Re: square wave harmonic theory (time domain)

On Wed, 30 May 2007 20:21:50 GMT, "Thomas Magma"
<somewhere@xxxxxxxxxxxxxxxxxx> wrote:

Hello,
I'm trying to determine if the higher harmonics of a low frequency square
wave are actually AM modulated. For instance, I can see the harmonics of a 1
KHz square wave all the way up at 100 MHz if I zoom into them on a spectrum
analyzer. Are those harmonics really there when the 1 KHz square wave has
finished it's transition and is in a steady state for half a millisecond? If
I was to sample this steady state with a ultra fast ADC and FFT the samples,
would I see the harmonics extending up through 100 MHz?

It's a bit of a mind bender when converting between the time and frequency
domain in the case of a square wave.

Thanks,
Thomas Magma


Assuming an ideal constant-frequency square wave, all the (odd)
harmonics are there all the time, invariant in phase or amplitude. It
is mind boggling.

In real life, any square wave has a little frequency modulation
(jitter, in the time domain) and that makes higher harmonics jump
around, essentially amplitude modulated sort of at random.

It would be hard to sample and FFT such as to resolve the 100,000th
harmonic of any waveform.

John

.

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