[sdiy] What is a quadrature oscillator good for?

[Grant Richter]

Hi Don, Jurgen,

After finishing up my design and really, really playing with the waveforms.
I opted to go with a thru-zero triangle core and dual precision clippers to
convert to trapezoids.

Ideally, a quadrature LFO could put out trapezoid (XY display as a square)
and triangle (XY display as a diamond) waveforms.

I have a theoretical question.

Is it easier to go from a trapezoid core to a triangle, or a triangle core
to a trapezoid?

I couldn't think of a trapezoid to triangle converter. But a modification of
a precision clipper shown at page bottom here:

http://www.musicsynthesizer.com/DIY/Grant/OTA.html

gave good results as a triangle to trapezoid converter. I used an NJM13700
with Rabc at 330K and buffered the voltage references to the diodes. Also
the 10K resistors shown around the 3080 can be 100K around a 13700, it's
just using the OTA as a voltage follower with a max output current of 100
uA, this cuts off sharply when it hits the diodes. The 1000pF and 220 are
required for stability, but don't seem to adversely affect slew rate up to 1
kHz.

Anyway, much respect and appreciation to you both for publishing many
thought provoking designs.

> I've performed similar tests, but not that one exactly.  I'll set it
> up and try it soon, and maybe get a scope photo.
> 
> I don't expect to find any problems though.  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.
> 
> -- Don
> 
> -- 
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com
> 
>