[sdiy] filtering out a saw-reset glitch in a triangle shaper

[Scott Nordlund]

If the glitch is more or less an impulse-type signal, it contains a wide 
range of frequencies that won't be completely eliminated with a simple 
filter.  You might try something with a limited slew rate, since that 
affects the output based on rate of change rather than frequency content.

It's also convenient because triangle waves don't require a particularly
high slew rate (actually the lowest slew rate of any wave shape, while an 
impulse requires the highest?) and will only be attenuated rather than 
distorted...

I imagine that if you set the slew rate to match that of the highest
frequency triangle wave that you would desire, it should do a much better
job of eliminating the glitch than a simple filter.  

But if the glitch is small enough, well, it won't really matter.

You could look at the Fourier series to see how much of a problem it is.

----------------------------------------
> Date: Mon, 7 Sep 2009 20:16:31 -0700
> From: dixon at interchange.ubc.ca
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] filtering out a saw-reset glitch in a triangle shaper
>
> I'm designing a triangle shaper for the 2164 expo VCO. This is a new thing
> for me, because all of my previous VCOs are triangle-core. The basis is a
> simple yet elegant precision full-wave rectifier circuit I found on the web
> somewhere. To mostly eliminate the saw-reset glitch at the apex of the
> triangle, I'm thinking of filtering the rectifier output with a passive LPF
> with a cutoff frequency of 50kHz (3.3k + 1nF). That, plus a couple of
> strategically placed 22p caps in the rectifier circuit itself, seems to all
> but eliminate the glitch in my simulations.
>
> So, here's my question: Is there some reason not to do this that I'm not
> thinking of, or is there some other way to reduce or eliminate this glitch
> which is blindingly obvious to everyone but me?
>
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