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

[rperley at nrao.edu]

A very interesting suggestion, which clearly would result in a beating
fringe of about the right period, and scale with frequency as observed...

I haven't checked the u-dependence yet, but if memory serves me right, I
had established that there was this relationship in the original dataset
taken a few weeks back.  I'll check this tomorrow with the current data.

It may not be seen to 'close' if -- as is likely the case -- the spur is
present on only a few antennas.  As an example, suppose only two antennas
have this spur.  Then only the single baseline between them will show the
beat.  No way to solve for that on an antenna basis.  As it turns out,
only a few antennas have detectable beats, so perhaps the failure to close
is a SNR effect.  Incidentally, the baseline shown in the plots (2 X 23)
is an adjacent pair of antennas on the North arm -- u is quite small for
this baseline.  Of all the baselines with a visible 'beat', this one had
the longest period.

I don't see how this mechanism could cause the beating fringe to be so
strong on only two of the subbands, and virtually invisible on the others
...  And why is it only on the LCP polarization?  Shouldn't it be in both,
if due to an LO spur?


> 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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