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Entry  Mon Jun 11 15:03:15 2018, anchal, DailyProgress, ISS, Loop Gain Transfer Function analysis on LTspice ISSLoopGainAnalysis.pdfISSLoopGainTFwithAOMDCG1.11.pdf
    Reply  Wed Jun 13 18:04:32 2018, anchal, DailyProgress, ISS, Loop Gain Transfer Function analysis on LTspice ISSMeasuredTF06-13-2018.pdfISSLTSpiceTF.pdf
Message ID: 2202     Entry time: Wed Jun 13 18:04:32 2018     In reply to: 2201
Author: anchal 
Type: DailyProgress 
Category: ISS 
Subject: Loop Gain Transfer Function analysis on LTspice 

I implemented the feedback part of this circuit and measured the transfer function on SR785. Attached is the measured transfer function. It is close to the one given by LTspice except that there is a nearly flat top between ~100Hz to ~1.3kHz while analytically this was supposed to be a smooth peak.
I checked with attenuating the input signal to make sure the flat top is not due to saturation, it is in fact two peaks close together. By changing a capacitor C10 (see the last post for the circuit)., we can move these peaks closer or further. However, making them move closer decreases the overall gain of the point, so I stuck to 4.7uF value.
I'll now test this circuit with a low pass filter modeling the cavity and some delay for modeling the AOM and see if the loop gain is as expected.


With the circuit of Johannes, I ran LT spice analysis with modeling cavity pole and AOM delay to get an estimate of how the loop gain would look like. Attached is a plot of the transfer function of loop gain and the circuit schematic used. Here I used this post of Johannes on elog to use DC Gain of AOM as 1.11 dB. But in case, this changes, I have a running code which will output the new unit gain frequencies and phase margins. I'll implement this into a board soon and move forward by optimizing the choice of elements with help from LISO.
I'm seeking input on anything which makes the frequency curve as shown here less than satisfactory.


Attachment 1: ISSMeasuredTF06-13-2018.pdf  24 kB  Uploaded Thu Jun 14 17:01:37 2018  | Hide | Hide all
Attachment 2: ISSLTSpiceTF.pdf  25 kB  Uploaded Thu Jun 14 17:01:53 2018  | Hide | Hide all
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