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<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Cheers,<o:p></o:p></p>
<p class="MsoNormal">Jim<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> evlatests <evlatests-bounces@listmgr.nrao.edu> <b>
On Behalf Of </b>Rick Perley via evlatests<br>
<b>Sent:</b> Wednesday, June 8, 2022 9:22 AM<br>
<b>To:</b> evlatests@aoc.nrao.edu<br>
<b>Subject:</b> [evlatests] Fwd: Bandpass comparison<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p>Thanks to all who responded to my original posting. <o:p></o:p></p>
<p>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.
<o:p></o:p></p>
<p>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. <o:p></o:p></p>
<p>'Keep it simple' seems to be the appropriate mantra! <o:p></o:p></p>
<p>Rick<o:p></o:p></p>
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<br>
-------- Forwarded Message -------- <o:p></o:p></p>
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<p class="MsoNormal" align="right" style="text-align:right"><b>Subject: <o:p></o:p></b></p>
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<p class="MsoNormal">RE: Bandpass comparison<o:p></o:p></p>
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<p class="MsoNormal" align="right" style="text-align:right"><b>Date: <o:p></o:p></b></p>
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<p class="MsoNormal">Wed, 8 Jun 2022 13:43:49 +0000<o:p></o:p></p>
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<p class="MsoNormal" align="right" style="text-align:right"><b>From: <o:p></o:p></b></p>
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<p class="MsoNormal">Rob Selina <a href="mailto:rselina@nrao.edu"><rselina@nrao.edu></a><o:p></o:p></p>
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<p class="MsoNormal" align="right" style="text-align:right"><b>To: <o:p></o:p></b></p>
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<p class="MsoNormal">Urvashi R.V. <a href="mailto:rurvashi@aoc.nrao.edu"><rurvashi@aoc.nrao.edu></a>, Rick Perley
<a href="mailto:rperley@nrao.edu"><rperley@nrao.edu></a><o:p></o:p></p>
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<p class="MsoNormal"><br>
<br>
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)<br>
<br>
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.
<br>
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. <br>
Cheers<br>
Rob<br>
<br>
<br>
-----Original Message-----<br>
From: Urvashi R.V. <a href="mailto:rurvashi@aoc.nrao.edu"><rurvashi@aoc.nrao.edu></a> Sent: Tuesday, June 7, 2022 6:31 PM<br>
To: Rick Perley <a href="mailto:rperley@nrao.edu"><rperley@nrao.edu></a>; Rob Selina
<a href="mailto:rselina@nrao.edu"><rselina@nrao.edu></a><br>
Subject: Re: Bandpass comparison<br>
<br>
> It would be good to know why theirs are so good, and ours so ... crappy looking ...<br>
<br>
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 ?<br>
<br>
Regards,<br>
<br>
- Urvashi<br>
<br>
<br>
On 6/7/22 3:12 PM, Rick Perley wrote:<br>
<br>
<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal">MeerKAT vs the EVLA.<br>
<br>
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.<br>
<br>
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...<br>
<br>
It would be good to know why theirs are so good, and ours so ... crappy looking ...<br>
<br>
Rick<o:p></o:p></p>
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