RE(2): spectrum of PWM

[Magnus Danielson]

From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
Subject: RE(2): spectrum of PWM
Date: Thu, 7 Dec 2000 16:49:27 +0100 

> 
> > 	>You should be able to model it using Bessel functions.
> > 	>
> > 	>Consider this:
> > 	>
> > 	>You have two sawtooth waveforms which are at exactly the same
> > 	>frequency (SHARP! THE same) which are added together 1:-1. One of
> > these
> > 	>are phase-modulated in the relation to the other. Now, as we change
> > 	>the phase modulation will the resulting pulse-width change. Now, we
> > 	>can handle this using Bessel functions in the frequency plane.
> > 	>I think scetching something along these lines will make you arive
> > at
> > 	>an exact solution.
> > 	>
> > 	>Is that sufficient for you?
> > 
> 	>Excellent. I think that's the solution !
> 
> I was too fast. That's the saw-based PWM (minus DC shift) that is modelled.
> Any ideas for the triangle-based PWM ?
> (With the "center of gravity" of each pulse being invariant, rather than the
> rising
> edge as in the saw-PWM case)

Isn't that simple ;)

You just model that the same way but phase-modulate it back into
center using the same modulator source!

Done.

OK, it's nasty, but who cares? ;)

What it really does to this brew is just alter the amplitudes of the
side-bands and (over)tone modulations.

Cheers,
Magnus