[sdiy] TZ triangle core

[Ian Fritz]

At 05:25 PM 3/1/2011, nicolas wrote:
>One of the fairly standard triangle core approaches is to do something 
>similar to this, but with the switcher and the integrator combined around 
>a single opamp.

I remember seeing that, but now I can't find the article.


>So I guess the difference you are proposing is to split those and make the 
>integrator with an OTA to give you a multiply option?

Yes, we're talking specifically about a TZ FM device.


>Sounds like it will work fine.

Turns out it doesn't. The reason is that it only will work with one input 
polarity.  The opposite polarity puts the integrator/ST out of phase, so 
the system runs immediately to the rail.  <headslap>


>The only reason you will get voltages slightly outside the schmitt trigger 
>limits is due to slow switching times vs high oscillator frequencies if 
>you use an ordinary opamp as your comparator.

There could also potentially be a startup problem, no?  But the main 
problem is the usual the singular point where the core wants to reverse 
from the change in frequency and from hitting the 5V limit, both at the 
exact same time.  It's hard to predict what the outcome of that collision 
would be.  That's why I'm thinking a secondary containment might be needed.

It looks like a soultion to make the core work might be to generate a 
trigger pulse *every* time the signal tries to leave the +/-5V range and to 
feed these to a T-type flipflop to enforce the reversal.  This is similar 
to my Saw core TZ FM VCO, where every +/-5V crossing discharges the 
integrating cap.  That's the next thing I'm going to try.

Ian