[evlatests] tests, April 27 2006

[Ken Sowinski]

An attempt to investigate the sawtooth in phase at antenna
13 was foiled because the subrefelctor was stuck at X band
preventing work at K band.  Nevertheless an attempt was made
at X band and is described below.  It is expected that the
sybreflector will be working again tomorrow.

All antennas worked at X band; 14 and 16 at L, C, K and Q
as well.  14B needed delay tuning at all bands; the rest
of the delays were fine because they were set during
startup last night.


Antenna 18
The response from IF C is about half what we expect and
the SD voltage is pegged at 10 volts.  Sampler RMS and
TP look OK, so further investigation is warranted.


Q band
Collimations were tweaked for antenna 14 and 16 and they
might now be good enough for a proper collimation observation.
13 should be checked when it working once again.


Curiousity
Both 14 and 16 are doing something funny at K band, and matbe
Q as well but that is harder to see.  Amplitudes and phase
against a VLA antenna are both wobbling around in a smooth
fashion with a period of a few minutes.  Peaks in amplitude
do not coincide with peaks in phase.  14 and 16 together
are much weaker than expected from looking at each antenna
against a VLA antenna.  To allow an investigation some data
was taken at K band from 1925 to 1957 IAT.


Phase sawtooth
I tried to investigate this at X band where the effect is much
smaller.  Our original explanation that the wrong chirp rate
was being programmed into the DDS was confounded by a bench
test which showed that the original L302 implementation drove
the DDS output at the expeected rate.  We still do not understand
whay the data sheet and the part disagree.  If you take that
result at face value we can be
1) using the chirp with the wrong sign,
2) not using it at all,
3) changing it by a factor of two (or so) in the L302.
To try to distinguish these possiblities three tests were run
changing the chirp parameter for AC and leaving BD unchanged
as a control.
1.  2007 to 2028 IAT
Standard software l302.ramp=-.000282 cycles/sec/sec

2.  2030 to 2040 IAT
Set the chirp prameter to be identically zero.  For
IF BD, ramp=-.000330

3.  2041 to 2059 IAT
Set the chirp parameter to the negative of the calculated value.
l302.ramp=+.000367

These tests are sufficient to distinguish 1) and 2), and maybe
rule out 3).  Vivek is looking at the dat as we speak.


Collimations
An attempt to do a collimation pointing run was abandoned because
antenna 13 worked only at X band and the strong winds.


Holography
The rest of the time has been given over to an attempt at holography.
The first problem found was that the .evla script refused to run.
This was because not all necessary files were included.  'raster.py'
was included but not 'vlaraster.py'.  I have editted the file to
take care of this but obs2script needs to be fixed.  We will see
if there are other problems as well.