thru zero VCO questions

[Scott Gravenhorst]

Don Tillman <don at till.com> wrote:
>   Date: Mon, 13 Sep 1999 14:48:52 +0200
>   From: Rene Schmitz <uzs159 at uni-bonn.de>
>
>   At 00:40 13.09.99 -0700, Don Tillman wrote:
>   >I'm thinking of having two integrators running through four states:
>   >
>   >   state 1: integrator 1 rises, integrator 2 holds at -Vref
>   >   state 2: integrator 1 holds at +Vref, integrator 2 rises
>   >   state 3: integrator 1 falls, integrator 2 holds at +Vref
>   >   state 4: integrator 1 holds at -Vref, integrator 2 falls
>   >
>   >This way you can run the oscillator forward and backward, however
>   >gradually, without hitting any funny states.
>
>   Well, this sounds interesting. But I'm not sure I got the idea. By
>   quadrature you mean that one integrator crosses zero while the
>   other one hits the reference?  And the above states are just for
>   the moments where the references are hit?!
>
>I didn't explain that very well...
>
>By quadrature I mean having two integrators generating trapazoidal
>waves 90-degrees out of phase; during the time one is slewing from the
>negative reference to the postive reference the other is holding
>still.  (You can convert them to your favorite waveform later.)  The
>four states are four quarters of the full cycle, and each state
>transition is simple and can occur in the forward or reverse direction
>depending on the input polarity.  
>
>It's sort of like the analog equivalent of counting in binary Gray
>Code: 00, 01, 11, 10.  (Gray Code is used for some binary counting
>applications; the order of the numbers is mixed up so that only one
>bit changes per transistion.)
>
>Like I said, I haven't worked out the details yet.

We trust that you will, Don...  (c:

This reminds me of the positioner feedback system used on some old
DEC disk drives.  A glass slide with laser etched lines was used to
generate a sine and cosine signal as the positioner moved.  Because
both sine and cosine were present, it was fairly easy to get speed
AND direction.

What Don is proposing is going the other way around, and I think he's
got something here.  With just sine (or just cosine), you can define the
"speed" of an oscillator (or a spot rotating on the edge of a circle),
but you cannot know the direction.  But use BOTH and voila, speed and
direction.

Question:  What effect would mismatches between the two integrators have
on harmonic content through it's range of frequencies?  My guess is none 
since each integrator will be charged and discharged at a given rate 
defined by a single control voltage applied to both.


-- Scott Gravenhorst
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