[sdiy] problem with 13700 flter upper range

[David G. Dixon]

Maybe your voltage source was very non-stiff?

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Scott Gravenhorst
> Sent: Tuesday, January 27, 2009 9:01 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] problem with 13700 flter upper range
> 
> simon.oo.o at xs4all.nl wrote:
> >harrybissell at wowway.com schreef:
> >> You really want to drive the OTA Gm inputs with a current source rather
> >> than voltage. Try looking for the EFM VCF1 that was adapted from the
> >> LM13700 data sheet... that would do it.
> >
> >As the Gm pins of the OTA are connected to current mirrors the voltage on
> >them will be practically constant; about 0.7V above the negative rail.
> The
> > control voltage range is *much* larger than the variation of that
> >voltage, therfore the Gm current will be proportional to the control
> >voltages with little error. I don't see how using a "real" current source
> >could improve much. An exponential converter however...
> 
> That is what I thought too, however when I first started playing with
> LM13600, I did
> some experiments playing around using a resistor to feed the Iabc pin - it
> was anything
> but linear.  When I switched to a real current source, the device did what
> it was
> supposed to do and quite accurately.
> 
> 
> -- ScottG
> ________________________________________________________________________
> -- Scott Gravenhorst
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