[evlatests] Noisy C-band antennas?

[Rick Perley]

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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>>
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