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February 24th 04, 06:52 PM

(Avery Fineman) wrote in message ...

Unsymmetric digital signals haven't been found to have much more
even harmonic content than odd harmonit content when viewed on a
spectrum analyzer...until the digital signal pulse width is VERY
short compared to its repetition time.

Huh? Try a 1/3 -- 2/3 ratio. NO third harmonic (or sixth or ninth,
etc); second is the greatest amplitude harmonic at fully 1/2 the
fundamental amplitude. Fourth is larger than the fifth. For a
symmetric square wave, there's no second and the third is only 1/3 the
amplitude of the fundamental. Easy to verify with a spectrum analyzer
or through a Fourier series. As the pulse width goes to zero, the
fundamental and all harmonics go to equal amplitudes. At 5% pulse
width, for example, the harmonic amplitudes decrease monotonically as
harmonic number increases, out to the twentieth, which is a null, but
even the 12th is fully 50% of the fundamental.

This is useful info if you're trying to design a simple amplifier-type
doubler; adjusting the conduction angle to approximately 1/3 will give
you lots of second but little third, making it easier to filter the
output. But if you weren't thinking about it and tried to make a
tripler, and accidentally made your conduction angle 1/3, you might
wonder why you were having so much trouble getting good tripler
efficiency.

Cheers,
Tom