[evlatests] P-band switched power observed at S-band
Walter Brisken
wbrisken at nrao.edu
Fri Jun 17 18:51:35 EDT 2016
Not actually sure what VLITE uses for calibration. My read of Dan's email
is that the _receiver_ would be turned off. If only the cal signal
disappeared that might be different. I seem to remember the strong
in-band interference by the MUOS satellite has made use of swichted power
for calibration much more difficult than expected.
If the P-band cal signal is changed and VLITE wants to use it, then some
software and firmware changes in VLITE are implied...
-W
On Fri, 17 Jun 2016, Barry Clark wrote:
> Do the VLITE pipelines really rely on the pdif equivalent?
> The least destructive form is to shift by 90 degrees. Then you
> can just double the pdif and take the loss of SNR on the pdif.
>
> On 06/17/2016 04:36 PM, Walter Brisken wrote:
>>
>> The VLITE folks would not particularly like that, especially ramping up
>> to VLASS...
>>
>> -W
>>
>> On Fri, 17 Jun 2016, Dan Mertely wrote:
>>
>> > Not being a cooled FE, the P-band receiver
>> > would be easy to turn off, on request. -Mert
>> >
>> > On 6/17/2016 3:48 PM, Barry Clark wrote:
>> > > A nice piece of work. We should do something about the problem.
>> > >
>> > > I still worry that as well as the cal, some receiver noise from
>> > > the P band might be leaking in. It can't be much, as Fraser says
>> > > it didn't show up on stress test gains, but it might be more
>> > > sensitive to use this approach to see if S band psum changes
>> > > when the P band receiver is turned off.
>> > >
>> > > The simplest fix for most purposes is to run the S-Band Pcal
>> > > at half the frequency as the other cals (or twice the frequency,
>> > > or displaced in phase by 90 degrees). Paul will hate this
>> > > suggestion - it does nothing to help the pulsar problem.
>> > >
>> > > On 06/17/2016 02:42 PM, Paul Demorest wrote:
>> > > > hi everyone,
>> > > > > While looking into various switched power issues recently, I
>> > > noticed
>> > > > that the state of the low-band (4/P) cal switching causes a
>> > > significant
>> > > > change in switched power (aka Pdif) measurements done at S-band.
>> > > This
>> > > > is not a subtle effect; for several antennas the S-band Pdif
>> > > changes by
>> > > > a factor of ~1.5 to 2 (!) when the low-band cals are switching.
>> > > > > Note this is _not_ the same effect as the gain modulations that
>> > > lead to
>> > > > apparent "Pdif compression" as we have also been discussing
>> > > recently. As
>> > > > far as I can tell, the low-band cal switching has no detectable
>> > > effect
>> > > > on the amplifier gains at S-band. Rather, the effect observed
>> > > here is
>> > > > that when the low-band cals are switching, there is an extra
>> > > amount of
>> > > > switched power added to the S-band Pdif.
>> > > > > This is easy to test by separately controlling the state of the
>> > > P- and
>> > > > S-band cal switching and plotting the resulting Pdif values vs
>> > > time, as
>> > > > produced by the correlator and recorded in the SDM switched power
>> > > table.
>> > > > The attached three plots show the results of this for three
>> > > different
>> > > > antennas. In these plots, the red labels and dashed lines show
>> > > > which
>> > > > cals were enabled at different times during the test. All cals
>> > > > other
>> > > > than P and S were disabled the entire time. Note that at each scan
>> > > > boundary (black dashed lines) the system temporarily reverts to the
>> > > > default state (both S+P cals on) until I send a command to change
>> > > > it.
>> > > > > The different antennas show a wide range of behavior: ea01
>> > > looks great
>> > > > - a consistent S-band Pdif is seen independent of P-band, and the
>> > > Pdif
>> > > > level goes to zero when the cals are turned off. In contrast, for
>> > > ea03
>> > > > the amount of switched power coming from P-band seems comparable
>> > > to that
>> > > > coming from the S-band cal itself! (ea03 was the worst case in this
>> > > > test.) The third one shown here, ea05, is somewhere in between
>> > > with a
>> > > > ~ 10% effect.
>> > > > > This is summarized for all antennas in the table below, which
>> > > shows the
>> > > > ratio of the P-band-only to S-band-only Pdif values, sorted by the
>> > > IF-A
>> > > > value. The starred antennas are those with 4-band MJPs installed
>> > > > (at
>> > > > least according to the war room white board); there does not seem
>> > > to be
>> > > > much correlation between this and the Pdif values.
>> > > > > I have not yet looked at this effect on receivers besides S, or
>> > > checked
>> > > > carefully for frequency dependence within S-band (this test used a
>> > > > single 128 MHz subband centered at 3.0 GHz); I may look into this
>> > > some
>> > > > more in the near future. Other questions, comments, suggestions
>> > > > are
>> > > > welcome.
>> > > > > Cheers,
>> > > > Paul
>> > > > > ----
>> > > > > P/S Pdif ratios
>> > > > > Ant IF-A IF-B IF-C IF-D
>> > > > ea03* 0.758 0.763 1.355 1.350
>> > > > ea07 0.503 0.492 0.512 0.506
>> > > > ea09* 0.416 0.422 0.292 0.286
>> > > > ea15 0.400 0.400 0.282 0.295
>> > > > ea25 0.247 0.299 0.015 -0.014
>> > > > ea21 0.244 0.248 0.457 0.498
>> > > > ea26 0.151 0.158 0.125 0.116
>> > > > ea12* 0.136 0.133 0.494 0.474
>> > > > ea05* 0.119 0.122 0.077 0.074
>> > > > ea14* 0.112 0.116 0.147 0.146
>> > > > ea28 0.097 0.102 0.065 0.065
>> > > > ea23* 0.086 0.087 0.167 0.179
>> > > > ea04 0.074 0.077 0.061 0.057
>> > > > ea16 0.072 0.072 0.073 0.070
>> > > > ea13* 0.065 0.057 0.029 0.030
>> > > > ea24 0.061 0.060 0.060 0.060
>> > > > ea27* 0.054 0.054 0.057 0.059
>> > > > ea17 0.050 0.048 0.069 0.050
>> > > > ea06* 0.044 0.044 0.049 0.049
>> > > > ea22 0.039 0.036 0.054 0.063
>> > > > ea20 0.038 0.037 0.048 0.051
>> > > > ea19* 0.023 0.024 0.008 0.008
>> > > > ea18* 0.002 0.001 0.006 0.006
>> > > > ea01* -0.000 -0.000 -0.000 -0.000
>> > > > ea10* -0.001 -0.000 0.004 0.004
>> > > > ea11* -0.008 -0.007 -0.021 -0.021
>> > > > >
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