JH's VCO, wavetable source

[Martin Czech]

Another idea:

Well, PLLs like 4046 can go up to 4-10MHz (at least I read a spec sheet
about that).  The problem is of course the "sproiiing" step response,
or lag.  Now the typical pll is set up in a way that the vco is totally
dumb, ie.  without closing the loop totally wrong. This need not be.
You can do a high frequency oscillator with say 10% absolute maximum
error to the ideal scale. That should be possible. If now a pll is added
I guess the the loop gain can  be much smaller, or in other terms the
corrective control voltage can be much smaller then usuall, because it
doesn't have to be full scale.  The wide band comparator will still try
to pull the vco to 0 Hz deviation.

This way the vco will lock to the (xtal digital divider) frequency, but
you can also do sweeps etc because the influence of the PLL is limited to
a small pull. This will of course change the sweep characteristic a bit,
I don't know if this will hurt. Maybe even the xor small band comparator
will work, giving less jitter.

m.c.

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