AW: [sdiy] ICL8038

[Czech Martin]

I have the feeling that this IC has some problem
with wide range sweeps. And AFAIR you need a PNP expo pair, because current
has to come from the power rail. But I may be wrong.

When I was young (16) I built a nice VCO using a XR2206 and CA3046 expo
pair. I remember that it worked
imediately with a very wide sweep range, more then 1:10000. Of course, you
can't have sine and tri at the same time with this design, but the sine is
much cleaner than what I got out of the 8038.

No saw also, but sqa. A tri to saw converter is a no-magic, and the sine can
be fed into that, too.

I remember that some pll projects in my favourite mags always used the
XR2206 chip, never 8038.

So, perhaps you would like to have a look into the data sheet as well.

m.c.

> -----Ursprüngliche Nachricht-----
> Von: Tavys Ashcroft [mailto:bigtex at cwnet.com]
> Gesendet: Samstag, 9. Juni 2001 10:45
> An: Synth-DIY list
> Betreff: [sdiy] ICL8038
> 
> 
> I've been pretty interested in using the ICL8038 as a VCO for 
> non-precision stuff.  Right now I'm just getting started with the 
> whole synth-making thing, so I'm not too worried about perfection. 
> This chip looks really good, though, because I could use all three 
> waveforms, have a large frequency sweep range, and adjust duty cycle 
> all very easily.
> 
> Now I was just on the music machines page and a brief .txt file 
> mentioned an electronotes circuit which gives a much larger frequency 
> range and is much more useable as a VCO.  Does anyone here know where 
> or how I could find such a schematic, or where I would start out to 
> design something like this from scratch?
> 
> The description was as follows:
> 
> In typical Electronotes fashion there follows a "cheater" circuit for
>       the 8038 which bypasses the current-to-voltage problem 
> altogether and
>       injects exponential current directly into what would 
> normally be pins
>       for timing resistors, making the VCO exponential with a 30,000:1
>       voltage/frequency sweep. The outputs are Sine, 
> Triangle, Sawtooth,
>       Square, and Pulse (with PWM). This circuit uses an NPN matched
>       transistor pair, followed by a PNP matched transistor 
> pair, along with
>       5 op-amps and a FET.
> 
>       While this sounds attractive, if the performance of this VCO is
>       expected to be as good as possible, then the two 
> matched transistor
>       pairs will add up to about twenty bucks. This combined 
> with the cost
>       of the 8038 itself makes this design less attractive 
> than say one of
>       the later Electronotes designs which use only one 
> matched pair or even
>       a 3046 transistor array, plus a few op-amps, with 
> performance specs as
>       good as anything ever available commercially (and 
> better than most).
> 
> 
> If passing out electronotes articles is a no-no here (I seem to get 
> that drift), then where do I find electronotes articles?  I'm getting 
> much more interested in finding such a circuit as my plans-on-paper 
> are starting to want to become a real machine, and soon.
> 
> -Tavys
>