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

Roman Sowa modular at go2.pl
Tue Sep 5 11:27:32 CEST 2023


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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