[evlatests] Noisy C-band antennas?

Bryan Butler bbutler at nrao.edu
Tue Aug 29 19:42:02 EDT 2023 

I must have missed your presentation - apologies.

This map is great.  So the culprit for my observations is likely 
Mangus-Gray Hill, which crosses over about the right parts of the E and 
W arms.  For the ones that affect the N arm it's Davenport-Gray Hill or 
NM3SMIT...-Gray Hill.

The fundamental problem is Gray Hill, it seems.  Do we have any recourse 
here?  Has it always been there and they're just using it more now or 
something?

And I'd suggest getting these alternatives out to users sooner than the 
next OPT release.  Have the DAs tell folks to use them now.  This is 
almost a show-stopper when it hits.

	-Bryan


Pedro P.B. Beaklini wrote on 8/29/23 17:10:
> 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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