[evlatests] Fwd: Bandpass comparison

[Rick Perley]

Thanks to all who responded to my original posting.

I forward Rob Selina's response, as it neatly summarizes what I heard 
from others.   I also pinged the MeerKAT people about any 
'pre-calibration' -- there is none, except for parallel-hand and 
cross-hand delays.

The 'bottom line' is evidently in the simplicity of the electronics, 
with no long cables connecting disparate units, and no up/down 
conversions involving separate modules.   Our comlicated-looking 
bandpass spectra seem to be a result of the many different reflections 
bounding around in the electronics.

'Keep it simple' seems to be the appropriate mantra!

Rick



-------- Forwarded Message --------
Subject: 	RE: Bandpass comparison
Date: 	Wed, 8 Jun 2022 13:43:49 +0000
From: 	Rob Selina <rselina at nrao.edu>
To: 	Urvashi R.V. <rurvashi at aoc.nrao.edu>, Rick Perley <rperley at nrao.edu>



I did look at some of the MeerKAT block diagrams and confirmed that 
their S-band is direct sampled, so the analog chain is FAR simpler than 
EVLAs. (no up conversion, no multi-stage downconversion, no switches, etc)

For a bit of context - the LNAs themselves can be quite flat. Low Noise 
Factory input on our ngVLA specs suggest gain slope of less than 1dB/GHz 
at these frequencies.
MeerKAT's data still looks impressively good even given that, but the 
far simpler direct sampling architecture may make flatness on this scale 
plausible... I'm still a little suspicious of it though since I would 
have expected a bit more structure from the anti-alias filters, if 
nothing else.
Cheers
Rob


-----Original Message-----
From: Urvashi R.V. <rurvashi at aoc.nrao.edu> Sent: Tuesday, June 7, 2022 
6:31 PM
To: Rick Perley <rperley at nrao.edu>; Rob Selina <rselina at nrao.edu>
Subject: Re: Bandpass comparison

 > It would be good to know why theirs are so good, and ours so ... 
crappy looking ...

Online telcal solutions being applied vs not being applied ?  Do you 
know of MeerKAT applies them, or whether this is the natural and 
completely uncalibrated bandpass ?

Regards,

- Urvashi


On 6/7/22 3:12 PM, Rick Perley wrote:
> MeerKAT vs the EVLA.
>
> The two plots are on the same scales -- dB for the amplitudes, linear 
> for the phases.  RCP  or 'X' in the upper half, LCP/Y in the lower.
>
> The small-scale fluctuations in the VLA spectra (barely visible in 
> these plots) is due to the reflection between the OMT/horn and the 
> subreflector.  I don't know the origin of the rather large spectral 
> power changes in the VLA spectra...
>
> It would be good to know why theirs are so good, and ours so ... 
> crappy looking ...
>
> Rick
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