[sdiy] LM13600 tri-VCO

[Scott Gravenhorst]

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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