[sdiy] What is a quadrature oscillator good for?

[Don Tillman]

   > Date: 11 Sep 2003 17:42:24 +0200
   > From: jhaible at debitel.net
   > 
   > But I wonder if this problem is solved with a quadrature _triangle_
   > core, such as proposed at 
   > http://www.oldcrows.net/~jhaible/tonline_stuff/hjfs2_qv.gif

I had considered this approach, but didn't use it for two reasons:

One is that the bipolar control voltage splits into an absolute value
current and a polarity, and any offset voltage in that area will show
up as an imperfect transition across zero Hz.

The other is that any offset voltages in the integrators will cause
the triangle outputs to slew at wildly different rates at frequencies
near zero Hz.  Possibly even slewing in different directions.

That situation is one of the main reasons for basing my VCO on a
trapezoid core; with a quadrature trapezoid only one capacitor slews
at a time.

   > I haven't checked this in theory, but in all these years that I
   > have this Frequency Shifter, the VCO _never_ got stuck anywhere.

Perhaps an offset voltage generating the control currents keeps the
stuck state from occurring.  That's a wild guess.

  -- Don

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