[evlatests] Fwd: Bandpass comparison

[Jim Jackson]

Another thing to keep in mind is I don’t think South Africa has a nationwide high power S-band satellite radio system to deal with.

Cheers,
Jim

From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Rick Perley via evlatests
Sent: Wednesday, June 8, 2022 9:22 AM
To: evlatests at aoc.nrao.edu
Subject: [evlatests] Fwd: Bandpass comparison


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><mailto:rselina at nrao.edu>

To:

Urvashi R.V. <rurvashi at aoc.nrao.edu><mailto:rurvashi at aoc.nrao.edu>, Rick Perley <rperley at nrao.edu><mailto: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><mailto:rurvashi at aoc.nrao.edu> Sent: Tuesday, June 7, 2022 6:31 PM
To: Rick Perley <rperley at nrao.edu><mailto:rperley at nrao.edu>; Rob Selina <rselina at nrao.edu><mailto: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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