Re: [CAD_CAM_EDM_DRO] Capicitor tuning VCO/DRO

On Wed May 12 15:01:46 2004 Roy J. Tellason wrote:

>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... :-)

I've heard they were used in some of the earlier IBM clones with some other
part number, too.

Assuming we're still talking dual VCOs, and not 14 stage ripple counters, you
may have just made a sale. I wish they were the faster LS series, but at least
these would be good for initial testing. Care to part with some of them?


>> 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?


Not precisely, but I do have a formula for figuring with.

C=300/f Where C is in microfarads, and f is in Hertz.

From the info I've managed to glean about these chips, the original MC4024
topped out at about 20 MHz, and the 74424/74LS424 went up to 25/30 MHz
respectively.


>> 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...

I figure two kinds; Something to keep oil/coolent/swarf off the works, and
maybe some sort of shield braid over that to shield the vari-cap from outside
noise. This might not be needed for a hand crank mill, but might for a CNC
unit.

The basic idea is to use insulated mounts to attach the wire or tube to the
table, and the other to the platform the table is mounted to. The length of the
wire and tube being roughly equal to the table travel, and the output frequency
being counted for positional readout. Additional circuitry/counter chips for
setting the zero, etc.

Bill

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