I read in sci.electronics.design that Reg Edwards
wrote (in
et.com) about 'Extracting the 5th Harmonic', on Fri, 12 Mar 2004:
According to Fourier, at some mark-space ratios of a square wave certain
harmonics may be missing from the spectrum.


For a waveform like this (use Courier font):
_____
/ \ /
_____/ \____________/

with rise-time f, dwell time d, fall time r and period T, the harmonic
magnitudes are given by:

Cn = 2Aav{sinc(n[pi]f/T)}{sinc(n[pi][f+d]/T)}{sinc(n[pi][r-f]/T)},

where sinc(x)= {sin(x)}/x

There seems to be a number of opportunities for a harmonic to 'hide' in
a zero of that function.
--
Regards, John Woodgate, OOO - Own Opinions Only.
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The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk