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In Spectre you have the Noise analyses - you have to place a port on the input and sweep a variable (probably frequency in your case).
You can also do this by runing a PSS analyses - this is more complex and you can apply 2 tones on the input.
You should be able to find tutorials and examples in Cadence Help!
i had a try! in spectre, the result of simulation of using noise analysis between add a AC source and place a port on the input are different. i don't know anything wrong. if place a port on the input, how to fill out the parameter option? thanks.