[sdiy] DCO with OTA fiddlings (semi-beginner question)

[Walker Shurlds]

Ah! That's a great idea. I had a few ideas floating in my head about this this morning but they're all terrible in comparison.

I really want to test my own take on this now with an adjustable amount of feedback as a kind of "behavior" control. Sonically it may be less interesting than I'm imagining though.

(Another idea I haven't thought through yet: adjustable lowpass cutoff after triangle.)

Walker


On Mar 23, 2016, 07:03 -0500, Roman Sowa<modular at go2.pl>, wrote:
> OTA charging capacitor is independent of timing determined by MCY, so
> they will *never* be the same, whatever you do. It will always drift.
> The only way is to create feedback path which is not existing at the moment.
> Take DC component out of the triangle wave, by filtering it to very low
> frequencies, for example with 100k/10u RC filter, followed by inverting
> amplifier, and route that to [+] input of the OTA, that presumably is
> now connected to GND. It should keep the triangle within rails.
> 
> If you want this DCO to go very low, in sub-hertz frequencies, above
> described method will fail, so you can add comparators that check if the
> triangle is going to much up and down. Comparators feed your MCU and it
> then decides if to add a bit of positive pulse duration or negative,
> like one clock cycle here or there from time to time.
> 
> Hope all that is clear without schematics
> 
> Roman
> 
> W dniu 2016-03-23 o 11:09, Steve pisze:
> > Hey there.
> > 
> > I, a software guy dabbling in electronics, recently read a bit of how
> > OTAs work and started to experiment.
> > From an inspiration of some years ago, on Tom Wiltshire's website
> > describing how the Juno DCOs worked, I thought myself: Hey, why not do
> > that with an OTA, which might even enable to do some more funny stuff...
> > Feeding a pulse wave from my microcontroller of choice...
> > So I first built the simplest of things, which works fine: a ramp DCO
> > using an OTA to constant-current charge a capacitor and abruptly
> > discharging it with an NPN for the last percent or so of the waveform
> > period.
> > The amplitude compensation that's necessary to keep the wave at the same
> > peak is done via the current input of the OTA with a signal from the MCU.
> > 
> > I then proceeded to do a triangle wave with pretty much the same setup,
> > and a minor change:
> > I fed a 50:50 pulse to the OTA, swinging positive and negative. My
> > thinking was, I first charge the capacitor with a certain current, then
> > discharge it with the same current inverted, for the same time, and
> > should thus land at zero voltage again. (no dedicated discharge
> > transistor involved)
> > 
> > But this quickly wanders off towards one of the rails - I thought, ok,
> > probably imperfections in parts, a slight bias towards one direction, so
> > I just put a 10Meg resistor across the capacitor, large enough to not
> > deform the wave.
> > This seemed to do it at first, but it's very frequency dependent, when
> > it looks good at 200 Hz, it will have a large offset at 800 Hz or so.
> > (this was actually breadboarded, not just in spice)
> > 
> > This is a bummer, as I thought I could even build something that morphs
> > between saw and tri seamlessly by feeding a different duty cycle pulse,
> > switching not only the polarity but also magnitude of (dis-)charging
> > current for the rising (a) vs. falling (b = (-1) * (1 - a) = a - 1 :
> > using the OTA also as subtractor) ramp parts of the period to get the
> > same overall time. But given that the simpler scenario of a perfect
> > triangle doesn't even work, I'm not so sure anymore :-)
> > 
> > Can someone give me some pointers here?
> > 
> > Regards,
> > Steve
> > 
> > 
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20160323/6f082184/attachment.htm>