Op Amp integrator automatic amplitude/frequency correction - Page 1
Forum section: electronics / projects

author: cteffects
author_rank: Newbie
timestamp: May 23, 2015, 02:01:12 am
content: Hey all! I'm designing an analog music synthesizer, and I'm using standard multistage Op Amp Integration to generate triangle waves and "sine" waves from a square wave input. However there is naturally an inverse relationship between amplitude with respect to frequency. Does anyone have any ideas how I could compensate? I would ideally like to make the output as linear as possible over the audible frequency range.

author: acbern
author_rank: Frequent Contributor
timestamp: May 23, 2015, 11:00:28 am
content: if you use fast and high input impedance opamps you should not have that problem. low impedance designs also help in general.other than that, it would hep if you were more specific re. your exact problem (schematics, parts selected...).

author: sync
author_rank: Frequent Contributor
timestamp: May 23, 2015, 11:43:51 am
content: Is it a classic subtractive synthesizer? They usually using sawtooth oscillators from which are other waveforms are made. Starting from a square wave makes it more difficult.

author: rs20
author_rank: Super Contributor
timestamp: May 23, 2015, 11:54:53 am
content: Quote from: acbern on May 23, 2015, 11:00:28 amIdeal integration causes amplitude (& frequency) changes when the input frequency changes.

author: cteffects
author_rank: Newbie
timestamp: May 23, 2015, 01:50:58 pm
content: Quote from: sync on May 23, 2015, 11:43:51 amNo, it's just a simple 4x waveform generator going from Sawtooth > Square > Triangle > "Sine" (really a piecewise quadratic curve). Sawtooth to square is trivial since I can just use a comparator on it with respect to ground, but as soon as I start integrating, things get hairy. Since, like the above post says, even ideal mathematical integration produces amplitude changes with respect to frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2015, 01:55:45 pm
content: Yes, you want to *start with* the triangle wave. You can always make a square later, via nonlinear process (comparator). That is, you'd never make a square by differentiating the triangle, that'd be silly. But you have a whole lot harder time making a triangle from a square, without using some bizarre compensated integrator.If you absolutely must (e.g., to sync with an external clock), you might be better off with:a. Triangle oscillator, PLL locked to sourceb. Integrator with peak amplitude detection, OTA controlled gain (amplitude locked loop, I guess you could say)They're more-or-less equivalent because of the direct ratio (frequency and voltage being proportional), which should have interesting consequences actually, as a PLL is generally considered more complex than a mere integrator and gain stage. Or conversely... the amplitude loop should be as complex as the PLL, since after all, it's trying to regulate on a widely variable signal, so the compensation will never be perfectly well defined.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2015, 01:56:50 pm
content: Quote from: cteffects on May 23, 2015, 01:50:58 pmAha, invert half the sawtooth. Problem solved!

author: sync
author_rank: Frequent Contributor
timestamp: May 23, 2015, 02:11:16 pm
content: Quote from: cteffects on May 23, 2015, 01:50:58 pmYou don't need an integrator.Saw to square/rectangle: comparatorSaw to triangle: full wave rectifierTriangle to sine: sine shaper.But on a subtractive synth a sine wave is fairly useless. I think I never saw a sine wave VCO on a synth. Only for LFOs.