spiral pi coil

[godigit1]

For all who are measuring coil self resonance, keep this mental modal in mind. The oscillations of the coil discharge are based on the coil inductance and the total capacitance as seen by the coil.

Keep this mental model in mind.

If a coil has no capacitance, which is impossible, but good for a mental model, the fly-back pulse would discharge with no oscillations. Then, RX sampling could immediately occur. However, in real life science, the capacitance comes from:
1. Coil turn to turn parasitic capacitance
2. Coax cable from circuit board to coil capacitance
3. Coil to shield capacitance
4. TX circuit capacitance which is mostly MOSFET COSS capacitance that can be reduced by adding a series capacitor between the MOSFET and the coil.

Before the RX circuit can be turned on, the TX oscillations need to drop to near zero to allow the RX circuit to detect the target signal and not the end of the TX signal decay or any residual eddy currents in the coil wire itself.

The value of the damping resistor is directly related to the total capacitance seen by the coil to quickly damp the oscillations. Higher capacitance means lower damping resistor values; and lower capacitance means higher damping resistor values and potentially faster sampling.

Measuring the self resonance is a good way to build the fastest coil possible, meaning the earliest potential sampling.

How to reduce capacitance?

Reduce inter-winding turn to turn capacitance by the wire size, insulation dielectric constant, thickness, and winding technique.
Reduce coli to shield capacitance by using a shield with less area than a solid shield or space farther from the coil or both.
Reduce coax capacitance by using the lowest capacitance shield wire or shortest wire.
Add a capacitor in series with the coil and MOSFET output to reduce COSS.

Faster sampling means detecting smaller and lower Time Constants (TC) targets.


I hope this helps?

Joseph J. Rogowski

Hi Mr Rogowski,
Thanks for your reply. I appreciate the input and love the way you use visualization or the thought process. Most people do not express the value of thought and imagining the results versus just looking at the math. My problem has been that my imagination was uninformed. But Im getting the picture more and more thanks to all the input from members like your self.

Your fast coil article was my first go to when I started to learn hobby electronics. It lead to many experiments and one interesting shielding method.
That was i wound a bundle inside a braided copper shield I literally threaded each wrap through the sheild.

On SRF and sampling if I may ask?
If you have a SRF wave form that is completly clear of the coil in less than 10us could you sample that coil at ten Us without a damping resistor as the SRF oscillation is gone before first sample?
Here is a wave form of one of the spirals Ive wound. pretty much clear at 5us
This coil is 30 awg
no space 308Uh coil.
5.2 ohms
26 wraps.
33.664 Pf with 30 ince leads.



Best Regards.

[godigit1]

[QUOTE=Elliot;260528]Hi godigit1, I had an idea today after reading your post and reading other feedback from my posts. May i suggest using a little bit of copper foil tape in your shield near the cable gland area - say 1" or 2cm long, and then use either nickel shield or carbon paint (or graphite spray) for the rest of the coil shielding - ensure you over-spray the copper area a bit (say by 1cm) to ensure an electrical connection? That way, you can get the best of both worlds - you'll have copper to solder your shield drain wire to, and you'll have the benefits of the remaining shielding material![/QUOTE

Hi Elliot, Thanks for the input I had not thhought as copper tape as a attachment for the shield wire. It may be the way to go.
I was thinking of using a small spray of nickle like a dime in diameter and seeing if I can solder to that then use carbon for the shield.
I also found this product that looks interesting.

https://shop.evilmadscientist.com/productsmenu/831
Its metal impregnated versus a solid sheet. Double sided so you could attach to shield then wire then coat with a little more carbon.??
Just thinking.
Also thinking I can get away with imcomplete coil coverage by maybe using a fractal antenna design painted inside the coil cover for a ground shield just maybe.
Thanks,

[godigit1]

On SRF and sampling if I may ask?
If you have a SRF wave form that is completly clear of the coil in less than 10us could you sample that coil at ten Us without a damping resistor as the SRF oscillation is gone before first sample?
Here is a wave form of one of the spirals Ive wound. pretty much clear at 5us
This coil is 30 awg
no space 308Uh coil.
5.2 ohms
26 wraps.
33.664 Pf with 30 ince leads.



