Matt, Jenne

On Thursday (Apr 28) at about 6:20pm local, while Rick and Keita were in the PSL, Jenne and I made some measurements of the ISS first loop PDs.  Rick told us that there was no light on the ISS PDs at the time of our measurement, so we were hoping to get a dark spectrum to compare with the ones that Sheila and I took on the 24th and to add to the ones that Peter took.  I took a bunch of photos of the results, but later wasn't sure of the state of affairs for each photo, so I went out again today to get a better organized set of measurements.  At present, there is no light on the ISS PDs, so this is a DARK noise measurement.  The aim here is to get high-frequency (10kHz - 10MHz) noise measurements which are complementary to the ones taken by Even and Stefan on Friday using the digital system.

I started by cabling-up a readout system which included a fast o'scope, an SR785 for sub-100kHz spectra, and an RF analyzer for MHz spectra.  To avoid loading the OP27 outputs with 50 Ohms, I made a Pomona box with a 4.7kOhm resistor and 33nF cap in series for the RF path.  When loaded with 50 Ohms (by the RF analyzer) this gives -40dB of attenuation and a 1kHz high-pass.  (The 100 Ohm output impedance of the ISS board or ISS PD doesn't cause problems.)

The first 2 pictures show the output of the PDA monitor from the ISS board.  The same 36.6kHz rep-rate noise bursts that Sheila and I saw are present, though they don't look clipped the way they did when the system was running.  In the RF spectrum, these bursts appear as a comb of lines spread from ~100kHz to ~1.5MHz with a 36.56kHz spacing.  There was nothing interesting above 2MHz.

Photos 3-5 show the result of disconnecting the input.  The noise bursts go away, but since most of the noise is above 100kHz, the SR785 spectrum looks pretty similar (just the peak at 36.6kHz is gone).  The result was similar when I terminated the PDA input on the ISS board with 50 Ohms.  This, and what follows, appears to exonerate the ISS servo board.

Photos 6-8 show the result of placing a 100Hz low-pass in the line coming from the PSL to the PDA input.  The idea here is to attenuate the MHz content on the signal path by > 60dB so that we can see what is on the ground.  The pomona box I used was something that Stefan made for his coil driver measurements: 1.6kOhm and 1uF.  This may not be the optimal way of doing this, but if the RF were all on the signal side it would go away... which it did not.  The RF is much smaller (factor of ~5), but still clearly visible in the time series and the RF spectrum.

Finally, photos 9 and 10 show the output of PDA directly from the PSL.  While qualitatively similar to the monitor output shown in photos 1 and 2, it has more RF junk.  The sub-100kHz spectrum also has a lot more content in this configuration, and while watching it I noticed that there were features moving around with time.  (I made a video, but I can't post it to the log.  Ask me if you want it.)  This smells like RFI to me.