[sdiy] LM13600 tri-VCO

[Jay Schwichtenberg]

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.
> >
>
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