We modified on Saturday 14th of April, the LC filter of the actuator board of the frequency laser servo.
Previous values:
L1 = L2 = 100 microH (L1 and L2 are in parallel)
R25 = 390 Ohms in parallel with 270 Ohms = 160 Ohms
fc = 370 kHz (Cpiezo = 3.7 nF)
Q = 0.73
New values:
L1 = L2 = 680 microH (L1 and L2 are in parallel)
R25 = 390 Ohms
fc = 142 kHz (Cpiezo = 3.7 nF)
Q = 0.78
We tested the open loop TF. The oscillation at 300kHz disappeared.
We succeed in adjusting a unity gain frequency above 20 kHz.
We noticed that sometimes, an oscillation at about 6.5 kHz appeared. It does not seem to be due to an instability servo loop because the gain margin at this frequency is about 10dB.
We noticed also an oscillation at 10kHz with the signal analyser HP 35670 which is very stable (exactly 10.000 kHz) and does not vary with the servo gain.
We will continue to investigate about the 6.5 kHz spurious oscillation in order see if its frequency varies with the servo gain.