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Filtering the P-band cal signal is possible, but is a big hassle
because all the receivers would have to be modified and retested in
the lab. My preference would be to turn off the P-band cals when not
observing with this band, unless the VLITE folks really have to have
it on.<br>
<br>
-Wes<br>
<br>
<div class="moz-cite-prefix">On 8/12/2016 3:06 PM, Paul Demorest
wrote:<br>
</div>
<blockquote cite="mid:f9f40fe0c06ce94d45894f386bb89883@nrao.edu"
type="cite">hi all,
<br>
<br>
A while back I sent some results from a test that compared S-band
switched power measurements done with the P-band cals switching vs
off (see evlatests email on 2016/06/17). That previous test was
done using a single 128-MHz-wide spectral window centered near 3
GHz. I've since repeated the same test using the full receiver BW
at both L and S bands.
<br>
<br>
Results for a few antennas are attached. The top panel of the
plots shows the ratio of the P-band cal signal to the L or S cal,
measured in two different ways: 1) the usual switched power table
values; 2) WIDAR autocorrelations, which provide better frequency
resolution. These two methods seem to give consistent results.
The bottom panel shows the raw "on-off" autocorrelations with
either the P-band cals on (red) or with the in-band (L,S) cals on
(blue).
<br>
<br>
The main takeaway from this is that this P-band cal leakage has
lots of structure on ~10-100 MHz frequency scales. My previous
ordering of which antennas were "good" or "bad" mostly reflects
whether or not they happen to have a big spike near 3 GHz. For
example ea03 (the worst one last time) would have looked great if
I'd done the original test near 2.5 GHz instead. When viewed
across the full 1--4 GHz, all the antennas have similar features
somewhere in the band.
<br>
<br>
This may not be a big problem for current observing practices,
where flux calibrators are observed frequently and the switched
power is mainly used as a kind of intermediate scaling. I think
this effect would be an issue if we were to move towards a scheme
where the switched power is used to set flux scales. Also while
the average power level increase across >100 MHz bands is not
too big, over small freq ranges this may be large enough to
noticeably affect Tsys.
<br>
<br>
In general this seems like a messy thing to be happening and I
think we should either filter the P-band cal signal appropriately,
or else just turn off the P-band cals during non-P projects. I
realize VLITE is always observing P-band, but it's not clear to me
whether or not they actually make use of the switched power
signal; would be good to get a definitive answer to that.
<br>
<br>
Cheers,
<br>
Paul
<br>
<br>
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