AW: quadrature osc

[Haible Juergen]

	>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. 

It looks similar at the first glance, but it's actually very different.
The second order differential equation uses a continuous voltage
to drive the integrators, thus all the amplitude problems you 
described. My oscillator (and the Tietze/Schenk circuit from
which I've derived it) uses comparators to drive the integrators.
There is also a regulation path (not for amplitude, but for offset
voltage of the 2nd integrator), but the trick is that this path will be
completely inactive when the initial error is corrected. No time
constants involved.
With the 2nd order differential equation method, you must make 
a compromise between response time of your regulation, and
waveform distortion. This is easy for high VCO frequencies, but
almost impossible for LFO applications.
The cos**2 + sin**2 method you mentioned is the one exception
to this dilemma. No time constants involved here, too.
I'd be interested in results from somebody who has tried it.

JH.