A fundamental problem of exponential VCOs?

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Thu, 10 Jul 1997 17:30:35 +0100

   I found that I run into one problem quite often: When I detune the VCOs
   so much that I get a decent beat rate of 0.2 to 1 Hz (which I find very
   pleasant), the VCOs sound very detuned in higher octaves,
   *regardless* of the scale adjustment of each VCO.

I know this problem well.

   We get *rid* of this by generating an exponential current which charges
   some capacitor directly. The only way to *introduce* an offset here
   would be an offset *current*. Maybe some leakage current has some
   (positive) effect, but it's really hard to control such small
   amounts of current. (If 1kHz corresponds to some uA, then a 0.2 Hz 
   difference would correspond to some nA ... ) (Method A)

I think this is a very reasonable approach; it's elegant in that it
solves the problem in a very direct manner.  It also gives you linear
FM, which is a wonderful feature for a VCO.

   Another way to introduce an offset is using an exponential
   voltage-to-voltage converter (just drive the collector current into
   a resistor instead of the VCO capacitor), and use this voltage to
   drive a linear VCO. 

I'm going to claim that this approach will drift too much to be
worth it.

A third possibility is to build a seperate module that performs the
Vout = log(exp(Vcv)+Vdetune) operation.  But that's not elegant at
all. 

  -- Don