TF from all components except the sevo is measured (collectively). This will help us to determine what kind of servo(UPDH) we want for ACAV.
TF from other components in ACAV loop except the servo (collectively) (frequency discriminator, marconi, amplifier) and fit]
TF from ACAV loop without servo. The red curve is fitted with 1pole at 50kHz with 70kHz delay (exp(-i*f/70kHz)) . Since our bandwidth of interest extends upto only a few hundred kHz, this fit is good enough for a model. It starts to diverge from the data at 300kHz.
Note: The TF looks ok, it is flat as expected from most of the parts (frequency disc/Marconi/amplifier). The pole and time delay is from the AOM. We can see the phase changes as we change the AOM position so that the beam is closer to the PZT side. We gain ~5 degrees from adjusting the position.
[current updh schematic]
The TF has a pole around 50kHz. C18 with 3300pF gives a zero at that frequency and cancels the pole. We are designing the TF of the servo that is suitable for our need (UGF ~100kHz, with 1/f roll off at UGF, and ~45 degree phase margin, and, 1/f^2 at low frequency)
Problem with UPDH, on stage 2, the resistors' values might be wrong. We will check and fix it tomorrow.