[sdiy] LM13600 tri-VCO

[patchell]

    I think I have exactly what you want.

    Download:http://www.silcom.com/~patchell/smb1/smb.pdf  (big file, lots of
good stuff).

    and then goto page 8.  This should be a schematic of an LFO that I did
using an LM13700 for the VCO part, and a TL072 for the comparator.  Works
great!  To change the frequency range, just change the value of the integrator
capacitor...and, if you don't want to use an integrator, and want to use the
internal buffer, just run the output of the comparator to the + input of the
OTA instead of the - input as shown....

Henri Kovalainen 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.
>
> 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.
>
> 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.

--
 -Jim
------------------------------------------------
* Visit:http://www.silcom.com/~patchell/
*-----------------------------------------------
*I'm sure glad Merry Christmas comes just once a year
* -Yogi Yorgensen
------------------------------------------------