Q about using a coil to excite another ...

[GT Blocker]

I decided to ask this question as a seperate thread. Hope no one minds-
Can my ML Eureka Gold be used to excite my home made HH coil for impedence and resonant freq testing? If so, which freq would be the best to use, 6.4, 20, or 60kHz?
To cut it short, Can I use a VLF detector to find my resonance freq with my scope? BTW - a few details as to how to go about it and the scope settings I need to start out with would be nice.
FTR - I have a BK 675A LCR metrer, Kenwood 150mHz Quad trace scope, frequency meter, and BK precision 3300 pulse generator (& DMM,etc) to work with. I'm used to digital work and PIC chips (gotta love that 16F84!). Hope I didn't ramble on too long...ltrs, thx again

[Carl-NC]

Sounds like you have a pretty good bench, except for a signal generator. You really need this to find coil self-resonance, because you want to be able to sweep the frequency while looking for a peak (or null, depending on how you do it). Also, you want a sig gen that gets up to several MHz, as a good PI coil can have self-resonance beyond 1MHz.

Your BK3300 might work just fine for this, because you will still get amplitude change vs frequency, and you will also get a change in waveform shaping. Give it a try. If not, hit eBay and get a cheap 5MHz sig gen, which is incredibly useful otherwise.

- Carl

[GT Blocker]

'K, Carl, Here's what happened...

Hey again, Carl,
I built two identically sized and shaped coils (9"x3" - rectangular).
Coil '1' is made of approximately 38 winds of 30awg 7-strand Teflon coated.
The measured inductance of coil 1 is 530uH and the resistance measures 7 ohms.
Coil '2' is made of approximately 30 winds of 26awg 7-strand Teflon coated.
The measured inductance of coil 2 is 253uH, resistance is 2.1 ohms.

I am running my BK3300 pulse generator past my frequency counter into coil 1 to excite coil 2. I placed coil 1 on top of coil 2, hooked the scope to coil 2 and started sweeping. Here are my results:
466 kHz - shows .80v (wave form 'clean')
1.76mHz - shows 1.48v (wave form 'clean')
3.79 mHz - shows 3.4v (wave form 'clean')
+4mHz approx 3.4v (wave form shows beats/ringing no increase in amplitude).

Neither coil is shielded, ATM.
So here's the question, is 3.79mHz the resonance frequency of coil 2? (Assuming I didn't screw up my measurements...)
I'm guessing the highest amplitude without a screwy waveform is what I'm looking for? Anyway..I hope I got it right...
Carl? Anyone else?
Thanks, Gregory

[GT Blocker]

I think I got it...

Last night I woke up with an idea and got out of bed to fire up the lab.
I move the frequency counter over to the scope and the coil I was testing, instead of monitoring the freq on the 'exciter' coil.
The new results seem more believable:
1.83MHz @ 1.5v peak to peak. Clean wave form.
I think I was looking a harmonics or something before.Now there are no more odd wave forms and wild numbers.
Anyone know if that would explain my odd readings?
Did I finally get it right? Thanks, Gregory

[porkluvr]

too complicated

You are very fortunate to have a REAL lab. I am stuck with a "virtual" test bench (aka LTspice running on my PC).

Why do you use an exciter coil? There can be no doubt that the two coils will interact with each other, thus affecting your measurement. The self resonant frequency for L2 may actually be higher than what you are seeing. I looked at bbsailor's fig 12, and it looks simple and direct. Why not use the test setup exactly as bbsailor demonstrates?

http://geotech.thunting.com/pages/me...s/FastCoil.pdf

Think about this: aren't you adding stray capacitance to L2 by having a chunk of copper (that being L1) laying on top of it? There is also a mutual inductance between the two coils.

[Carl-NC]

When you test for resonance frequency, you want to test only the single coil. If you use an induction setup, you really have no idea what the response (and the peak)
is due to. Could be the TX coil, could be the RX coil, could be the mutual coupling.

- Carl

[GT Blocker]

Done (and interesting)

Ok, porkluvr and Carl,
I reconfigured the test set-up to the one in BBSailor's fast coil article. I started this thread about one coil to another, but as you must have already known, things get a bit weird. However, the results are interesting.

As per the first test setup I had in the thread, the mean of the results appeared to be 1.76 MHz.

My second test setup gave me 1.83 MHz.

The BBSailor setup yielded 1.7 MHz.

Though I'll trust the final result, it is interesting that they all hover near similar values. Strangest of all to me is that the first test setup, with the wildest results, had one very close value to the last (proper) setup.
weird.....Thanks for the help and input! Gregory