[evlatests] R/L phase differences.

[Vivek Dhawan]

             1. Summary of previous tests:

3MHz continuum, IF1 = IF2 in frequency, source= Orion maser.

Q and K band showed short-term changes in (RR-LL) phase of ~5 deg.

Apart from the polarizer D-terms, such effects may be caused by cross-
talk between various paths in the RF chain (unlikely, since these are
well padded with isolators); or in the LO output fanout of the L301 or
L302 (neither of which are as well isolated).

Similar variations are seen in the (IF1-IF2) phases, K & Q. This is
(R1-R2) phase, so the D-terms cancel exactly as IF1=IF2 in frequency,
implicating the LO fanout path.

Most antennas show something, not just a few bad apples. Looking at
VLA vs. EVLA differences, a puzzle (or clue?) emerges:
VLA antennas are just as bad as the EVLA at K band. But
VLA antennas are good (in the noise, <1deg) at Q band.



            2. New tests:  6.25MHz continuum, 3C84, L, C, X bands.

L,C,X all show the same variations as above, at about 1-2deg. VLA ants
are a bit better, but the SNR is getting low, I can't really tell.

There are a couple really bad outliers, at all bands. Possibly, the
T304's are old models or something like that: EA16 IF D; EA17 IF B.
Perhaps also EA14 B,D (intermittent?)



            3. Next step: in consultation with Hayward & Jackson,
               the plan is, in rough sequence,

A. Do Ka band for completeness. (Test scheduled for tomorrow)

B. Put in isolators on the L302 outputs of a select few antennas, see
    if that cleans up X band. I pick EA24 and one (or more) of EA1,2,3,4.
    as hardware permits. Most useful if both L302's are done on the same
    antenna. Isolators can be left in place for normal use.

C. Put in isolators after the L301 power splitter at Q band. Also can
    be left in for normal use. See if this cleans up Q band. Same antennas
    as in A, to ensure good L301/T304 isolation.

D. More invasive diagnostics - Feed same RF input to several T304's to
    narrow down the location of trouble, e.g. cables vs. T304's. This must
    be undone for normal astronomy.

Vivek.