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

Mike Beauchamp list at mikebeauchamp.com
Tue Aug 29 17:52:08 CEST 2023



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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