[sdiy] Help needed with ASM based VCO

[Ian Fritz]

At 09:56 AM 2/18/06, René Schmitz wrote:

>I know I keep repeating this, but anyway: There is no point in makeing the 
>discharge faster than the OP can handle. This only leads to overshooting 
>behaviour. There is an *optimal* Ron (actually Idss, but they are related 
>to each other) for the given opamp slewrate.
>
>(Its actually a mixture of both Ron and Idss. It depends on the threshold 
>voltages, if you always keep some voltage across the cap its Idss (if you 
>never enter the ohmic region), if you discharge all the way to GND, you 
>enter the ohmic region at some point, with a much slower discharge(!).)

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.


>I've been playing with MOSFETs for discharge for quite some time, the 
>earliest examples are even on my site (VCO1 and VCO2). But I never quite 
>liked the behaviour at the transition. I mean those dirty oscillations 
>around the discharge, like you show also on the scope shots at your site. 
>I suppose this is really due to charge injection. With a careful balance 
>of Idss and cap size to the slewrate of the opamp, you can get a much 
>cleaner transition with a JFET. Which is beneficial IMO for waveshaping 
>the sawtooth into a triangle.

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.  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.

   Ian