[evlatests] Failures to Tune/Fringe

[rperley at nrao.edu]

A seven hour observation of a single source (3C345) at all eight bands was
taken Friday evening with the primary purpose of determining the elevation
dependence of the antenna gains.   Besides this goal, many other system
characteristics can be determined from this rich data set.

The run consisted of 37 short observations at each band -- along with 37
reference pointing determinations.  The large number of band changes
permits analysis of the reliability of the frequency changes, and of the
reproducibility of the antenna gains.  This note lists the problems noted
in the system's ability to change band.  As noted below, there are two
bands -- C and Ka, for which we have quite a significant issue.

I give below the basic statistics of the success in getting fringe power
upon a band change.  The order is that utilized in the observations, and
was chosen to minimize subreflector motion time.

The loop started with a C-band pointing scan, done at 7 GHz.  There then
followed, in order:

1) C-band (4376 and 6626 MHz)  (all frequencies are those of the low
frequency edge):

There were 69 failures for fringe power to appear in the BD IFs only -- AC
had only 2 failures.  The failure rate (69/(27*37) = 6.9% is by far the
highest I've seen in tests like this.  The distribution of failures over
the antennas is essentially uniform -- only two antennas had no failures
(5 and 7), while two had six failures (10 and 14), and two had four (2 and
24).  Since no band change was involved here, these failures must be due
to the LO.  None of these failures was flagged by the system.

2) S-band  (2496 and 3456 MHz).  No failures.  (Nine antennas are not
outfitted with the S-band receiver).

3) Ku-band: (13136 and 16736 MHz). Four failures, all in both IF pairs at
the same time.  Eleven antennas are not yet outfitted.

4) K-band:  (25436 and 19436 MHz).  One failure, on both IF pairs.

5) Ka-band:  (36436 and 29436 MHz).  There were 50 failures to give
fringes -- all of these on the BD IFs only.  The distribution here is not
uniform -- six antennas had no failures (2, 5, 7 9 11 19), two antennas
had five (20 and 24), two with four(14 and 25), the others with two or
three.  There were no tuning failures on the AC side.  Two antennas (4 and
8) provided no fringes on either IF through the entire run.

6) Q-band:  No tuning failures noted.

7) L-band:  No tuning failures noted.

8) X-band:  Thirteen tuning failures, all in both IFpairs simultaneously. 
However, four of these involve antenna 8, which is special (it failed to
fringe partially or wholly at all other bands, and has not been counted in
the preceding lists), so we should really consider only the seven other
failures:  two of these are with ea24, and two others with ea13.

Examination of the switched power shows the usual problems with the
switches -- application of PDif usually -- not not always -- corrects the
loss of amplitudes.  This will be the subject of another, later, note.

The key point is the extraordinary failure rate in the BD IFs for C and Ka
bands.  Any ideas for why?