[sdiy] problem with 13700 flter upper range

[cheater cheater]

Most interesting.

On Tue, Jan 27, 2009 at 6:56 PM, Simon Brouwer <simon.oo.o at xs4all.nl> wrote:
>
> 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
>> -- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
>> -- FatMan: home1.gte.net/res0658s/fatman/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- When the going gets tough, the tough use the command line.
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>>
>
>
> --
> Vriendelijke groet,
>
> Simon Brouwer
> -*- nl.openoffice.org -*- http://www.opentaal.org -*-
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>