I installed LISO, a circuit simulation software, into the control room linux machines.
I also installed a PCB CAD called Eagle to serve as a graphical editor for LISO.
I put a brief explanation in the wiki.
http://lhocds.ligo-wa.caltech.edu:8000/40m/LISO
As a demonstration, I made a model of the FSS PC path and did a stability analysis of the op-amps.
The first attachment is the schematic of the model.
You can find the model in /cvs/cds/caltech/apps/linux/eagle/projects/liso-examples/FSS
The second attachment shows the stability analysis plot of the first two op-amps when AD829s are used.
The op-amp model is for the uncompensated AD829. The graph includes the bode plots of the open-loop transfer function of each op-amp.
If the phase delay is more than 360deg (in the plot it is 0 deg because the phase is wrapped within +/-180 deg) at the unity gain frequency,
the op-amp is unstable.
It is clear from the plot that this circuit is unstable. This is consistent with what I experienced when I replaced the chips to AD829 without
compensation.
Unfortunately, I don't have an op-amp model for phase compensated AD829. So I can't make a plot with compensation caps.
The third attachment is the stability analysis of the same circuit with AD797. It also shows that the circuit is unstable at 200MHz, though I
observed oscillation at 50MHz.
Finally, I did an estimate of frequency noise contribution from the noise of AD829.
First I estimated the voltage noise at the output of the board caused by the first AD829 using LISO's noise command.
Then I converted it into the input equivalent noise at the stage right after the mixer by calculating the transfer function
of the circuit using LISO.
Within the control bandwidth of the FSS, this input equivalent noise appears at the mixer output with the opposite sign.
Since we know the calibration factor from the mixer output voltage to the frequency noise, we can convert this into the frequency noise.
The final attachment is the estimated contribution of the AD829 to the frequency noise. As expected, it is negligible. |