DanielLam
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For FFT to work well with a rectangular window (sometimes misinterpreted as no windowing), you need to coherent sample.
2047 is not a prime number.
Here is the formula, you should set your input frequency as Fin = (Prime # * Fs) / Number of samples taken
In your case, you can try Fin = (5 * 40 MHz) / 8192 Here I chose the prime number 5, you should pick numbers ABOVE 3.
Also, if your circuit has transient effects, I would run the simulation a little longer and take the later 8192 pts. For example, run for 8500 pts, and take the last 8192.
Btw, you should be fine with perhaps a 256 or 512 pt FFT just to check functionality. So you can save simulation time.
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