Oh yeah... we're into it now, are we? ;-)) 

I attached a quickly made Excel spreadsheet (it's in the zipfile)
because I guess many readers here can open it and I don't have my good
software anymore as I'm retired ;-) The graph from the sheet is below
this text, you can't miss it (it's a bit big isn't it). Now, count the
tops in the positive half of the period... there's 5 of them. That's
the fundamental 1 kHz plus 4 odd-numbered harmonics. So, quickly back
to the scope and count those tops so you can calculate up to which
harmonic your DAC went to create that square wave, which gives you the
max. frequency used!

So you only need the odd harmonics of the fundamental, but you need all
of them to get a perfect square and even then it'll never really be
perfect. 

So: fundamental + 4 odd-numbered harmonics. The first harmonic is the
fundamental itself so we have 1f + 3f + 5f + 7f + 9f. When you look at
the formula's I put in, you can change it into 1f + 2f + 3f +4f +5f, ie
both odd and even harmonics. See what you get when you change it! The
Excel SIN() function is ugly because it take radians as it's argument
instead of degrees... so I needed to go from degrees to radians, which
is why you see PI() etc. Just change the harmonic numbers. Be careful,
the harmonic number is in the formula twice and both must be changed.

cheers,
Nick.


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