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<p>Thanks to all who responded to my original posting. <br>
</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. <br>
</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. <br>
</p>
<p>'Keep it simple' seems to be the appropriate mantra! <br>
</p>
<p>Rick<br>
</p>
<div class="moz-forward-container"><br>
<br>
-------- Forwarded Message --------
<table class="moz-email-headers-table" cellspacing="0"
cellpadding="0" border="0">
<tbody>
<tr>
<th valign="BASELINE" nowrap="nowrap" align="RIGHT">Subject:
</th>
<td>RE: Bandpass comparison</td>
</tr>
<tr>
<th valign="BASELINE" nowrap="nowrap" align="RIGHT">Date: </th>
<td>Wed, 8 Jun 2022 13:43:49 +0000</td>
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<th valign="BASELINE" nowrap="nowrap" align="RIGHT">From: </th>
<td>Rob Selina <a class="moz-txt-link-rfc2396E" href="mailto:rselina@nrao.edu"><rselina@nrao.edu></a></td>
</tr>
<tr>
<th valign="BASELINE" nowrap="nowrap" align="RIGHT">To: </th>
<td>Urvashi R.V. <a class="moz-txt-link-rfc2396E" href="mailto:rurvashi@aoc.nrao.edu"><rurvashi@aoc.nrao.edu></a>, Rick Perley
<a class="moz-txt-link-rfc2396E" href="mailto:rperley@nrao.edu"><rperley@nrao.edu></a></td>
</tr>
</tbody>
</table>
<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 class="moz-txt-link-rfc2396E" href="mailto:rurvashi@aoc.nrao.edu"><rurvashi@aoc.nrao.edu></a> Sent: Tuesday,
June 7, 2022 6:31 PM<br>
To: Rick Perley <a class="moz-txt-link-rfc2396E" href="mailto:rperley@nrao.edu"><rperley@nrao.edu></a>; Rob Selina
<a class="moz-txt-link-rfc2396E" 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>
<blockquote type="cite">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<br>
</blockquote>
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