[evlatests] P-band switched power observed at S-band
Barry Clark
bclark at nrao.edu
Fri Jun 17 18:43:01 EDT 2016
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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