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Entry  Wed Jan 30 15:07:32 2019, awade, anchal, HowTo, FSS, Shot noise in FSS loops? 
    Reply  Fri Feb 1 14:59:01 2019, awade, anchal, HowTo, FSS, Noise tallies CTN_FSS_south_blockdiagram.pdf
       Reply  Fri Feb 1 20:20:11 2019, awade, HowTo, FSS, Modifying North FSS EOM path 
          Reply  Tue Feb 5 11:40:02 2019, awade, HowTo, FSS, Modifying South FSS boost 
             Reply  Tue Feb 5 20:39:52 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis FSS_Modified_Analysis.pdfFSS_Mod_Analysis.zip
                Reply  Wed Feb 6 19:13:06 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis FSS_Modified_Analysis.pdf
                   Reply  Wed Feb 6 21:30:24 2019, awade, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis EOM_Vrms_FunctionOfLowerFrequencyBoundOfTF.pdfTTFSS_lisomodel_FSS_Mod_Analysis.ipynb.zip
                      Reply  Thu Feb 7 10:33:27 2019, anchal, Summary, FSS, Complete model of TTFSS box with TF and crossover analysis 
Message ID: 2303     Entry time: Tue Feb 5 11:40:02 2019     In reply to: 2302     Reply to this: 2304
Author: awade 
Type: HowTo 
Category: FSS 
Subject: Modifying South FSS boost 

Shortening the capacitor leads on U7 boost

I checked the boost stage on U7 of LIGO-D040105-C on the south path box (37 MHz).  It appears that Craig had changed its value to 6.8 nF to match the North path. 

The resistor was actually 5.088 kΩ, instead of 5.6 kΩ as listed. The resistor was changed from the radial thick film (5.088 kΩ) to a thin film surface mount type (5.6 kΩ). I moved the capacitor from the switch (at the end of the wires) to immediately at the board. The switch on the side of the box now only has two wires to short the capacitor when we want to disable boost. This should mean that there is an integrator with a pole at DC and a zero at 4.2 kHz and won't have any funny feedback impedance due to long wires and switch. This now matches the North path.

Adding gain to RF PD

Anchal is going to figure out how to increase gain of south FSS RFPD from ~1.5 kΩ to 2.5 kΩ of the north path.  In the meantime I've added a mini circuits low noise amplifier to the output of the RFPD. This has a noise figure of about 3 dB and a gain of +24 dB.  I put a 20 dB attenuator after the amplifier.  I think the extra gain stage will only add a few mHz/rtHz to the input referred noise, so should be fine for now.  Once Anchal has solved the crossover TF optimization he'll have time to fix this.

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