[evlatests] Noisy C-band antennas?

[Bryan Butler]

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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