[sdiy] Compensation for triangle amplitude getting bigger at higher frequencies?

Mattias Rickardsson mr at analogue.org
Tue Aug 29 22:01:19 CEST 2023

When building a noisegate using TL074 as a comparator, I was surprised to
see that it was just as inappropriate as everyone had been saying all those
years. Short transients were undetectable. The op-amp output simply didn't
toggle directly on comparator detection, but was stuck for a little while
in its saturated state.

After redesign with the cheapest comparator IC instead, everything worked
as a charm. Just do it! :-)


Den tis 29 aug. 2023 17:53Mike Beauchamp <list at mikebeauchamp.com> skrev:

> On 2023-08-29 09:38, Scott Bernardi via Synth-diy wrote:
> > I've breadboarded a VCO with an OTA (as switch) - Integrator - Schmitt
> > trigger triangle / square wave generator at its core. I've noticed that
> > at higher frequencies, the amplitude of the triangle wave gets larger.
> > Like the output of the schmitt trigger is delayed in switching.  I'm
> > using parts of a TL074 in the integrator and schmitt trigger.
> > Is this because the rise and fall times of the TL074 become a more
> > appreciable part of the waveform time?  TL074 has 13 V/uSec slew rate,
> > which traverses the ~22v in about 1.7uSec, or the +/-5v trigger points
> > in a bit under 1uSec.  Even at a top frequency of 16kHz, that's about 2%
> > of the total period.
> > Is there a way to compensate for this?
> Maybe someone can chime in on this that knows more, but my guess is that
> if you're seeing the amplitude increase at higher frequencies then the
> 1V/OCT tracking is probably getting bonked as well.
> I built one VCO with a TL07x as a comparator and the tracking would get
> progressively worse at higher frequencies. I've also built a VCO with a
> discrete schmitt trigger like on Thomas Henry's VCO-1, and saw no
> tracking problems in fact the tracking was accurate well above human
> hearing range.
> Mike
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