[evlatests] Antenna 14 Measurements from 20 Sept 2005

[Jim Jackson]

FYI - The 8-12GHz filter shown on the block diagram as being in front of 
the T304 are indeed now inside the T304.

Travis - I don't understand the apparent gain variations in the T304. Do 
any of your measurements show this?   When you characterize the T304's, are 
you looking for fast variations this small?  If not, how do we test this in 
the lab?

Jim


At 08:22 AM 9/21/2005, Rick Perley wrote:
>    Bob Hayward and I spent most of the day making fundamental sensitivity
>and stability measurements in Antenna 14.  The original goal was to track
>the sensitivity (via the 'Y-factor' between hot load and cold sky) through the
>signal chain.   As always, the tests uncovered unanticipated problems that
>may, or may not, be related to our original goal.
>
>    1) Up-Front Tests.  We did these in LCP (as the power meter is
>single-channel).
>    We did a hot/cold power ratio at the RF, using a 4750/600 MHz
>filter.  The Y-factor was found to be 9.3 dB -- a value 1.7 dB below
>what we measured in February of this year (and the discrepancy will
>increase when we account for the differing temperature of the hot
>absorber between February and September).   This represents a
>significant apparent loss in sensitivity!
>
>    We next did an efficiency test, observing Cygnus A and Virgo A
>in an on-off mode.  These data have not been reduced -- I'll get this
>done today.  Visual inspection of the traces however revealed a
>curious power instability while on Cygnus A -- but not seen in the adjacent
>sky observation, nor on Virgo A, nor on the hot load calibration.
>    As a side issue, we note that we had considerable difficult in aborting
>the scripts to end these tests.  Neither we, nor the operator, could get
>the abort command to be accepted.  Approximately 30 attempts were
>made for both tests before the tests were in fact aborted.
>
>    2) Test before and after the T304.   We elected to use IF `D', as this was
>the best behaved channel in the astronomical data I took last week.
>    We elected to check the Y-factor at the output of the T304 first, rather
>than at the input.  LOs were set to put the desired 4722 MHz center frequency
>in the 1750/150 MHz filter we attached to the output of the T304.    We 
>immediately noted the output power was very unsteady -- sporadic,
>and fast (~100 msec) power fluctuations of ~0.3 dB were noted, as well as 
>erratic drifts
>of similar size over time scales of tens of seconds.  Yet at other times, 
>the power
>was semi-stable.  Weird behavior.
>    We measured the Y-factor as 9.4 dB -- the same as at the RF.
>    We moved the power meter to the output of the LSC converter, using a 
> 11000/100
>MHz filter.  These data are perfectly stable (as they were in February 
>when we did
>out earlier tests).  Crisp and clean.
>    We then spent considerable time trying to track down the origin of the
>instabilities which appear to arise within the T304.
>    - We noted that the 8 -- 12 GHz filter which the block diagram located 
> on the
>wall of the vertex room shows should precede the T304 was not present, so we
>added one to see if any different in behavior could be found.  No changes were
>seen.  *** I now can see why -- the updated block diagram on the wall opposite
>my office shows this filter to be within the T304 module -- We need updated
>block diagrams in the antennas!!!
>
>    We noted that even with the power input to the T304 limited to 100 MHz BW,
>the output power instability remained.
>    The instabilities go in both directions (i.e., the `pops' can be down 
> as well as
>up).   Changes in power level as much as 1.2 dB were seen -- with the power
>fairly stable both before and after the jump.  The jumps can be extremely 
>fast --
>faster than our time resolution of 100 msec.
>    All these data were recorded, and I can show them to anybody interested.
>    At 6:10 PM we quit for the day.  We were unable to get to the backend
>measurements, so these must wait until Thursday.
>    Rick
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