peter.king@LIGO.ORG - posted 11:37, Wednesday 15 April 2015 - last comment - 15:16, Thursday 16 April 2015(17885)
TTFSS work
Continuing on from yesterday's TTFSS work. All notches were removed, and the transfer function was measured (see NONOTCHS.tif). The peak at ~760 kHz is of no big concern. However the peaks at around 1.77 MHz are. These were notched out with C50 (1-65 pF) and series with L2 (220uH). C50 was adjusted to suppress the two peaks. The transfer function was re-measured (see OLTF.tif). The unity gain frequency is around 450 kHz. Currently the common gain is set to 27 dB and the fast gain to 5 dB. Should the loop break out into oscillation, reduce the fast gain to 0 dB, wait until the oscillation stops and then increase it back to 5 dB. Rick, Peter
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I looked at the FITS estimate for the optocoupler. The demonstrated FITs for the part is < 5555. Which if I understand things equates to a mean time to failure (MTTF) of just over 20 years. In a nutshell, I do not know why the part failed. Let alone two of them.
Because this looks to be the "final" configuration for a while, I again post an annotated version of the open loop gain transfer function for the PSL's (table top) FSS. In summary, - The unity gain frequency is 449 [kHz], with a phase margin of 58 [deg]. - The feature at 1.8 [MHz] (believed to be a resonance in the EOM) has been notched out, but with a pretty skinny / high-Q notch. - This [creates a new / pushes up the] feature a little higher in frequency, but the gain margin appears to be OK for now. - The 770 [kHz] feature is one of those "lucky" resonances where the zero is before the pole in frequency, so we win phase instead of losing it. So, there's no need to compensate for it, there's plenty of phase margin, and we aren't going to do anything about it. - In the future, we'll install a lower Q / wider notch to completely suppress this feature entirely without creating any new features, but at this time that's been deemed of lower priority.
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