[evlatests] Noisy C-band antennas?

Tue Aug 29 19:10:38 EDT 2023

Hi Guys,

I am sending attached the source of the problem. A map of the microwave 
link that crosses VLA. I am also attaching my presentation at the last 
VLA/VLBA test/commissioning meeting, where I briefly updated those 
issues. The problem can be solved using a mix of 3-bit and 8-bit 
samplers instead of the usual 3-bit samplers.

As Lorant mentioned in today's meeting, on the next OPT release, we will 
provide the users with an alternative resource to be used at C-band 
during A and B configuration. We will also add an explanation about what 
is going on to the user's guide. I am also sending a draft of the text 
that will be added attached; it is still being revised by the team to 
reach its final version.

I hope that solves your major concerns. But of course, we can still work 
on new tests if a deeper explanation is needed.

Cheers

Pedro

On 8/29/23 16:26, Rick Perley via evlatests wrote:
> This problem must be highly time-variable.  I have eight wide-band, 
> 3-bit, C-band observations on hand, taken between 22 June and last 
> week -- and *none* of them show any problem in the calibration gains, 
> or any hint of any problem.
>
> We really need to find the source of this problem...
>
> Rick
>
> On 8/29/23 16:23, Bryan Butler wrote:
>>
>> 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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>>>>>
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