[sdiy] Help needed with ASM based VCO

[Chris Manders]

Hi René and Ian

The original FET I was using was (no laughing!) a
2N3819. I realise now why it wasn't working.

Perhaps I should add that I am self taught, and
learning all the time :)

I would assume that the capacitor is being fully
discharged now. As you mention René, I do have a few
issues with the triangle waveshape - The bottom of the
slope drops down sharply and then shoots back up - It
is difficult to trim it into shape using the controls.
Perhaps I should reduce the value of the discharge
capacitor now (18pF).

Chris

P.S. Nice to see you replying René - It was your
website that partly inspired me to build the synth I
am currently constructing. In fact, it was literally
yesterday that I knocked up the Yash (LM398) Sample &
Hold circuit as detailed on your site - It works well
:)

--- René Schmitz <uzs159 at uni-bonn.de> wrote:

> Hi Ian, Chris and all,
> 
> Ian Fritz wrote:
> 
> > Careful there.  You don't get something for
> nothing.  The J108 has much 
> > higher leakage current -- a factor of 30 compared
> to the 2N4391.  (And 
> > the sheet I have doesn't even specify the
> capacitance.)  So you are 
> > trading off some low-end performance.  Maybe you
> don't care, which is 
> > fine, but the original FETs (2N4859 or 2N4391)
> were carefully chosen to 
> > give the best overall performance.
> 
> 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(!).)
> 
> > But an even better solution seems to be MOSFET
> switching, such as in the 
> > designs I put up a couple of monthe ago.
> > http://home.earthlink.net/~ijfritz/xfer.htm
> 
> 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.
> 
> Cheers,
>   René
> 
> 
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 
> 


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