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

Scott Bernardi scottbernardi55 at gmail.com
Tue Sep 5 16:06:29 CEST 2023

I solved my problem by using a LM319 comparator as the Schmitt trigger.  I
tried several things, LM311, discrete transistors (that was weird in that
it worked fine when driven by an external triangle, but as soon as I hooked
it to the integrator output, no.  And I've used that circuit in an LFO
Here is my core VCO circuit:

I'm working on a triple VCO, where the panel components are on a 18HP PCB,
and three oscillator boards plug into the back of it.  It will have a mixer
for the three outputs.

On Tue, Sep 5, 2023 at 6:19 AM Roman Sowa <modular at go2.pl> wrote:

> What are the odds... I just had a visit from my friend who brought an
> oscillator looking exactly like this, with big tuning problems. Triangle
> core, OTA, TL074 as Schmitt. So I had a chance to confront my mumblings
> with the real world.
> The triangle amplitude started to rise at about 600Hz, and climbed more
> than 1V above rails at 10kHz. Integrator starts to change direction
> about 2-3us after the Schmitt has toggled so there's some lag in OTA
> itself. But the Schmitt trigger is a real bugger, at high setting it
> starts to move only when its input was overdriven by 400mV. That creates
> most of the amplitude rise and frequency going flat big time.
> I tried to add that feedback cap on the Schmitt, 33-100p works best, but
> the improvement was just by one semitone. So the only option was to use
> Rossum compensation and now it's somewhat OK in most used frequency range.
> Roman
> W dniu 2023-09-05 o 11:27, Roman Sowa pisze:
> > I suspect you have taken those rise/fall times from datasheet and not
> > actually measured them.
> > TL07x may indeed swipe its output so fast, but only in certain
> > conditions, when the input is overdriven and output unloaded. If it
> > works as a comparator, it will think for a while what to do in this
> > unfortunate situation of passing threshold, then slowly deciding that
> > maybe it should move its output a litlle up or down.
> > If you add a small capacitor in the feedback of the Shmitt trigger
> > opamp, it might help.
> > There might also be some parasitic capacitance at OTA inpt, so even if
> > Schmitt changes immediately, the OTA will still push the current in
> > previous direction for a short while. That's easy to check if you look
> > at Schmitt waveform superimposed on integrator waveform
> >
> > Roman
> >
> > W dniu 2023-08-29 o 15:38, Scott Bernardi via Synth-diy pisze:
> >> 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?
> >>
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