Re: Help Finding Overtones, Again

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Phil LaBudde wrote:

> I obtained another hand-me-down HP oscillator, this time a
> model 220A Square Wave Generator that goes to 10MHz.

Well, that ought to do the trick.

> Wimpy output, at only about 1V P-P or so...

Should be fine for this job.

> Anyhow, I found NO evidence of ANY overtone when using
> it to base drive the secondary coil. I.e., voltage across
> the generator stayed absolutely the same with or without
> the coil as a load.

> ...Despite all the ringing, I expected SOMETHING to happen
> in the 1-2MHz range you had predicted.

Yes, it will be happening, but you'll have to apply the scope
probe directly to the top of the coil.    This will alter
the frequencies a little, but they'll stay in the same ballpark.

You should see a nice pure sine wave on the scope as you sweep
the generator through each overtone, peaking at a few tens of
volts at the coil top.  As each signal appears, you can use the
finger sliding trick that Antonio mentioned to find the voltage
anti-nodes.  They're the points on the coil where the response
is most altered by application of a finger.

If your scope has an external trigger option, feed the trigger
input direct from the HP220.

Once you've located the resonances with the scope directly
attached to the coil top, you can then try the probe hooked
up to a bit of wire placed a foot or so from the coil top.
The signal will be less and you'll have to crank up the scope
gain to compensate, but now the resonances should be close
to the predicted values - and somewhat sharper.

That HP220 sounds like a useful piece of kit. With square
waves to 10Mhz you'll have harmonics to VHF so it'll come in
handy for many things.

As regards monitoring the drive voltage as you've been doing,
you may not notice the small drop in drive voltage as you
pass through the resonances.  The coil base impedance will
likely be several hundred ohms so if the generator is offering
50 ohms or less, the coil will not be pulling the generator
output voltage down very much.  You may see no more than
a slight change in the waveform as you pass through the
resonant frequency.

If still no luck, lay out a square metre of aluminium foil
beneath the coil to act as a ground plane.  Connect it to
the generator ground terminal.  Without this, presumably the
returning displacement current of the coil's E-field is having
to fight its way through the room/building to the mains earth
in order to get back to the generator.  This additional impedance
(picture a complicated RC network formed from tables, chairs,
carpets, floorboards, etc) is effectively in series with the coil
and generator, further obscuring the coil response.  In those
circumstances the foil sheet offers a direct return path for the
coil capacitance.

--
Paul Nicholson
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