setting up the gain for intensity servo
1) TF of current actuator and PMC trans PD:
The TF goes down at 40 dB per decade(1/f2). It seems to be a good idea to have the UGF around 103 where the phase margin will be compensated a bit and the slope will go with 1/f.
- We want to build a servo that looks like a band pass. Our servos can be separated into three parts
TF (whole servo) = TF of current actuator + TF of SR560 + TF of our intensity servo box, or
H(s) = A(s) * B(s) * C(s), we can write H(s) in polynomial terms. TF of current actuator can be measured and fitted to determine its Laplace Transform.
TF of SR560 is a band pass. Then we can calculate the TF of our intensity servo and bulid it.
The fit and its parameters of current actuator is plotted below.
The poles and zeroes are
pole 1, simple: 14.7 kHz; 1/(s + f_pole *2pi)
pole 2, complex: 6.39 kHz, Q = 0.227; w2 / (s2 + w*s/Q + w2) ;w = 2 pi *f_pole
pole 3, complex: 7.25 kHz, Q = 0.642;
pole 4, complex: 89.14kHz ,Q = 1.143; (This one is less important, since the expected UGF is ~ 30 kHz)
zero: 390.4 Hz, Q=0.433, (I'm going to treat it as a simple zero) s - f_zero *2pi
The calculation details are in the matlab code, file bandpass.m.
The TF of the servo will be fitted by LISO.
2) CLG of the whole thing
3) PMC trans PD noise spectrum, (before and after the intensity servo)
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You have to post here the TF between the laser current actuator and the PMC trans PD. Then, you should think about how to make an effective servo using a single SR560 (i.e. what poles).
Then when you hook it up, measure the CLG of the whole thing and see if the PMC trans PD noise spectrum goes down as expected.
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