Thank you for your reply.
I've tried the "arbitrary reference voltage" settings and it works. Thank you.
But, it seems this function doesn't support in R5.20 with my old ADC-216.
There is a function in the Audio Precision (APWIN) to lock the input signal level to 0dBr by pressing "F4". - Just a little suggestion.
It is also a pity that, my new PicoScope 5203 (R6.1) cannot support log scale (x-frequency) in FFT analyzer.