The OMC DCPDs have proven to be useful for monitoring the test mass acoustic modes around 15 kHz, but there is a lot of low-pass filtering in the readout chain that render them less useful for monitoring higher frequency acoustic modes. This is now being changed with modifications to the electronics that will provide separate, faster channels for PI monitoring:
- In-vacuum pre-amp: this has 2 poles at 16 kHz (these will remain, since in HAM6)
- Whitening board: 1 pole at 14 kHz; 1 pole at 18 kHz. These will be eliminated in the new chain.
- Anti-alias filter: 3rd order Butterworth low-pass at 10 kHz. These will be eliminated in the new chain.
The attached plot shows the magnitude response of the low-pass filtering in the previous case, and with the poles removed from the whitening and AA channels. It is no wonder that no PI modes have been seen above the 15-16 kHz grouping, as there is 40 dB of relative attenuation already at 25 kHz.
I also attach a 1kHz - 10 MHz transfer function of the in-vacuum DCPD readout that Koji measured at Caltech:
Here are the transfer functions with the AA notch included. I had forgotten that the notch is a passive twin-T type, which by design has a Q = 1/4, so it is quite wide and should be taken into account. In future the AA notches should also be removed.