Looks like I can.
Ive got my test detector up and running finally. I hooked up the 390Uh test coil.
My damping resistor set up goes to 1645 ohms.
For this coil it seems not enough. It acts a little different than my other coils on damping.
I get a nice Knee as you damp the coil but I can keep cranking up the damping without it looking overdamped It just gets faster.

I have to make a set up with more resistance to check this out.

Heres the kicker When I removed the resistor set up to measure damping value I forgot to put it in and the next night I fired up the MPP and started waveing my nickle in front and I was like man its hitting better tonight for some reason.
Then looking at my board i noticed No damping resistor installed.
Ive done this before but usually the detector does not work due to the large amount of ringing at the 0 volt line very unstable.
On this coil the ringing is at shut off and almost gone before the sample window.


Here is a shot of the coil with no damping resistor.


Has anyone else experianced this ???
Thanks,

[green]

On SRF and sampling if I may ask?








Heres the kicker When I removed the resistor set up to measure damping value I forgot to put it in and the next night I fired up the MPP and started waveing my nickle in front and I was like man its hitting better tonight for some reason.
Then looking at my board i noticed No damping resistor installed.
Ive done this before but usually the detector does not work due to the large amount of ringing at the 0 volt line very unstable.
On this coil the ringing is at shut off and almost gone before the sample window.


Here is a shot of the coil with no damping resistor.
[ATTACH]47401[/ATTACH]

Has anyone else experianced this ???
Thanks,

A guess at what might be happening. MPP input resistor connects to amplifier - input, can act as Rd. I would connect the scope probe to the ground lead making a loop and hold near the coil to see if the coil damps similar to your amplifier out. Then disconnect one end of the input resistor(no damping resistor and no input resistor) to see if the coil oscillates.

[godigit1]

Hi Green,
Thanks for your reply,
Im not sure how you mean to make the test loop. Do you mean just loop the probe wire?
By input resistor i think you mean the 1 k on the preamp. If I remove this will the fly back harm my amps?
Thanks

[green]

Hi Green,
Thanks for your reply,
Im not sure how you mean to make the test loop. Do you mean just loop the probe wire?
By input resistor i think you mean the 1 k on the preamp. If I remove this will the fly back harm my amps?
Thanks

The input resistor connects the coil to the amplifier. If you remove it the amplifier can't see the coil volts so it shouldn't harm the amplifier. Leave the diodes on the input for safety.

including a picture of loop

[bbsailor]

Tips for measuring coil self resonance.

The output from a signal generator is typically about 50 ohms. Put a 1 meg ohm resistor between the signal generator output in series with the coil hot lead. Put the other coil lead to signal generator ground. Doing this frees the coil to easily reach resonance because the 1 million ohm resistor isolates the coil high impedance at resonance from being loaded down by the signal generator low output impedance. To keep the oscilloscope probe and leads from loading down the coils impedance at self resonance, put a 1 pF capacitor in series with the probe and the coil hot lead. Attach the probe ground to the common ground connection of the coil and signal generator. You can make a 1 pF capacitor, called a "gimmick" capacitor by twisting about 1 inch of insulated wire together and use the capacitance between the wires to again insulate the coil high impedance at self resonance from the oscilloscope probe capacitance.

Now that the coil is hooked up properly it is time to get your reading. Ensure that your signal generator can go above 1 MHz if you are trying to make fast coils with low capacitance, as your resonance may well be above 1Mhz. With the oscilloscope probe attached through the 1 pF gimmick capacitor, sweep the frequency and observe the oscilloscope screen and when you get near resonance you will see the sine waves peak. When you stop sweeping the frequency at the peak of the sine wave display, take note of the frequency on the signal generator. That is your coil's self resonant frequency and if you know the coil inductance you can go to on line electronics calculators, enter the coil inductance and resonant frequency and deduce the coil's capacitance. Keep notes of these results with each coil style, coil wire size, coil wire insulation thickness and coil wire insulation type (dielectric constant), shielding material and distance from the shield to the coil wire. Each variation will give you different capacitance levels and offer you good insight in construction techniques to reach your goals.

