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

[Brent Carlson]

Rick,

On 17/01/2013 12:06 PM, Rick Perley wrote:
>    Brent:
>
>    The amplification of the effect at the bottom edge of subband #1 is 
> likely from the bandpass correction.  The  attenuation is caused by an 
> anti-aliasing filter within the T304.  This suggests that the effect 
> is added in after this filter.  But then this would seem to contradict 
> the indication that the effect is proportional to the correlated 
> power, which (I think) would cause the wobble amplitudes to decrease 
> after that filter.  On the other hand, if the effect is related to 
> some complicated aliasing from some non-linear process, then the 
> filter effect would be avoided.
>    We do see, very faintly, wobbles at other IFs.  However, in nearly 
> all cases, these are only seen in the two or three channels on each 
> side of the subband filter 'notches'.
This would indicate that it is a correlator effect since, presumably, 
the sub-band band-pass correction would amplify the effect in a similar 
manner as you indicated for the T304.  Curious, though, why it doesn't 
show up or is much weaker in some sub-bands and is weaker in ~1/2 of 
sub-band 2...

Setting up non-beating corr chip integration times would be the first 
test I'd run...

--Brent

>    I hope these clues help ...
>
>    Rick
>
> Brent Carlson wrote:
>> Hi Rick,
>>
>> It is strange that even though it shows up in sub-band 2, it is not 
>> uniform across all channels in that sub-band.
>>
>> A possibility is that in A config, with a few Hz of earth-rotation 
>> phase rate, that the corr chip dump/blanking time is beating with the 
>> net phase rate in the correlator...an effect that is known to cause 
>> issues.  I don't believe that corr chip integration times are set to 
>> mitigate this effect as pointed out in NRC-EVLA Memo #032.  I also 
>> don't know if 3C84 for these observations is located such that very 
>> low delay rate effects, if not taken out by post-correlation 
>> "discrete-step delay correction", can become evident as amplitude and 
>> phase oscillations--although you'd think this would show up the same 
>> in all sub-bands and be increasingly worse moving away from the 
>> center of a sub-band, and normally an acceleration signature is seen 
>> as well.
>>
>> I also note that it seems worst at the lowest edge of sub-band 1, 
>> which turns out to be the highest I/F frequency region feeding into 
>> the sampler (i.e. since it is Nyquist zone 2, with resulting 
>> frequency sense flipping, unless the plots are already corrected for 
>> this), and is normally caused by sampler clock cross-talk into the 
>> sampler.  Don't know what this means exactly, if anything.
>>
>> --Brent
>>
>> On 17/01/2013 10:57 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! 
>>

-- 
Brent R. Carlson
Brent.Carlson at nrc-cnrc.gc.ca
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