[evlatests] Slow Wobbles -- back with a vengeance!

[Barry Clark]

The thought that comes to mind is a problem with an LO spur offset
by 128 MHz on the LO in the T303 converter.  But this would result
in a wobble whose period is inversely proportional to the u
coordinate.  Are you sure this isn't the case?  I notice the
period changes from bottom to top in your plot, from about 29 seconds
to about 33 seconds, which is about the way u is changing.
(Also this would close, if the calculation were done with
bandwidth << 128 and time << 30s.)


On 01/17/2013 11:57 AM, Rick Perley wrote:
> I reported, a couple weeks ago, that we had baseline-dependent slow
> (periods of seconds to minutes) oscillations in amplitude and phase on
> some baselines. These showed up in Ku-band data on 3C84 (a strong,
> unresolved object) in A-configuration. The magnitude of the effect was
> modest -- a few percent and a few degrees. The effect was seen only on
> IF 'C', and it was clear that it was a non-closing effect (i.e., cannot
> be removed by antenna-based calibration). A subsequent test, done at the
> same band, with the same source, with the same setup, failed to find
> these 'wobbles'.
> But ... They're back! About ten days ago, we observed 3C84 at Ku and K
> bands (alternating), while still in A configuration. The duration of
> this test was 6 hours, and the goal is to find our 'ultimate' dynamic
> range -- and to uncover any remaining subtle effects produced by our
> system. Overall, the data quality is simply amazing. Closure levels
> (except for what I'm about to report on below) are effectively
> unmeasureable -- likely less than 0.01%.
> But in reviewing the data carefully, the 'slow wobble' problem found
> earlier is present throughout the entire new dataset. But since we have
> two frequencies in this long dataset, some new and hopefully useful
> information has been found.
> 1) The strong slow periodicity is seen in both K and Ku bands for each
> baseline on which the effect can be seen. The periodicity seems
> unrelated to the baseline coordinates. (This has yet to be proven). Some
> antennas are more likely to show wobbles than others. There is no
> spatial relationship between antenna pairs showing the effect. There is
> no apparent relationship with antenna number or f-shift. 2) For both,
> the wobbles are only seen in IF 'C'. One can make a case for them being
> very faintly visible in the others IFs, but the effect is at least an
> order of magnitude lower.
> 3) The wobbles are seen identically in amplitude and phase, with the
> same period. Viewed in the complex plane, the 'wobble vector' circles
> about the (stationary) visibility vector.
> 4) The effect is not visible on a secondary source that was observed.
> This source has 1/15 of the flux of 3C84 -- from this we conclude that
> the wobble effect is multiplicative, not additive, and is proportional
> to the correlated power -- not the total system power.
> 5) The effect is strongest by far in subband #2 (#1 in CASA-speak). It
> is also easily visible in subband #1, and faintly visible in subbands 5
> and 6. The wobbles are contiguous between subbands 1 and 2 -- which Ken
> says is evidence that the baseline boards are not to blame. (See the
> attached figures).
> 6) The period of the wobbles is shorter at K band than Ku band, roughly
> at the ratio of the sky frequencies. For the baseline 2 x 23 (see
> attached figures), the wobble period at K-band is 20 seconds. At
> Ku-band, the period is 30 seconds. The periodicity of the wobbles is
> different on all baselines, varying from a few seconds to a few tens of
> seconds.
> 7) The amplitude of the wobbles is much higher in K-band than in
> Ku-band. At K-band, the effect is about 5% of the source flux, and 3.5
> degrees in phase. (pk-pk). At Ku-band, the effect is about half of this.
> The latter two characteristics make us wonder what happens at Ka and Q
> bands. A short test this evening -- if there is time available -- might
> help here.
> I have attached four plots to show the effect -- amplitude and phase for
> Ku and K bands. All plots are of the baseline which shows the effect
> most strongly: 2 x 23.
> About the only thing I'm willing to conclude from this is that the
> effect is not caused by an external agent, such as RFI.
> Speculations as to origin, or suggestions for further testing, are welcome!
>
>
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