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

[Donald Tillman]

The easiest solution for this situation is to note that a triangle wave has both positive and negative trip points... have, say, the positive trip point triggered by the triangle crossing the positive reference voltage, and have the microcontroller provide the negative trip point.

One keeps the wave in position, the other keeps the frequency precise.  The negative peak voltage of the triangle will not be as accurate, but hey...

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com


> On Mar 23, 2016, at 3:49 AM, Steve Taylor <synth-diy at toot.org.uk> wrote:
> 
> 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.
> 
> 
> On Wed, 23 Mar 2016 10:09:59 -0000, Steve <sleepy_dog at gmx.de> wrote:
> 
>> 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