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

[Ian Fritz]

Another approach is to make a simple counterglitch network to cancel the spike.


At 11:48 AM 9/8/2009, David G. Dixon wrote:
>I've simulated this, and it works like a dream.  I connected the collectors
>directly to +15V, connected the inverted and non-inverted saw through 22k
>resistors to the bases (this resistor value is pretty arbitrary), and
>connected the emitters through a 180k resistor and a 1nF cap to -15V.  The
>only important values are the emitter resistor and cap, and, of course,
>increasing one has the same effect as decreasing the other.  The selection
>or RC is a compromise:  It must be large enough to filter out the glitch,
>but not so large that it skews the resulting triangle wave, at least within
>the useful audio range.  With and RC of 180us, the glitch is mostly gone,
>and the triangle is symmetrical up to about 10kHz.  Beyond that I don't
>really care.
>
>The parts count of this technique is a little bit lower as well: one fewer
>opamp, and only the one capacitor.  The glitch isn't cleaned up quite as
>effectively, perhaps, particularly at high frequencies, and there is the
>skew issue, but on balance, I think this might be the preferred approach.
>
> > Connect two transistors as emitter followers with the emitters tied
> > together. Feed the second transistor with inverted signal. By connecting a
> > small capacitor to the output, you can easily control the slewrate
> > asymmetrically. The transistors can charge the capacitor very fast, but
> > the resistor/current sink at the output has only finite current available
> > for discharging it. Now arrange input sawtooth polarity so that the glitch
> > at triangle output goes down and adjust resistor/current sink and
> > capacitor to taste.
>
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