[evlatests] P-band switched power observed at S-band

[Dan Mertely]

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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>
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