[evlatests] P-band switched power leakage into L/S (wide-band version)

[Bryan Butler]

i thought i sent around a note from tracy previously - they (VLITE) are 
not currently using switched power.  they have been trying to work it 
into their pipeline scripts, so in principle would like it to be on if 
possible.  but they understand that if it's affecting VLA data then we 
may need to turn it off.

	-bryan


Paul Demorest wrote on 8/12/16 15:06 :
> hi all,
>
> 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.
>
> 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).
>
> 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.
>
> 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.
>
> 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.
>
> Cheers,
> Paul
>
>
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