[sdiy] More about the Aries VCO

[harrybissell]

The LM311 is a very GOOD comparator... but it can be pretty fussy as well. It is
prone
to oscillation unless you do very careful layout (which is true about most
comparators...)

It has the advantage of ground referencing a differential signal, thanks to the
extra pin for
the output section.

A lot of folks don't know to short pins 5 and 6 together if you are not using
them... this can
solve a lot of troubles.

However... in some cases where long input leads cannot be avoided, or where you are
trying
to compare somewhat noisy signals... the 3080 is a good choice. I got introduced to
it as comparator
in the Etherwave Theremin P/V converter (where I said ... what is this KOOK doing
here ?  :^). Of course
Bob Moog is just full of juicy analog circuit design tricks... and I found that
indeed he really DID know what
he was doing.  So I stole the technique :^P

H^) harry

James Patchell wrote:

> I myself have also used the LM311 as a comparator in both saw (ASM-1 type)
> and triangle VCO's with excellent results.  Don't know why, but in
> breadboards, I always manage to wire the power supply pins up
> wrong....(somehow, I always got them correct on PC boards....)....
>
> At 11:56 PM 6/14/2004 +0000, harrybissell wrote:
> >The 3080 has a slew of like 80V/us which puts it above opamp class (as
> >comparator)
> >but below the std LM311 etc. It is a very well behaved comparator and i
> >have used
> >it
> >often as such...
> >
> >H^) harry
> >
> >Ian Fritz wrote:
> >
> > > At 04:04 PM 6/14/2004, JH. wrote:
> > > >Now that I know the internals of the expo onverter, I've taken a
> > closer look
> > > >at
> > > >the whole thing.
> > > >
> > > >This CA3080 based schmitt trigger is very clever! Must be pretty fast
> > *and*
> > > >precisely bouding at fixed level without saturating.
> > > >(Does anybody know at what current it is working, i.e. the value of the
> > > >resistor at pin 5 of U4 ?)
> > > >
> > > >I still wonder if it is *that* fast (and if the transitors have such low
> > > >rbb) such that
> > > >it doesn't need any HF tracking compensation ?!
> > >
> > > I haven't looked at the Aries, but the 3080 as Schmitt trigger was used by
> > > Bernie in the ENS-76 series.  See EN#75, p. 10, or the builder's
> > > guide.  The integrator is a 3080 driving a 1 nF cap buffered by a 3140. The
> > > Schmitt trigger output is diode clamped.   There is no hf
> > > compensation.  The tuning wasn't measured to high precision, but it looks
> > > like it goes sharp (!) by 0.5% at the high end (24 kHz).
> > >
> > >     Ian
>
>          -Jim
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>
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