[sdiy] What is a quadrature oscillator good for?

[Don Tillman]

   > Date: 12 Sep 2003 17:06:53 +0200
   > From: jhaible at debitel.net
   > 
   > > This circuit really doesn't mind going through zero Hz,
   > > bouncing off zero Hz, or sitting at zero Hz.  Although sitting
   > > close to zero Hz does require trimming the OTA offset voltages
   > > accurately.
   > 
   > But doesn't the offset voltage change slightly with the amplifier
   > bias current? 

Ummmm, a little bit; yes, it does.  

That means that the OTA offset adjustment is optimized for a
particular value of OTA bias current (and thus for a particular value
of the exponential control voltage).

In practice I haven't noticed this to be a problem, but I can't say
that I've specifically looked for it yet either.  

   > You're avoiding the split into sign and magnitude,
   > but you "pay" for this with the need of precise 2-quadrant
   > multiplication, where "ordinary" triangle cores just need the
   > OTA for switching.

True enough.  But those areas where an OTA is less than perfect won't
have a negative impact on the musical use of the trapezoid VCO.

For instance, any OTA offset voltage will just be a small frequency
jitter on half cycles.  This won't be noticible at higher frequencies,
and near 0Hz the half cycles are pretty far apart anyway.

Another example is OTA nonlinearity.  That would affect the linear
control voltage only, while the accurate tuning is done with the
exponential control voltage input.

   > I'm still convinced your design is far superior to others
   > for audio rate thru zero FM; maybe even the first one that
   > really does the trick! 

Thanks!

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com