programmer, Using Apple II for frequency generator - Page 2

Michael J. Mahon wrote:
>> He wanted to use it to "tune" a Tesla coil--he will want to use
> a continuously variable frequency to do that.


I need a continuously variable frequency generator up to 500khz, down
to 1khz.. This is to test/find the resonant frequency of the primary
windings
of the Tesla coil.

I knew there would be resolution problems with using the 6502, but I
didn't
know the gaps between frequencies would be so wide.

I have the circuit for a 555 variable oscillator... it would be nice if
I could
control the frequency via the Apple II..

Rich

Measuring the frequency with an Apple II is also a bit tricky, since
you have to count for a known period of time while observing the input
frequency. And, since you can't count and sample in less than 11
cycles, it's hard to measure frequencies that don't stay detectably
high and low for at least 11 cycles, or a period of 22 cycles, which
would be under 50kHz.

What you really need is what amateurs call a "grid dip meter"--a
variable frequency oscillator with a meter measuring the strength
of oscillations. If you hold it so that it couples to the circuit
or coil you want to test, then vary the frequency, there will be
a marked "dip" in the meter at the resonant frequency of the
tank circuit (whose depth and breadth provides information about
the Q of the resonant circuit).

The trick in making such measurements is to couple as loosely
as possible to the tuned circuit, since tight coupling can easily
"pull" the resonant frequency.

Google will provide lots more info on "grid dip meter".

-michael

Parallel computing for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."

Probably. I don't know the details.

--
Linards Ticmanis

Yeah... A few for sale, but very few! I think I'll buy one from
ebay... it seems like a handy tool

Rich

Be sure that the coils supplied cover the frequency range you need--it's
a little lower than most people's requirements.

-michael

Parallel computing for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."

This is easy enough to do. personally I would rig the 555 as a voltage
controlled oscillator and use a poor mans DAC to feed it from the
annunciators on the game port. you could use an op-amp as a summer or
simply an R-2R ladder to achieve this.

Of course, you'd need a lot more than 4 bits of resolution for your
application, perhaps 24, which could easily be accomplised with some
LS575's cascaded off one of the annunciators.

Even then, your resolution probably won't be fantastic, but it's a fun
little 2-4 hour project to muck about with, and you can have it up and
running in a few minutes on a breadboard.

Matt

I think it would be much easier to build up a little Colpitts oscillator
and then, if you really *must* use an Apple II somehow, use a counter
chain to prescale the frequency by a factor of 1024 or so and then time
the period with the Apple to calibrate it. ;-)

If he's looking for a resonance, it's best to excite the coil with
a fairly pure sinewave, otherwise he may detect resonances with
the harmonics and be off by some multiple of the fundamental.

Short summary--this is an analog problem, not a digital one.

-michael

Parallel computing for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."

Absolutely. I had constrained the problem to controlling an oscillator
with an Apple II, rather than actually suggesting an ideal solution to
the problem itself :-)

<att

Right, but a 555 is a *horrible* choice for an oscillator in this
application. Sinewaves are the way to go, not squarewaves or pulses.

And using a DAC to control the frequency of an analog timing circuit
is kind of like reaching behind your head to scratch your other ear. ;-)

It doesn't produce digitally repeatable results, and it doesn't allow
analog continuity of frequency control--the worst of both worlds.

Ah, well--I see that this horse is getting stiff... ;-)

-michael

Parallel computing for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."