Re: fourier anlysis of square wave...

Ron N. wrote:
On Nov 30, 6:00 pm, Jerry Avins <j...@xxxxxxxx> wrote:
Ron N. wrote:

...

64 cycles of period 16 (8 ones, 8 minus ones) fills 1024
samples (the clue was the fft result filling only odd
multiples of 16 bins).
Your test square wave had a frequency of fs/16. The harmonics should
fall at 3fs/16, 5fs/16 , 7fs/16 (ok so far), 9fs/16 (aliases, but 19 dB
down), 11fs/16 (aliases 21 dB down), and so on. Is that what you see?

The harmonics fell in those bins (and only those bins),
but the higher harmonics did not drop off at 1/bin_number.
Maybe the harmonics from the negative frequencies were
wrapping around from the opposite side of the fft and
interfering constructively in the middle. That would be
aliasing. But nothing appeared in non-harmonic bins. So
this aliasing would not be the cause of the OP's observation
of seeing "non-harmonic tones" in his FFT plot... unless
his square waves were not periodic in the FFT aperture,
in which case the negative frequency harmonics would not
overlap the positive frequency harmonics, but appear
in between them (in addition to the rectangular windowing
artifacts).

That's what I figure happened. Even the positive harmonics, when folded back wouldn't land on harmonic frequencies because the folding frequency wouldn't be centered.

Jerry
--
Engineering is the art of making what you want from things you can get.
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.

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