We suspect the wired behavior of SHG's mechanical Transfer Function(TF) is caused by High Voltage Driver(HVD). So we decide to measure the TF of HVD.
We measured two HVDs. One is HVD used for SHG, the other is a new SHG. Each trace data is taken with two different frequency spans and overlapped. This is to make measurement precise.
As you can see from below, the TF of HVD truly has feature like low-pass.