I know you can make a square wave from the sum of sinusoidals, but does

this mean that if you look at a sine-wave that wasn't made by using

sinusoids (perhaps using a switch or an oscillating crystal to turn the

signal on and off) on a spectrum analyzer that you would see all of the

harmonics required to make up the square wave?

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This post got me to thinking about a related subject. I'm a hobbyist,

and my math is limited to one year of calculus, but I would like to

see if I have a correct conception of what's going on here. I can see

that any periodic function can be put through the Fourier transform to

obtain an infinite series of sin and/or cos terms to completely

describe the original function. This applies to electronic circuits,

musical instruments, vibrational analysis of bridge decks, etc.

So, when one sees a "perfect square wave" on the oscope, it is

actually always a mixture of sine waves of f(fundamental-the frequency

of the square wave as seen on the oscope),3f,5f,7f...... frequencies.

A more complex wave like that produced by a violin string would look

different than either a sine wave or a square wave, because the

mixture of waves producing it are not at the amplitude/frequency

required by the Fourier transform to produce a square wave. If I were

to see what looks like a very low distortion sine wave on the oscope,

I can infer that this is a "true sine" wave, with very little

contribution from any higher harmonics, and not some weird lucky mix

of higher sin/cos frequencies that are significant compared to the

fundamental? Or would the use of a spectrum analyzer be required to be

sure?

For circuit elements like capacitors and inductors, whose reactance

varies with frequency, what happens when dealing with a square wave?

what frequency does one use in the reactance formulas, knowing that

you're dealing with a mixture of them? I would instinctively just put

in the fundamental frequency, but is this right? TIA for clearing any

of this up for me.