[sdiy] What is a quadrature oscillator good for?

[jhaible at debitel.net]

> 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?


In Don's circuit, you just add the "sin" trapezoid and the "cos" trapezoid
to get a triangle. But you only can do this if you have quadrature
signals, of course.

I was wondering about Don's statement about a triangle core not giving
enough information for failsafe operation (paraphrasing). And while
I haven't understood all the exact details, this sounds very reasonable
to me. Just thinking about a "reverse" command slightly before or after
the "normal" trigger points does convince me, without looking at
all different cases.

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
(and based on Tietze/Schenk Halbleiterschaltungstechnik) ?
Here, the second variable (cos output) is also fed back to the
schmitt trigger. This is mostly done in order to prevent the
second integrator from "running away" from a tiny DC offset on its
input. But maybe this also solves the "reverse-near-trigger-point"
problem, as a pleasant side effect? I haven't checked this
in theory, but in all these years that I have this Frequency
Shifter, the VCO _never_ got stuck anywhere.
But I'm not using it as a audio rate modulated thru-zero-FM VCO,
so the transitions thru 0Hz are always slow. (And the "direction"
comparator oscillates, but gently & controlled, near 0Hz, which
doesn't hurt in the FS application.)

Now as I think about building a stereo Frequency Shifter, I also
consider to improove the circuit further. For one, I will
make the schmitt trigger and direction switching much faster,
to improove HF tracking. As a second idea, I think about
a HF tracking _compensation_ that allows a certain delay of the
schmitt trigger and other components. Maybe the relative slowness
prevents it from getting stuck? My first goal here is tracking
1V/Oct on _one_ side (positive frequency), and a smooth transition
thru zero. But making it go thru zero "gracefully" _and_ fast
would be an extra improovement, for use as a thru-zero-FM VCO
(independent of FS application). So Don's circuit is interesting
for me as well.

JH.


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