Capicitor tuning VCO/DRO

Hi folks;

I've got a question about tuning capacitance. I've had some thoughts about
doing a DRO or otherwise, positional feed back to servo, using a frequency
counter circuit, fed by a VCO. I've never been able to find a source for the
prospective chip for doing this, but if I do, I'd like to play with it.
Originally the chip came out as the MC4024, (Motorola chip, not CMOS), and was
later designated as the 74424/74LS424. Its freqyuncy range is zip to 25-30 MHz,
and while its a VCO, it can also be controlled across its full range with just
a capictor.

My thought is to make a vari-cap by way of taking some stiffish, springy, steel
wire like a guitar string or piano wire, insulating it, and running that down a
piece of narrow guage tubing. Capacitance can then be tuned by how much wire is
slid in or out of the tube.

Now here is the meat of the question. Whatever the range of capacitance this
would amount to, is there a way to tune the ends of the above discribed
vari-cap such that it would then be able to feed the proper full range of
capicitance to the chip, without running above or below the capacitance required
by the chip, in a reasonably linear manner? Maybe the capacitive equivalent of
an R2R network with one side going to ground? Another, (ordinary), vari-cap, or
other circuit element(s)? I'm pretty sure this would require shielding, which
might affect things, too. Thanks in advance.

Bill


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