Dear friends, I changed some options in my netlist and found quite different results.
First, I changed the .option accurate=1
Then, I simulate AC like .AC DEC 1000 1e-6 1G.
The, the first the step of the graph I posted disappeared!!!! It seems to be a flat band from 1e-6 to the -3db frequency, about 137MHz. So, the problem seems come from the simulator and its settings.
So, which one can I trust, and how to set the simulator to get the trustable resutls.
Thank you in advance.