[sdiy] problem with 13700 flter upper range

[Simon Brouwer]

Scott Gravenhorst schreef:
> 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.

Any idea why?

With a control voltage range of e.g. 10V the control voltage/ Gm current
relation should be reasonably linear over a frequency range of 1:100 or
so.

Below a voltage/current that corresponds to a few hundred Hz the relation
would become less linear and more exponential but that might actually be
desirable...


> 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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-- 
Vriendelijke groet,

Simon Brouwer
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