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

[Steve Taylor]

Hi Steve,

the integrator gradually drifts off without the reset to zero used by the  
saw. Think about the maths of integration. Any tiny error will eventually  
integrate to infinity. I wonder whether a DC servo loop might be able to  
control that but I haven't tried it yet.

Steve.


On Wed, 23 Mar 2016 10:09:59 -0000, Steve <sleepy_dog at gmx.de> wrote:

> 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