AW: expo converters

[jhaible]

I'm currently experimenting with exactly this: expo current source for
LM13700 with HFT trimmer.
I'm using an EP1015 quad pnp array, with one pair for the expo converter,
and the second
pair for temperature compensation. High time to finally try Rene's method !
My circuit is a little different, but the principle is the same. It looks
excellent in simulation,
but I'm gonna test it in reality first before I publish the circuit. It's
part of the tri / saw
VCO Iwas talking about earlier.

JH.

----------
> Von: macdonald at evenfall.com
> An: synth-diy at mailhost.bpa.nl
> Betreff: expo converters
> Datum: Sonntag, 7. Februar 1999 06:27
> 
> I have just been experimenting with a LM13600 based VCO (sawtooth/pulse
> circuit in figure 16 of the National datasheet).  I used an expo
> converter similar to the one in Tom G's VCO5a, (two 3906's with the
> emitters tied to V+).  I can get decent V/oct performance at low
> frequencies, but the oscillator goes flat as the frequency climbs.  I
> understand this is common in VCOs and can be compensated for ("high
> trim").  I guess I have a few questions:
> 
> Does anyone know of an LM13600 VCO circuit that has high trim circuitry
> that I can look at?  I haven't been able to find one.  Or maybe it can
> be done in the expo converter circuitry.
> 
> I substituted a 500pf cap (actually 2 1000pf in series) for the
> specified 50pf item (smallest ones I have - time to order some more
> parts).  Do larger cap values increase the high frequency "droop"?
> 
> Is there a general or common approach to implementing "high trim" or is
> it a circuit-specific problem?  
> 
> Why do some VCOs not have high trim (for instance the ASM-1)?  Is it
> because these VCOs have a very fast discharge time and thus there is
> less error at high frequencies?
> 
> Thanks,
> 
> Chris MacDonald
>