[evlatests] Non-closing wobbles still exist!

Fri Dec 28 02:27:40 EST 2012

My feeling - it is no more at this point - is that it is not in the correlator
but it is an  'interfering'  signal of some sort. Perhaps in the UX converter.

e.g. an aliased portion of an external signal in the UX converter that is
more-or-less rejected by each antenna hardware, so some have it worse
than others.

Rick said he thinks it is downstream of the T304 but I don't remember
why. And the fact that the wobbles are weak on the weak source makes it
not an additive but a multiplicative mechanism.

Sleeping on it now...

On Thu, December 27, 2012 17:55, Bob Sault wrote:
| Rick,
|
| WIDAR-based wobbles that were previously worked on should be the same
| in RR and LL (although there could be a phase offset between the
| wobbles on the different polarizations). So at first blush it is not
| that.
|
| Best regards
| Bob
|
| --
| ----------------------------------------------------------------------
| Phone:    +61-2-80651793
| Email:    rsault at nrao.edu
| Web:      http://www.ph.unimelb.edu.au/~rsault
|
| -----Original Message-----
| From: evlatests-bounces at nrao.edu [mailto:evlatests-bounces at nrao.edu] On
| Behalf Of Rick Perley
| Sent: Friday, 28 December 2012 11:45 AM
| To: evlatests at aoc.nrao.edu
| Subject: [evlatests] Non-closing wobbles still exist!
|
|     I took an hour's data last week on 3C84 , and some nearby calibrator
| sources, in order to establish whether our 'PDif Compression' problem
| affects the visibility amplitudes.  What has happened since then
| illustrates perfectly my rule 'When we look for something, we find
| something else'.  Note in what follows that we are in A configuration --
| the baselines are long, and the fringe rates fast!
|
|     To make this test as clean as possible, I utilized Ku-band, in 8-bit
| 1 GHz mode, with the A/C IFs at 14.5 GHz, and the BD IFs at 17.5 GHz.
| There is no visible RFI in either band.  I used the standard setup -- 8
| subbands in each IF pair, each of 128 MHz width, with 2 MHz channel
| resolution.
|
|     The data in IFs A, B, and C are as perfect as any I have ever seen.
| An image of 3C84, made in a single subband, gives over 500,000:1 dynamic
| range -- and this with only 18 minutes of data!
|
|     But subband 'D' (LCP at 17 -- 18 GHz) is something quite different.
| Here I find, only many baselines, a strong, perfectly sinusoidal signal,
| covering multiple subbands, but particularly strong in the first two.
| The basic characteristics of this 'wobble' are:
|
|     1) In LCP only, on a single (analog) IF 'D'.  The effect is much
| stronger on the lower two subbands within this IF, but is visible on all
| eight.  The largest wobbles are seen in the 20 channels at the bottom
| end of the first subband (following the bandpass correction).  This is
| true on *all* baselines which show the effect, which I claim is strong
| evidence that the interfering signal is being introduced *downstream* of
| the anti-aliasing filter located in the 1-2 GHz path of the T304.
|     2) It is equally visible in both amplitude and phase, with the same
| period and SNR.  Visualized in the complex plane, the visibility vector
| terminates on a circle, centered on the correct amplitude and phase.
|     3) The amplitude of the wobble is typically 1 -- 2 % -- but on some
| baselines, it is as high as 6%.  On phase plots, the wobble is up to +/-
| 5 degrees.
|     4) The wobble period can be amazingly long (given that we're in A
| configuration, in Ku band) -- up to 2 minutes!  About ten baselines have
| periodicity of longer than 30 seconds.  Many longer baselines have
| periods of 5 seconds or less.  It is possible that all baselines have
| the effect, but that the period is too short for most to be seen with
| the 1 second averaging.
|     5) The effect does not close.  This is evident from inspecting the
| wobble patterns from any triplet of antennas.   The program CLPLT (which
| plots closure phases) shows this beautifully.
|     5) There is a tendency for shorter baselines to have longer
| periods.  The cluster of four antennas near the end of the north arm (ea
| 1, 2, 23 and 28) all have high amplitude and long periods.   However,
| counter-examples can be found.  And short baselines near the center of
| the array show short periods,  or no wobbles.
|     6) Some longer baselines have clear beating patterns.
|     7)  There is no clear spatial relationships between high beat
| amplitudes and baseline/antenna length or position.  Antenna ea04 -- at
| W56 (way, way out there) has large, visible 'wobbles' on nearly all of
| its baselines.  But neither of the two antennas on each side of it (17
| and 21) show any visible wobbles at all.  I think this alone makes an
| external origin impossible.
|     8) All the monitoring data (PSum, PDif) are normal and clean.  The
| autocorrelation spectra are clean.  The bandpasses are clean.  There is
| no evidence of an external signal within the 1.024 GHz bandpass.
|
|     I consulted with the available gurus  (Ken and Vivek), with
| essentially no agreement, other than the effect is unlikely to be driven
| by an external, radiated signal.
|
|     I attach some amplitude and phase plots (encapsulated postscript),
| from subband #2 of the C and D IFs.  The blue trace is RCP (IF 'C'), and
| red is LCP (IF 'D').  The amplitude plots are first, the phases follow.
| The antenna numbers and locations are written on the plots.
|
|     Any ideas?  We're desperate!
|
|     Rick
|
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