Did I calculate the Self Resonant Freqency of my coil correctly?

[yatahaze323]

Hey Everyone,

I recently built a new 3dss coil for my minipulse and I'm unsure if I measured the self resonant frequency correctly.

Here's info on my coil, 24 awg teflon tin coated stranded wire, 8.5 inch 3dss coil, measures 370 uH.

Here's exactly what I did to measure the SRF,
1. I hooked up my coil to a function generator and to the oscilloscope.
2. I set the oscilloscope probe to 10x
3. I set the function generator to sine wave and adjusted the frequency until the wave form reached its max peak.
4. I then measured the frequency using the oscilloscope and I came up with 150 khz.

Does 150 khz sound correct or did I not measure this correctly?

Thank you everyone for your help!

[kt315]

something wrong. you have to sure get 550-600 kHz range res freq.

[godigit1]

Check out the new mirage thread .

Elliot has a thoughrough coil construction guide. He has a set up for SRF with a signal generator.
I have been using a working PI coil to Get SRF.

Im not totally sure how accurate using a live coil is it would be good to compare. If the live coil is accurate enough then its so much easier to do.
On a 3Dss ive tested im getting SRF in the Mhz range.

[godigit1]

something wrong. you have to sure get 550-600 kHz range res freq.

Hi kt315,
Is there a thread or documentation you could point me to on choosing 550 to 600 khz as SRF?

All my coils are way higher by my testing but testing the same way on a Pro coil built for the TDI it has a SRF of 500khz.
and a whopping 361 pf of capacitance.

Im thinking this coil has a cap in it .
SRF does not change with or without shield attached.
Thanks

[yatahaze323]

Thanks Kt315 & Godigit1,

I figured it was to low of a frequency to be accurate. I'll check out that mirage thread and see how they doing things.

Thanks again!

[Carl-NC]

1. I hooked up my coil to a function generator and to the oscilloscope.

Show how you hooked it up. Something's wrong, somewhere...

[kt315]

Is there a thread or documentation you could point me to on choosing 550 to 600 khz as SRF?
---
certainly i have to give you a link on ITMD page... but that absent. so just this comment.

---------------------
Posted by: bbsailor (---.proxy.aol.com)
Date: March 18, 2005 09:12PM


Eric,
I'll bet you had to do some pretty creative things to get the coils to break the 5uS barrier. I can only speculate about the coil parameters. With the best coax you have about 120pf in the 2.125m coax; about 100pf in the coil-to-shield capacitance, with a self resonance somewhere between 600 and 650 Khz on a 300 uH inductance coil. I would certainly like to know the coil details on this super fast machine? I had to be very careful with reducing all capacitance to get my coil working a 7.33 uS.
In previous posts you mentioned that about 10 uS is about the fastest practical salt water beach speed due to the response of the salt water. Now that you have worked this new PI machine at 5 uS, is it possible that we will be seeing a new breed of beach/prospecting machines?
Congrats on breaking the 5 micro-second barrier.
bbsailor

[yatahaze323]

Show how you hooked it up. Something's wrong, somewhere...

Hi Carl,

I'm not near my setup right now but I hope the attached drawing(as crummy as it is) shows how I connected everything.

Thank you for your help!

[kt315]

wrong. you must put 1MOhm resistor in serial to 'hot' wire end to generator. o-scope must be connected directly to coil.

[Carl-NC]

KT is right, you need a resistor to the sig gen. Optionally, you can also add a series cap to the oscope, probably won't make a difference unless you are trying to measure an aggressively fast coil.

Here is a diagram:

[yatahaze323]

Thank you KT315 and Carl, I'm gonna try again and see if I can get this right!

[Elliot]

I'll share the Mirage process and test set-up I use to measure SRF and CPar here as well, as its topic relevant. How to Construct the Mirage 300uH Mono Coil.pdf

Now, onto a related matter. My above-mentioned test coil (with shield, but not potted in the ABS shell) capacitance is 84.7 pF. The Canare coax cable is 15pF per foot (I have 2 metres attached right now for my test bench purposes = 98.4 pF). My total coil (potted) and cable inductance is 304.5uH. The SRF= 555kHz. The total calculated CPar is 270pF from CPar = 1/[(2*pi*SRF)^2 * L)]. Therefore, I can only deduce that the combining of the coil to the coax cable and the potted search head adds (270 - 84.7 - 98.4) = 86.9pF. I think Godigit1 has a good idea when talking about using expanded polystyrene foam and other plastics (Non ABS) for the search head filler/cover.

[kt315]

323, i read the connection of o-scope to a coil directly is bad idea and right circuit on this pic

[Elliot]

323, i read the connection of o-scope to a coil directly is bad idea and right circuit on this pic

This is better I believe:

[kt315]

This is better I believe:

[ATTACH]47424[/ATTACH]

better will be that gives above the resonance frequence value. 150kHz bad, 550kHz good, 1Mhz best.