[sdiy] problem with 13700 flter upper range

[Scott Gravenhorst]

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?

Well, I think it's partly because I used a pot to provide the voltage that then went into
the resistor attached to the Iabc pin.  The current source I used
http://home1.gte.net/res0658s/fatman/vcngvco.html is at the top of the schematic.  This
is a linear CV current source, so it looks different from your "normal" expo source. 
I'll try to explain how this works, focus on the right side where there is an opamp with
a darlington pair (just a really high gain transistor) base-emitter junction in the
negative feedback loop of the opamp.  The desired current is initially created by the
input current from the opamp to the left of that flowing into the summing node of the
circuit.  The feedback loop then forces a current to cancel the input current through the
darlington pair E-B junction.  Since a transistor is a current amplifier, the
collector-emitter current is then set base-emitter current.  The source of the current is
actually the summing node zero volts or ground flowing into the the negative supply that
is found in the Iabc circuit of the LM13600.  In this way, the current into Iabc is
accurate and easily controlled as a proportion of the voltage at the circuits input. 
This is called a voltage to current converter (and it is linear).  Thus, if the input
voltage doubles, so does the current.  Using a resistor and a pot approximates this in a
crappy way, but the opamp doing the conversion makes it much more accurate.  With pot and
resistor method all you can really say is that if the voltage at the pot's wiper
increases, so does the current through the Iabc resistor, but as I found out empirically,
it is not very accurate.  Maybe a really stiff voltage source would work better - but I
think that requires a very low resistance pot.

>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.

I think that depends on how "stiff" the variable voltage source is - and a 10K pot isn't
very stiff.

>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...

My needs were for creating a current to control the pitch of a musical VCO (for a FatMan
- that's why I wanted linear CV converted to linear current).

>
>> 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.
>>
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>>
>>
>
>
>-- 
>Vriendelijke groet,
>
>Simon Brouwer
>-*- nl.openoffice.org -*- http://www.opentaal.org -*-
>
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-- 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.