Judging the output of a filter - FFT

A square signal (variously pre-filtered / buffered), enters the same type of filter trying to get the best approach to a sinewave of 3960 Hz. Corrected.

For four different topologies, LTSpice gives the attached FFT. My questions:

Which one is the best in terms of purity no matter if that "best" is still a poor one? Could anyone tell briefly what are the main points to know which one is?

How do you judge the height of the harmonics against the height of the fundamental? Or is it the wrong way to see this?

Additional comments appreciated.

I find a Bode plot to be more useful than FFT. The FFT is very sensitive to how many cycles fit in the window, and if window matches the period of the fundamental of the signal being analyzed.

MikeMl said:

I find a Bode plot to be more useful than FFT. The FFT is very sensitive to how many cycles fit in the window, and if window matches the period of the fundamental of the signal being analyzed.

Click to expand...

Suudenly I seem to be in square one Mike. Gracias for replying.

I did not know about that window problem so I have to redo the whole thing.

Could you elaborate briefly about the Bode plot? LTSpice, is it capable? Have to look at that tonight.

atferrari said:

...

Could you elaborate briefly about the Bode plot? LTSpice, is it capable? Have to look at that tonight.

Click to expand...

Example:

here is a write up to get good results and how to read the log file to get distortion measurement.

https://www.audio-perfection.com/spice-ltspice/distortion-measurements-with-ltspice.html