Re: [CAD_CAM_EDM_DRO] Capicitor tuning VCO/DRO

On Wednesday 12 May 2004 02:11 pm, Bill Vance wrote:

> 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.

I have some of those chips (the original numbers), but have never done
anything with them. They were used in later production runs of the Commodore
64 computer... :-)

> 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.

This is reminding me somewhat of the old Hammond organs, which used something
of the sort to control pedal volume. An actual variable capacitor was also
used for the expression (volume) pedal for the whole organ.

> 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?

This is getting into RF design. :-) It should be possible to trim both ends
of the range, but to have it be linear in between is going to be more
difficult.

> Maybe the capacitive equivalent of an R2R network with one side going to
> ground? Another, (ordinary), vari-cap, or other circuit element(s)?

Putting one in parallel is going to give you your minimum capacitance.
Putting something else (fixed or trimmer) in series is going to set your
maximum, which is going to be _less_ than either of the series elements
(just like resistors in parallel, you can use the same formula).

Do you have any sort of values in mind for what the acceptable range is going
to be?

> I'm pretty sure this would require shielding, which might affect things,
> too.

If it does require it, then it sure will -- it'll likely increase stray
capacitance all over the place. In any event, it's going to require some
mighty careful construction...