quadrature osc

[Martin Czech]

Yesterday I grabbed into a book shelf and found an old Elektor Audio
Special: Low distortion sine generator. The idea was to set up a second
order differential equation (ie. state var filter). It looks very
much the same then J.H.'s triangle quadrature oscillator. But it gives
directly cos and sin function.  Of course, there is a problem: It needs
amplitude stabilisation.  This can be done by introducing a nonlinear
resistor into the damping path, thus giving some distortion. OTOH one
could use the following equation: cos(x)**2+sin(x)**2=1. Ie. squaring
cos and sin output and adding this will give a constant value that
is only dependend on the sin and cos amplitude, ie. an instanteneous
measure of the amplitude. It should be therefore possible to regulate
the amplitude (with vca). There was once a hybrid "chip" that exactly
did this.  The problem is of course the squaring, any errors here will
introduce a second order harmonic into the pure sin/cos output.

Any experiences with this old cicuit out there.

m.c.