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

[Paul Demorest]

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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