[evlatests] Effects of T304 attenuator settings on bandpass shapes

[Rick Perley]

    I checked the plots.  There is no suggestion that 7<->8 for the 
input attenuator causes any particular effect on bandpass.  There is 
*dramatic* evidence that the 15 <-> 16 change for the output attenuator 
causes a significant change in bandpass.    Indeed, this explains why 
antenna #1 does not show the large changes that were so easily visible 
for antennas 8 and 25 in the plots shown this morning. 

The BPLOT files are rather large.  I'll just bring down the printouts...

   

Michael Rupen wrote:
> The "0,0" levels for the three antennas in this test were as follows:
>
>             Input attn     Output attn
>      ea01        7             18
>      ea18        9             14
>      ea25       12             15
>
> Rick, would you send around a PDF/PS of your BP plots?  I'd like to see
> whether the 7->8 and 15->16 changes had any greater effect, as Jim (?)
> thought they might.
>
>                 Michael
>
>>     Earlier WIDAR tests have shown that changing the T304's attenuator
>> levels causes changes in the bandpass amplitude shape by ~1%.  To
>> determine which attenuator is responsible, whether particular settings
>> have a more dramatic effect, and to otherwise illuminate the
>> characteristics of the problem, Ken and Michael ran a specific
>> experiment.   In this, they stepped through five different input and
>> five output attenuator settings, spending one minute on each pair.  This
>> was done on three antennas (1, 18, 25), the others were fixed.  The
>> observations were of 3C84 in one continuous observation.  The attenuator
>> pairs which were sampled were (in order):  (0,0), (-2,0), (-1,0), (0,0),
>> (1,0), (2,0), (0,0), (0,-2), (0,-1), (0,0), (0,1), (0,2), (0,0).  All
>> values are in dB.
>>
>>    The data were calibrated and the bandpasses determined for each
>> antenna, for each of the paired settings listed above.  The AIPS program
>> BPLOT was used to show the *differential* results.  (This means that the
>> average of all bandpasses is made and subtracted.  The results displayed
>> are the fractional deviations from the average).
>>
>>    Results:  Not as simple as had been hoped.
>>
>>    1)  The three antennas which had the attenuators changed clearly
>> show larger changes in their bandpass shapes.  The maximum deviations
>> are ~2.5%.   No commonality nor trends are seen in the bandpass changes.
>>    2) The seven antennas with fixed attenuators showed much smaller
>> deviations, much less than 1% for nearly all.   Changes that are seen
>> are gradual, indicating some physical change over time.  Antenna 19 is
>> by far the worst amongst these, with a 1.5% change over 13 minutes.  The
>> amplitude pattern has a ~100 MHz periodicity, indicating a standing wave
>> set up over ~1.5 meter length.
>>    3).  Two of the changing antennas -- 18 and 25 -- showed by far the
>> biggest deviations (>2%) with the setting (0,1) and (0,2) -- the output
>> attenuator seems to have the biggest effect.   This is also true for
>> antenna 1, but the amplitude of the bandpass change for this antenna is
>> very much less (~1%).
>>    4) For the three antennas with attenuators that changed, the four
>> (0,0) settings all showed different bandpasses.  These changes are
>> typical for the temporal variation of the seven antennas with unchanging
>> attenuators.
>>
>>    It thus appears likely that we have two simultaneous effects
>> confusing the issue:  A temporal change, operating over timescales of
>> tens of minutes, and an attenuator-depending effect.  Both are of
>> similar magnitudes -- up to 2% in bandpass amplitude.
>>
>>    All the above applies to the amplitudes only.  I'll determine the
>> phase effects after running FRING to remove residual delay effects (this
>> should keep my computer busy over the lunch hour ...)
>>
>>
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.cv.nrao.edu
>> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>>