[evlatests] ACU Test Results -- Short Steps

Rick Perley rperley at nrao.edu
Wed Mar 18 14:15:00 EDT 2015 

     We ran (what I hope are) the final ACU tests yesterday.  These come 
in two flavors -- this report is on the first.

     For readers with limited time and/or patience, here is the 'Bottom 
Line':

     *The new ACUs perform magnificently, almost completely eliminating 
the overshoot inherent in the old ACUs, as well as significantly 
reducing the 2.2 Hz resonance ringing in the old system.*

     Details:

     The test is to use the standard pointing mode to measure the 
antenna response function to short steps.  This was done by executing 
pointing mode observations of 3C345, when it was at an elevation of 45 
degrees, and an azimuth of 290.  (Hence, this does not test for the 
resonances which may occur when the rate of change of Azimuth or 
Elevation is zero.  However, this test was done last week, with the 
result that the dEl/dt =0 resonance, discovered last year, was not seen).

     The pointing test was done at four bands.  The antenna throws 
(offsets) were as follows:

     Ku:  1.9 arcminutes in elevation, 2.7 arcminutes in Azimuth (these 
are antenna, not sky, coords)
     X:  3.0 and 4.3 arcminutes, respectively
     C:  5.6 and 8 arcminutes, respectively
     L:  17 and 24 arcminutes, respectively.

     In pointing mode, the antennas are instructed to step, by the 
quantities given above in +El, -El, +Az, and -Az.  Each step lasts 20 
seconds.  Hence, the motion from +El to -El moves the antenna by twice 
the values listed above -- similarly for the +Az to - Az motion.  The 
cycle concludes with an 'on-source' observation.  Two full cycles were 
executed for each band.  The data dump time was 0.1 seconds, which gives 
us a nice clear view into the antenna behavior.

     The data were calibrated in the standard way.  CALIB was then run 
for all observations, and the data plotted using the 'PODB' function in 
SNPLT:  This inverts the gains, squares them (to get a power) and plots 
them in dB.  Hence, -3 represents a 'half-power' value on the plots.

     Attached are 16 plots, showing the antenna powers (in dB, w.r.t. to 
the on-axis value of 0.0) for the two antennas with the new ACU (14 and 
21), matched up against two 'representative' antennas with the old 
ACUs.  Each plot has two panels -- the old ACU plot is on top, the new 
ACU is below.  Each is on the same scale, permitting easy comparison.

     The plots show power versus time for the RCP polarization, starting 
just before the command to move arrived, and extending 15 to 20 
seconds.  The initial power level is usually ~ -4 dB -- corresponding to 
slightly beyond the half-power.  For 'L' and 'Ku' bands,  you'll note 
the before and after powers are a bit different in Azimuth -- this is 
due to the beam squint which splits the R and L beams in the azimuth 
direction.  The plots for LCP have this offset reversed.
     The plot names give the band (U, X, C, or L), the motion (El or 
Az), and the new ACU antenna (14 or 21).  Each plot has two panels, the 
lower with the new ACU performance, the upper with an representative old 
ACU.

     The performance for all bands is similar, and is summarized below:

     A) In Elevation:

     The old ACUs show an overshoot, extending 4 to 10 arcminutes, 
depending on the band (highest at L-band, least at Ku-band).  In 
addition, a strong 2.2 Hz ringing is seen, lasting up to 10 seconds.  
This ringing has an amplitude of ~ 1 arcminute, and is most easily 
visible at the higher frequencies, since the beam size is smaller.

     The new ACUs almost completely eliminate both phenomena.  The 
profiles show that the antenna accelerate smoothly (hence, crossing the 
beam 0.3 seconds later than the old ACU antennas), then decelerate 
smoothly.  The entire move is completed in 3 to 4 seconds, even at 
L-band.  Nice!

     B)  In Azimuth:

     The old ACUs show the same large overshoot as in elevation.  The 
2.2 Hz oscillation is not as strong as in elevation, but is present 
(mostly seen in the jagged gains as the antennas go through and come 
back from the overshoot).   There is a notable, but not large, slow 
resonance of ~ 10 second period.

     The new ACUs almost completely eliminate the overshoot and the slow 
resonance.  The 2.2 Hz resonance remains, but is only visible at the two 
higher frequency bands, and is of small amplitude.   For all bands, the 
step from half-power to half-power is completed in 3 seconds, except for 
the small ringing at the higher bands, which remains visible for 1 to 2 
seconds more.




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