwidth (we have seen this before)
than on time elapsed since March 01. No offset is seen in the monitor
point  (VLA_GPS to clock) in the master LO.

The ~2msec offset is not apparent on the VLA internally, but now we
must find out how and why it happens.  It may also be that the regular
VLA data is fine, but the VDIF stream has slipped relative to it.
I don't know if that is even a legal thought.
Tests must be devised, anyone have ideas?

I will set up another real-time session soon.

Relatively less significant mysteries remain:
The 4 subbands have (sometimes) different delays and amplitudes,
especially on bk178 (centaurus A) but that I am attributing to the
channels being badly phased. Chan 1 is sometimes lower amplitude.