[sdiy] More VCO fun

[David G. Dixon]

> >Only 1nF, with a ramp height of 5V.  It should drain out in a jiffy!
> 
> Yes it should.  Is something else limiting your measurement?  You are
> using
> a high enough frequency scope and good probes?

There's nothing wrong with the scope (a Tektronix).  I tested a few
different JFETs and they were all the same.  3.3us (33pF * 100k) seems to be
the preferred reset RC in my circuit.  More just slows the VCO down without
getting the reset point any closer to ground, and less puts me back to
incomplete reset and all of its attendant problems.

> >By the way, subsequent simulations suggest something new (or at least not
> >shown in your schematic): Putting a small cap (say, 47pF) between the
> >comparator inputs helps dramatically to bring the reset right down to
> ground
> >at significantly lower RC constants, and just generally cleans up the saw
> >reset response.  I'm definitely going to add said cap to my circuit as
> soon
> >as I get home tonight to confirm.
> 
> I can't remember seeing an improvement with that cap.  Nowadays I usually
> put diodes across the 311 inputs.

You were right about the caps.  They did more or less what the simulations
said they would, except that the reset point was still about +100mV instead
of 0.  In fact, nothing I did changed that.  Even too much RC still only
brought me down to 100mV.  Could this be the JFET?

So, what do the diodes do?

One thing that's different about this circuit with the LM311 is that the
ramp transition is a little bit ragged, and at high frequencies there are
sinusoidal wiggles at the top and bottom of the saw.  A very small cap
across the negative feedback loop of the saw shifting opamp fixes the
wiggles (implying that it is that one which is oscillating) but also rounds
off the transition somewhat, and puts a big old spike in the triangle and
sine waves (which is inaudible, but still...).  The ragged saw edge makes
the bottom corner of the triangle a bit gnarly as well, but actually this
oscillator sounds a little bit better than my other ones (to my ears), so
I'm not going to argue with it.