[evlatests] Noisy C-band antennas?

[Bryan Butler]

Middle of E and W arms.  See attached.  Drew tells me they see it "quite 
a lot" - not sure if it's always the same antennas.

	-Bryan


Rick Perley via evlatests wrote on 8/29/23 16:14:
> Where are these eight antennas located?  In previous tests, there seems 
> to be little spatial correlation for the affected antennas -- in Pedro's 
> note (below), W36 and N28 are far, far apart.  Hard to see how a single 
> source can affect both so badly.
> 
> Also note that I claimed on June 29 that the effect was completely missing.
> 
> So -- do we really understand what is going on?  Perhaps a more careful 
> review of data taken at C-band may be needed.
> 
> Rick
> 
> On 8/29/23 15:52, Bryan Butler via evlatests wrote:
>>
>> Yesterday, I got a pipeline reduction report from the DAs on a science 
>> observation at C-band, observed on July 27.
>>
>> The situation seems really grim - upon digging further (well, asking 
>> for more information from James), this effect is seen strongly on 8 
>> antennas (1, 4, 5, 9, 19, 25, 26, 27).  Roughly 1/3rd of them.
>>
>> Shouldn't we be telling our C-band observers to observe in 8-bit, 
>> instead of 3-bit, to avoid (or at least lessen the effect of) this? If 
>> it really is compression, it's unclear (to me at least) that this is 
>> recoverable at all, and certainly not without a much larger 
>> uncertainty on the flux density scale.  If I had known it was this 
>> bad, I certainly would have observed in 8-bit, and taken the sqrt(2) 
>> loss rather than the 1/3 (potential) loss.
>>
>>     -Bryan
>>
>>
>> Pedro P.B. Beaklini via evlatests wrote on 7/11/23 16:29:
>>> Rick, and all,
>>>
>>> I did a test last Thursday on this. The thing is that the noise 
>>> depends on the antenna location. The compression is due to an RFI 
>>> source in the direction of stations W36 and N28 and often can be seen 
>>> in N32, N36, and W32 stations. For some reason that we are still 
>>> investigating, ea17 is more affected than others when it is on the 
>>> RFI path. But the problem can be seen in the other antennas in the 
>>> RFI path.
>>>
>>> On the current configuration, there is no antenna on W36 and N28. 
>>> EA17 is far away now, at N64, and no compression was seen in my test 
>>> on ea17, in agreement with your data. However, on the antenna closest 
>>> to W36, ea05 at W40, I could see a C-band compression on 3-bit. 
>>> Please, can you take a look at ea05 on your data? I also saw a new 
>>> noise source leading to compression on the east arm ( peak on EA04 at 
>>> E48). That is new to me and needs further investigation, but the 
>>> effect on data is similar.
>>>
>>> If someone wants to take a look at pdif varying in time on my tests 
>>> and past data, I put some shorts movies at. Each frame is 5s, time on 
>>> top in minutes
>>>
>>> /lustre/aoc/sciops/pbeaklin/tsys/C-band-issue/2023-06-06/pdif/
>>>
>>> As a last comment, I was not the first one to notice the issue. The 
>>> DAs, SRDP team, and the support telescopes group have been aware of 
>>> that for a while. I only decided to take a deeper look at it :-)
>>>
>>> Cheers
>>> Pedro
>>>
>>> On 6/29/23 14:55, Rick Perley via evlatests wrote:
>>>> Some months ago, Pedro noted that certain antennas were especially 
>>>> noisy (low SNR) for some antennas and spectral windows, when using 
>>>> 3-bit samplers. From my own data, I confirmed this.
>>>>
>>>> The recent data referred to in my last circular allows a check on 
>>>> this situation -- the data were taken with 3-bit sampler.
>>>>
>>>> The 'noisy' effect is completely gone!  All antennas, in all 
>>>> spectral windows, have exactly the expected sensitivity and gains.
>>>>
>>>> Pedro plans to take some data, to confirm.
>>>>
>>>> Rick
>>>>
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