Square wave harmonics

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From: John J. Welch (jjw@precipice.chi.il.us)
Date: Wed May 05 1993 - 23:10:55 EDT


>
> Hmmm, nobody caught the obvious error(s) in my otherwise good post or
> perhaps you are all such nice guys that you thought you wouldn't mention it :-)
>
> A square wave (with 1:1 mark:space ratio) has only odd harmonics. This
> means the basic ideas presented are OK (and the basic conclusion is
> unchanged -- you need a LPF on the output) but the numbers are wrong.
>
> The amplitudes of the harmonics of a square wave goes as 1/n**2 (did I
> get this wrong too?)
>
> The first spurious emission is the third harmonic which will be 1/9 of
> the fundamental in amplitude and 1/81of the power of the fundamental
> (power goes as the voltage squared). This implies the spurious
> emissions are about -19dBc. The FCC wants -40dBc or better.
>
> Anyone got a spectrum analyzer and a square wave oscillator to check
> this revised figure?

I caught it, but was a nice guy :-) (too busy to reply :-) )
Still think you're wrong - I beleive *power* goes as 1/n**2, as I can
barely see -20db distortion on a scope. I have an analyzer, and when
I looked at it I got approx -6ish for the 3rd. Will double-check on
Friday with the comm analyzer and a 20M oscillator.

Also, FCC regs want -30dbc if you're power out is <5 watts, if over 5
they want -40dbc *or* some small level, whichever is less.

-->jjw

ps if you're the one building a DDS rig, Analog Devices has been a real
bear to work with - AD9955 lead time is 22 weeks for samples, they won't
sell partial trays, the price for <100 pieces is *very* high, they don't
let *ordinary* people buy the eval kits and so forth. I've had much
better luck with Qualcomm...


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