# Re: [TCML] HELP Oscilloscope coil tuning! BIG TOPIC!

```Andrew -

Charles Hobson's website provides very useful information on making Tesla coil measurements using an RF signal generator, oscilloscope, and frequency counter. Test setups include measuring primary and secondary resonant frequency, measuring primary/secondary coupling coefficient, and a means of viewing ring-up and ring-down characteristics.

See: http://myweb.tiscali.co.uk/teslatutorial/MEASUREMENTSPR.htm

For measurement of primary resonant frequency, add a 5 or 10K ohm resistor in series with the signal generator output to avoid possible overloading the generator.

Regards,
Herr Zapp

--- On Mon, 4/20/09, Andrew Robinson <teslamad@xxxxxxx> wrote:
From: Andrew Robinson <teslamad@xxxxxxx>
Subject: [TCML] HELP Oscilloscope coil tuning! BIG TOPIC!
To: tesla@xxxxxxxxxx
Date: Monday, April 20, 2009, 5:52 PM

Ok, so I just got back from the lab on campus. I was playing with the new
digital scopes which I am going to be able to use to tune the new coil im
building. Ya Universities! Heres the thing... I'm no stranger to coiling,
but a newbie when it comes to using the scope. Can someone very carefully
explain process to me... That is, correct what ever it is that im doing wrong.
First off visit this link on my server to see a diagram I made real quick in
paint to show you my setup.

http://host.atomiklan.com/tesla/scope1.png            (Note: The frequency on
the multimeter means nothing, just a visual aid)

Ok so in this first image, im attempting to obtain the operating frequency of
the secondary coil with toroid. Correct me if im wrong, but the toroid,
secondary coil, air, and ground acts like a inductor capacitor circuit hence the
frequency. The toroid forms a capacitor with earth using air as a dielectric. Is
it possible to measure the frequency of the coil without the toroid? Im assuming
yes... but pointless. Ok so to make replies simple and easy i'll number all
my questions at the bottom of this page.

Ok so back to the setup. Here are my understood steps to tune the coil...

1:    I should use the tesla geometry formulas for an inductor and topload to
get the general area of the frequency of the secondary and toroid so I dont have
to sweep the entire range of signal generator.
2:    Attach the signal generator to the bottom of secondary.
3:   Turn on below area of expected resonance.
4.   Hang oscilloscope probe near secondary coil.
5.   Raise frequency on signal generator and watch scope till the amplitude of
sine wave maxes. At this point this is the frequency of coil. If you pass this
by continuing to raise frequency, the sine wave amplitude should drop again till
you reach another resonance point.
6.   Read the frequency where sine wave was max on the multimeter. (The
multimeter was only in setup to get a digital readout of analog signal
generator.)

Ok, now that we have the secondary coils resonate frequency, its time for the
primary tank circuit.

Diagram: http://host.atomiklan.com/tesla/scope2.png             (Note: The
frequency on the multimeter means nothing, just a visual aid)

1:   Disconnect the transformer and any filiters. You should only have the
sparkgap, capacitor, and primary in the circuit.
2:   Bridge the sparkgap.
3:   Wire sparkgap, capacitor, and primary in series like normal.
4:   Attach oscilloscope probe to one end of primary and im assuming the ground
of probe to tap point??? You should use a 10K ohm resistor on probe in this
setup.
5:   Attach signal generator in same manner. One to end of primary, and other
end to tap point. Then adjust the tap point on primary till you achieve the
exact same frequency as above.

Ok so my questions.

1. Can the secondary frequency be found without the toroid? Or is this
pointless?

2. When getting secondary frequency, do i leave the alligator clip on probe
attached to ground, or nothing?

3. What settings should I set on the signal generator as far as strength?

4. When I tried testing this setup today with a test coil, when i varied the
frequency, the amplitude went up but never peaked at the "correct
frequency". It would just keep increasing no matter what setting I set it
on. What am I doing wrong here?

5: Am I missing something as far as using the scope correctly?

6. In regards to tuning the primary, is my setup correct as far as where to
connect everything?

7. Do I use the 10K built into probe or do i need another resistor?

8. The scope probe goes on one end of primary with... where/what does the other
connect to? Ground of probe to other end of primary? Other end of primary into
another port on scope?

Thanks everyone for all your help!

Hopefully this will clear up any confusion using scopes not only for me but
others as well.

Thanks,
Andrew Robinson

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