[evlatests] R/L phase differences.

[Jim Jackson]

I've confirmed that Antennas 14, 16 and 17 have early T304's in IF's 
B&D. At some point over the next year they will be brought in and 
updated.  The various issues in these early T304 modules almost 
certainly make them unsuitable for these tests.  These early T304 
modules are in use in IF's B & D of various EVLA antennas and can 
usually be identified by the lack of functioning total power detectors.

Cheers,
Jim


At 05:50 PM 5/18/2009, Vivek Dhawan wrote:
>              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.
>
>_______________________________________________
>evlatests mailing list
>evlatests at listmgr.cv.nrao.edu
>http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests