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Tue Dec 4 23:16:35 2007, tobin, Update, PSL, ISS
Wed Dec 5 17:49:57 2007, tobin, Update, PSL, ISS
Wed Dec 5 18:22:03 2007, tobin, Update, PSL, ISS dark noise
Wed Dec 5 20:32:51 2007, tobin, Update, PSL, ISS dark noise
Thu Dec 6 19:30:43 2007, tobin, Update, PSL, ISS dark noise - 60 Hz!
Fri Dec 7 19:14:30 2007, tobin, Update, PSL, ISS dark noise - ground loop enlightenment
Tue Dec 4 23:16:35 2007
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I was confused to find that I could increase the ISS gain slider all the way from 15dB to 30dB without seeing much of any increase in gain in the measured open-loop transfer function. While making these swept-sine measurements, the saturation indicator almost never tripped, indicating it was seemingly happy. But then I noticed an odd thing: if I disable the test ("analog excitation") input, the saturation indicator trips immediately. I hooked up a scope to the current shunt test point (TP12). With the test input enabled, the loop closed, and the analog excitation port connected to the SR785, I see a a 5 Vpkpk, 2.55 MHz triangle wave there. It is there even if I set the SR785 excitation amplitude to zero, but it disappears if I disconnect the cable from the SR785.
I found oscillations at TP20, TP30, TP36, TP41, and TP42. Many of these are in the (unused) "outer loop" circuitry and currently lack compensation capacitors.