[sdiy] Help needed with ASM based VCO

[René Schmitz]

Hi Ian and all,

Ian Fritz wrote:
> I agree that the parts have to work together.  That's partly why my 
> MOSFET switcher has a pulse-shaping network.  Also note that the OPA2134 
> I use for the integrator has a 20 V/us slew rate, so a 1V discharge 
> takes only ~50 ns. The opamp is not necessarily the limiting component.

In your case, it surely is not. This was more a general remark.

> I believe that the oscillations shown are not too big of a problem -- in 
> fact, they may be induced or at least enhanced by the scope probe.  

That may be, I never investigated this deeper with circuits where the 
cap is part of an integrator. Maybe the capacitive load of the probe 
does eat up some of the phase reserve.

> After buffering with a good opamp the signal is quite clean.  Note that 
> these particular oscillations are at >10 MHz.  I require a clean signal 
> for waveshaping also, especially for something like my 5Pulser, which 
> can have problems with extra triggering.

It is a matter of taste. And I don't doubt that you get a decent signal 
out of your circuits. Its just that MOSFETs for me never really worked 
quite the way I liked it to be. I like the signal to be as clean as 
possible from the start. But to really do that you have to tweak the 
component values a bit for each incarnation of a circuit. (JFETs have 
component variations...) I've often added little series resistors to the 
discharge circuit to get a cleaner signal out.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159