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Wed Nov 14 12:47:19 2007, tobin, Update, PSL, ISS
Wed Nov 14 17:50:00 2007, tobin, Update, PSL, ISS
Wed Nov 14 12:47:19 2007
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With John's notch filter installed and the increased light on the ISS sensing diode, we were able to get a UGF of about 60 kHz with the gain slider set to about 20 dB. This morning we met with Stefan to learn his ISS-fu.
His recommendations for the ISS include:
Replace the cables from the board to the front panel connectors if this hasn't already been done.
Replace the input opamps with 4131's. Be sure to test both positive and negative input signals.
Check that all the compensation capacitors are in place and are 68 pF
Make sure all the feedback loops have high frequency rolloff
The ISS board reads the PDs differentially; make sure the PD sends differentially.
Add a big (ie 10uF tantalum) capacitor to the PD to suppress power supply noise
Add bigger power supply bypass caps to the ISS
I just took sensing noise spectra (from the PD DC bnc ports) and then took the photodiodes off the table to check that they have the negative end of the differential line connected to ground. (I placed black metal beam blocks on the table in place of the ISS PD's. Also, from the ISS schematic, it looks like it sends a differential output to the PD DC bnc ports, but we have been plugging them directly into the SR785 (grounding the shield). We should make a little BNC-doodle that separates the signal+shield to go into the A and B inputs on the spectrum analyzer.) Opening up one of the photodiodes, it appears that the negative line of the differential output is
connected. Will continue later this afternoon.