[sdiy] LM13600 tri-VCO

harry harrybissell at prodigy.net
Sat Dec 15 23:19:42 CET 2001


Hi Jay...

Problem 3 can be fixed with a full wave bridge rectifier, a zener diode, and
a series resistor. You can make a symmetric clamp with this method. The diode
bridge makes it so you can use one zener diode, so the clipping is (more or
less)
guaranteed to be symmetrical.  The zener should be larger than the diode drops
for good accuracy... maybe use about a 5 - 7 volt zener.

If you don't get what I mean, write and I'll sketch it up...

H^) harry

Jay Schwichtenberg wrote:

> I've been working on an LFO based on the design. It sould give you a very
> wide freq range so something is probably wrong. I sub-ed the schmitt trigger
> for an op-amp one and got about 1000:1 freq ratio. I haven't got the VCCS
> (voltage controlled current source) working yet, there's a range problem
> (seems to current limit). This is pretty much a straight forward design.
> LM13600 data sheet, Jungs op-amp cookbook or Electronotes will provide you
> with all the blocks/math you need to design this. The biggest problem I have
> over the freq range is the symetry of the waveform. There are 3 areas of
> error in the circuit. First there is the offset in the OTA (effects low
> freq), next the comparitor hysteresis isn't exactly symetric (2.6, -2.4
> volts) and the last is that the comparitor op-amp doesn't output voltages
> evenly (13.6, -13.1 volts). The last two are related since the hysteresis is
> from feedback in the circuit. If I trim all those out it works pretty good.
> Problem is I'm doing this in surface mount and wanted this as small as
> possible. So I need to work on getting the trimmers out and figure out why
> the VCCS isn't going up to 1ma.
>
> Good luck.
> Jay
>
> > -----Original Message-----
> > From: owner-synth-diy at dropmix.xs4all.nl
> > [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Scott Gravenhorst
> > Sent: Saturday, December 15, 2001 11:12 AM
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] LM13600 tri-VCO
> >
> >
> > Henri Kovalainen <hekovala at kovalainen.fi> wrote:
> > >Hello,
> > >
> > >I've been aware of the tri VCO design in the datasheets for some time
> > >now but as I'm currently designing an analog drum machine, I finally
> > >prototyped it a couple of days ago.
> > >
> > >The thing is that it would be very convenient if I could modify
> > the design
> > >so that the OTA on the right (which works as a comparator) would be a
> > >normal opamp instead. This way I could use the other OTA on the chip as a
> > >VCA.
> >
> > There ought to be a way to do that since the right hand OTA is
> > used as a fixed gain amplifier in a schmitt trigger arrangement.
> > (note the fixed Iabc)
> >
> > >However my attempts to do so have failed. The VCO does oscillate, but I
> > >lose the voltage control so that the only this that really affects the
> > >frequency is the capacitor. Altering Iabc of the OTA still causes slight
> > >changes of frequency but not much.
> >
> > Only the left hand OTA needs the control voltage, actually a
> > voltage controlled current source would be better, but even
> > with a resistor and a stiff input CV, you should be able to prove
> > the concept, even if it isn't linear.  If the circuit oscillates,
> > then I assume your schmitt trigger is working.  The left hand OTA
> > then controls the rate at which the capacitor charges and discharges
> > and therefore the system's frequency.
> >
> > The schematic you have posted is a duplicate of the data sheet
> > schematic.  Do you have a schematic of your modified version
> > on a website somewhere that we could see?
> >
> > >Therefore I presume that there's something profoundly wrong in
> > my thinking
> > >and I've decided to give up and ask you guys for help =)
> > >
> > >So if anybody knows how it should/could be done, all advice would be very
> > >much appreciated. You can find a schematic of the circuit at:
> > >
> > >http://www.kovalainen.fi/~hekovala/lm13600vco.gif
> > >
> > >Thank you in advance.
> > >
> > >| henri kovalainen   | peltokatu 19 b51   | hekovala at kovalainen.fi
> > >| phone: 0400-683750   90100 oulu finland   hekovala at paju.oulu.fi
> > >
> > >| Man's little Day in haste we spend,
> > >| And, from its merry noontide, send
> > >| No glance to meet the silent end.
> > >
> >
> > =========================================================
> > - Government: The other religion.
> > - The media's credibility should always be questioned.
> > - Lambs who lie down with lions are lunch.
> >
> > -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> > -- Linux Rex         | RedWebMail by RedStarWare
> > -- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html
> >
> >
> >
> >

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