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

[Simon Brouwer]

Hi, 

If it works with lower frequencies but gets worse at higher frequencies, it
might be caused by the pulse width being off by a fixed amount (e.g. slower
high-going or low-going edge in the circuit that converts from logic level to
positive/negative levels), as the error would be added f times per second.

Best regards
Simon

> On 23 March 2016 at 11:09 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_______________________________________________
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