[evlatests] notes for September 27, 2005

[Ken Sowinski]

                            EVLA Tests
                        September 27, 2005

There three goals for today.  Verify that the phase compensation
in the FIRs helps, verify that removing the water helped the
C band system on 16, and try another careful attempt at C band
sensitvity.  These were all achieved to some degree despite 
massive confusion and many random failures.

The hardware failures.
1.  L302-1 in antenna 16 just doesn't work anymore.  At various 
times it was out of lock or had entered standby mode.  I rebooted
it once to no avail.  It did work for a while in the morning,
but not for long.  I suppose it is worth trying to reboot it now
that antenna 16 is properly synchronized.

2.  14B is weak.  Rick and I suspect the downconverter because
this is true at both X and C bands.

3.  14D was totally dead at the T5, but seemed OK at the deformatter.
Eventually Rob and Jerry tracked this down to a faulty connector at
the D rack.  A temporary fix was made.

4.  Antenna 16 had indeed been out of sync for a long time.  Some
of the confusion today arose because the L305 sync_enable had been
left on.  HOwever after enoticeing this neither Rob nor I was able
to synchronize the antenna.  Eventually Rob adjusted the tick signal
level and he was able to get the antenna in sync again.

The software confusions.
This was mostly due to changes in the deformatter realted to the phase
equalization.  The sense of the bits that control band pass and spectrum
sense have been inverted so our software was always setting it wrong.
Suitable values for GAIN1 and GAIN2 have changed.  Optimal values now
seem to be about 32 and 800.  This will be further explored tomorrow.

Phase equalization
This works well for 50 MHz continuum; less well at narrower bandwidths.
Using correlation coefficient as a proxy for G/T I measured the response
of the antennas without and with phase equalization at X band in 50 MHz.
without:
14A  6.4      16A  --
  B  4.4           8.3
  C  8.1           --
  D  --            9.4
with:
14A  12.2          --
  B   6.7          12.8
  C  11.6          --
  D  --            14.1
Most VLA antennas had responses in the range of 13 to 16 so we are much
closer now than we used to be.  Clearly 14B has a problem.

C band G/T
With the phase equalization still in place C band was checked.  Power
levels and delays were set and it was found that 16 works much better
when the feed is dry.
14A  11.9    16A  --
  B   6.4          9.5
  C  15.3         --
  D  12           10.2
Most VLA antennas had responses in the range of 12 to 16.  Antenna 16
is still not as good as it ought to be.  Bob told me sometime this
afternoon that he thought that it was not yet completely dry.

Rick then took some data to try to estimate sensitivity.  He just
called it 'bizarre' and said he would summarize it tomorrow.

A few minutes were spent after that to verify that settings of 32 and
800 really were optimal for GAIN1 and GAIN2 in IF 14D.  This will have
to be looked at more comprehensively another day.


Next time we should have the software in better shape to deal with the
deformatters and then spend some time trying to track down all the
instabilities we have been seeing today.