Thank you for your kind reply.3 c) F: e- L& ~( W% M
I have ever seen a paper or something else to say that when the signal frequency approaches the sample rate, the beat frequency is easy to appear at Fsig-Fs/2, for example, this frequency is 17MHz. It is as if sampling frequency is 27MHz. It is really a puzzle.
I am really thankful to DennyT's reply, it is a reasonable and constructive explanation on this issue. I will try to pull higher supply voltage to see if it can be relieved or removed or not. Thank you very much.