Exp converters (silly idea?)

[mbartkow at ET.PUT.Poznan.PL]

>    From: "Thierry Rochebois" <tiar at canamcomputers.com>
>    Date: Fri, 20 Aug 1999 13:11:02 +0200
> 
>    Just a (maybe) silly idea.
> 
>    As the discharge curve of a capacitor is an A*exp(-t/tau),
>    maybe, it is possible to calculate an exp thanks to it with the
>    help of a linearly discharging capacitor (constant current) figuring -t
>    and comparing -t with the input. The comparator would trigger a
>    sample and hold and output the A*exp(-t/tau).
> 
> Sounds too good to be true, right?  The capacitor discharge equation
> looks remarkably close to what we want in a VCO.  And the curve starts
> out fast and slows down, it really seems like it could work.
> 
> It's just backwards.  The V and the T are the wrong sides of the
> exponential function.  ("Damn!")  Now if you were looking for a log
> VCO... 

Well, it depends on how do you use the exp relation. In fact this approach 
has been described several years ago in a Russian magazine in a series
of articles about analog synths (they are somewhere on my bookshelf). 
I personally have "invented" it even earlier and experimented with it 
in one of my first synth projects.

I had V/Hz VCOs and the exp converter was used only for the keyboard voltage.
In fact, the converter operation was strictly coupled with keyboard scanning.
The keyboard was a typical DEMUX/diode/MUX field found in computer equipment.
The scanning started from the highest key just a moment after charging a capactor.
During the key number countown the capacitor was discharged through a resistor
with a time const (R*C) set appropriately to match the scanning rate so the
voltage decreased by 1/2 for each 12 keys. Whenever a depressed key has been
detected by the scanning logic, the voltage from the capacitor was sampled by
a S&H. The scanning rate was about 10usec per key, AFAIR.

This circuit did not work perfectly. My main problems were probably related
to the capacitor tempco. That time I hade a little experience in pro electronics
and I abandoned this idea. I reckon, however, that it is possible to obtain
a great temperature stability by selecting an appropriate capacitor or even
combinig several different technologies.

regards,

mb


--

Maciej Bartkowiak, PhD
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