Dual coil for mpp

[dbanner]

I am thinking that one way to test the SRF of a PI coil( several inches in diameter), would be to use a sort of a dip meter principle, except that the dip meter would be modified to accomodate a large coil(several inches) for coupling to the PI coil. Dip meters usually come with small coils to attach to the dip meter, depending on the frquency band, but you could just the same wind a larger coil to attach to the dip meter circuit, so it can be coupled to the PI coil under test more easily. Once the dip meter dips, you then take it over to a scope or frequency counter to check the frequency of oscillation and that will be your SRF of your PI coil.
https://www.youtube.com/watch?v=_HwIL6mqq_k

[dbanner]

Here is the schematic for the Heathkit HD-1250 solid state dip meter, It's perfect for measuring SRF. A very simple circuit to construct. Q21 is a dual gate mosfet 3N201, 20V 50mA.
I think it might be useful to conduct some experiments with such a circuit. Not sure if it is possible to couple these small dip coils to a large PI coil, but worth a try maybe. I think accuracies to within 5% would be acceptable.

[dbanner]

Also, an LED could be used to indicate the "dip" or resonance instead of the micro amp meter.

[dbanner]

Here is another interesting circuit for coupling to a coil. I outputs a frequency equal in value to the resonant frequency of the coil it is coupled to. It seems it can be directly connected to an oscilloscope or frequency meter. Perhaps this circuit can be made into a sort of probe, such that when placed next to a PI coil, the SRF of the coil can be read.

[dbanner]

In calaculating SRF by using the sig gen method, I can forsee the scope probe and other elements affecting the final measurement when measuring SRF in the kHz range.
More experiments are required. Even more mathematical calculations which over complicates the task.
The extra capacitance undervalues the final measurement significantly.

[dbanner]

A coil could be wound for a conventional dip meter to permit operating down in the kHz range. this I think would give the most accurate reading of SRF, since you eliminate the extra capacitances introduced by the test leads when directly coupling the coil under test to both the sig gen and the oscillo probe.
To read the oscillating freq of the dip oscillator only requires coupling to the BNC of the freq meter via a short loop connected to the BNC.

[green]

A coil could be wound for a conventional dip meter to permit operating down in the kHz range. this I think would give the most accurate reading of SRF, since you eliminate the extra capacitances introduced by the test leads when directly coupling the coil under test to both the sig gen and the oscillo probe.
To read the oscillating freq of the dip oscillator only requires coupling to the BNC of the freq meter via a short loop connected to the BNC.

What is wrong with the PI excite, loop pickup method? Least effect of all the methods I've tried.

[dbanner]

By loop pickup method, I'm assuming you mean grounding the scope probe to the alligator clip? If so, then the capacitance of the probe itself. So depending on the scope you have and its probe, you see readings may vary abit.
Take for example 300uH coil, 100pF, calculated SRF 918.8kHz. Now increase the C to say 150pF, now the calculated SRF is 750.2kHz.
A significant difference. Working the other way around to calculate the self capacitance of the coil, well you can see the difference. The point is that both methods(direct coupling or loop method) are going to produce results which are not quite the same as coupling an open ended PI coil electro-magnetically to measure the SRF.
The dip meter will oscillate at precisely the SRF of the free floating PI coil( the PI coil is not connected to anything and is therefore electrically isolated and its parameters are not at all influenced by any external factors)

[dbanner]

The most accurate reading occurs when all extraneous cables and leads are eliminated from the measurement, and when dealing with microHenries and picoFarads, well.

[dbanner]

Upon reflection, the PI excite and loop pickup method would be good, and probably the best way to do it. The loop pickup is only coupled to the coil under test and not electrically connected to it. I get it. I was just exploring other possible ways.

[kt315]

Here is another interesting circuit for coupling to a coil. I outputs a frequency equal in value to the resonant frequency of the coil it is coupled to. It seems it can be directly connected to an oscilloscope or frequency meter. Perhaps this circuit can be made into a sort of probe, such that when placed next to a PI coil, the SRF of the coil can be read.

this is sure that Nokta does. they cover LC contour tuned on working frequency in demonstrate target . that gives hude 'depth' reply.
but that is just a trick of the frauds.

[godigit1]

I am thinking that one way to test the SRF of a PI coil( several inches in diameter), would be to use a sort of a dip meter principle, except that the dip meter would be modified to accomodate a large coil(several inches) for coupling to the PI coil. Dip meters usually come with small coils to attach to the dip meter, depending on the frquency band, but you could just the same wind a larger coil to attach to the dip meter circuit, so it can be coupled to the PI coil under test more easily. Once the dip meter dips, you then take it over to a scope or frequency counter to check the frequency of oscillation and that will be your SRF of your PI coil.
https://www.youtube.com/watch?v=_HwIL6mqq_k

Hi dbanner,
I had been thinking allong the same lines after watching another one of his videos.
Im also reading it can be used as a signal generator.
Here is a pretty detailed build.
http://users.tpg.com.au/users/ldbutl...esDipMeter.htm

Ive seen a couple different coils that were about 3 inches in diameter on the net seems it would be easy enough to make a custom coil for one.
Though I think green is correct as Im getting the same #s now that Im testing the correct way it would still be fun to play with.

[godigit1]

Ive finished my testing of the dual field as far a damping the coil goes.
I tried damping with both a damping resistor set up and with the calculated damping resistor from the patent formula.

Damping the inner coil with the damping resistor set up my value was 528 ohms.
the coil maybe was slightly underdamped but performed best there on small gold targets tests.
Final damping value for the two coils with this set up was 725 ohms.
this is the scope shot triggered from the Fet turn off so you can see actual coil speed.



I calculated the inner coil damping resistor at 454 ohms My series resistor set up came out to 447 ohms.
testing the coil with this value looked ever so slightly overdamped and target response was slightly deminished.
Final damping value using the calculated inner damping resistor is much better at 1.236k as well as coil speed is slightly faster.



I did allot of target testing both ways and end performance is so close depth wise it hard to tell but the damping resistor set up is maybe just a little more chatty.

The patent says the inner coil is isolated by the outer coil and indeed it is but I found a way to get a glimps at least.

With the 528 ohm set up i noticed if you hit the coil with a large target, for just a instant I would get flash of a underdamped resonate coil then it looked good.
Testing further with the 447 ohm damped coil, the resonation response was gone and like I had said the coil is less chatty and end performance as you can see by the scope shot is a bit faster.

I wanted to play with it a bit try to understand the coupling of the coils. It appears that after coupling, the inner coil was underdamped and I think thats the brief ringing I was seeing with the 528 ohm set up but only if you hit the coil with a bigger target would it show.

The patented method is of coarse the way to go.
Thanks All

[green]

Hi godigit, your scope pictures look good. Wondering how you get 454 oohms for the inner coil. Reply #97 you have 6.67MHz reply #65 135uH. Rd=pi*L*resonance, =2829 ohms.

[dbanner]

The formula in the patent is curious. Maybe it is tailor made for some coupling of the two coils.
The SRF you measured seems quite high. 454 ohm would correspond to a coil with much lower SRF.