[evlatests] P-band switched power observed at S-band
Dan Mertely
dmertely at nrao.edu
Fri Jun 17 18:52:54 EDT 2016
That was the way I understood Barry's suggested test.
-Mert
On 6/17/2016 4:50 PM, Paul Demorest wrote:
> 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
>>>> > >
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