[evlatests] P-band switched power observed at S-band
Paul Demorest
pdemores at nrao.edu
Fri Jun 17 18:50:27 EDT 2016
I think the suggestion is to power down P-band during a test to quantify
the effect on S-band Psum, not to do this routinely during observations
(right?). If it is causing a big Tsys hit, I guess we'd want to think
about ways to modify the HW to fix this. But first need to know how big
a problem it actually is.
Can the receiver power be controlled easily from software? Or does it
need manual button-pushing?
-Paul
On 2016-06-17 16:36, 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
>>> > >
>>>> _______________________________________________
>>> > evlatests mailing list
>>> > evlatests at listmgr.nrao.edu
>>> > https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>> > _______________________________________________
>>> evlatests mailing list
>>> evlatests at listmgr.nrao.edu
>>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.nrao.edu
>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>
>>
>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/evlatests
More information about the evlatests
mailing list