logarithmic F to V, tuning ref

klund at MIT.EDU klund at MIT.EDU
Sun Jan 5 02:02:06 CET 1997


I've been thinking about building a logarithmic frequency to voltage
converted (kind of an inverse V/Hz VCO).  Imagine using a cheap Casio
toy keyboard as a CV front end!

What I've thought of is to build a PLL using an exponential VCO.  The
problem is since the VCO isn't linear (duh!), that the loop gain depends
on the operating point.  Thus, it is difficult to properly compensate
the loop for acceptible dynamics (like no overshhot, as in no "boing"
when changing notes...) at all operating points.  However, I think there
is a way to do it, but not easily.  Before I jump head long into working
on this, has anyone else solved/thought about this problem?  Is there
another way to attack this?

Also, I've been thinking about how to build a stable tuning reference.
I'd like to come up with a box that just produces a 440 hertz tone.  The
only way that I've thought of for real stability is to use a crystal and
count it down.  Searching through the Digikey catalog, I was only able
to find one crystal that was an integer multiple of 440, 11 MHz.  That
still requires a 25,000 division, which will need a 15 bit counter.
That's not too complicated, but does anyone have another idea?

And while on the tuning topic, for a V/Hz VCO, what do you guys usually
tune zero volts to?

--> Kent.



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