Just so you know what impedance range you are working in, do this calculation. Coil impedance is XL. XL = 2 pi X F (frequency) X L (coil inductance) A 300 uH coil is 300 with the decimal point moved 6 places left to get into the H range or .0003H. Now just multiply 6.28 (2 pi) X .0003 and X 1,000,000 (1MHz) and get 1,884 ohms. However, at resonance the coil impedance is very, very high due to the quality factor or Q of the coil which can be 10 to 50 times the coil impedance.
Now we are playing in the impedance range of about 19K ohms to about 94K ohms so you see how the 1 meg ohm isolation resistor and 1 pF capacitor allows the coil to do its own thing and allows you to accurately see it.

This is how science is done. It is not a magic belief system but something that can be replicated over and over once you understand what is going on. Accurate measurements don't lie! The fun part about learning is beginning to understand what is going on and how all the different things done in your latest coil design are related.

Joseph J. Rogowski

[green]

Tips for measuring coil self resonance.

The output from a signal generator is typically about 50 ohms. Put a 1 meg ohm resistor between the signal generator output in series with the coil hot lead. Put the other coil lead to signal generator ground. Doing this frees the coil to easily reach resonance because the 1 million ohm resistor isolates the coil high impedance at resonance from being loaded down by the signal generator low output impedance. To keep the oscilloscope probe and leads from loading down the coils impedance at self resonance, put a 1 pF capacitor in series with the probe and the coil hot lead. Attach the probe ground to the common ground connection of the coil and signal generator. You can make a 1 pF capacitor, called a "gimmick" capacitor by twisting about 1 inch of insulated wire together and use the capacitance between the wires to again insulate the coil high impedance at self resonance from the oscilloscope probe capacitance.

Now that the coil is hooked up properly it is time to get your reading. Ensure that your signal generator can go above 1 MHz if you are trying to make fast coils with low capacitance, as your resonance may well be above 1Mhz. With the oscilloscope probe attached through the 1 pF gimmick capacitor, sweep the frequency and observe the oscilloscope screen and when you get near resonance you will see the sine waves peak. When you stop sweeping the frequency at the peak of the sine wave display, take note of the frequency on the signal generator. That is your coil's self resonant frequency and if you know the coil inductance you can go to on line electronics calculators, enter the coil inductance and resonant frequency and deduce the coil's capacitance. Keep notes of these results with each coil style, coil wire size, coil wire insulation thickness and coil wire insulation type (dielectric constant), shielding material and distance from the shield to the coil wire. Each variation will give you different capacitance levels and offer you good insight in construction techniques to reach your goals.

Just so you know what impedance range you are working in, do this calculation. Coil impedance is XL. XL = 2 pi X F (frequency) X L (coil inductance) A 300 uH coil is 300 with the decimal point moved 6 places left to get into the H range or .0003H. Now just multiply 6.28 (2 pi) X .0003 and X 1,000,000 (1MHz) and get 1,884 ohms. However, at resonance the coil impedance is very, very high due to the quality factor or Q of the coil which can be 10 to 50 times the coil impedance.
Now we are playing in the impedance range of about 19K ohms to about 94K ohms so you see how the 1 meg ohm isolation resistor and 1 pF capacitor allows the coil to do its own thing and allows you to accurately see it.

This is how science is done. It is not a magic belief system but something that can be replicated over and over once you understand what is going on. Accurate measurements don't lie! The fun part about learning is beginning to understand what is going on and how all the different things done in your latest coil design are related.

Joseph J. Rogowski

This is how science is done. It is not a magic belief system but something that can be replicated over and over once you understand what is going on. Accurate measurements don't lie!

Which methods do you consider magic belief, maybe any but the one you suggested?

[bbsailor]

This is how science is done. It is not a magic belief system but something that can be replicated over and over once you understand what is going on. Accurate measurements don't lie!

Which methods do you consider magic belief, maybe any but the one you suggested?

If you can explain another method and someone else can get the same results, this is not magic. Look at how Eric Foster designed detectors more sensitive to small low TC targets by using the concepts of a "Lock-in" amplifier where multiple RX signals were integrated while the coil was over the target and many integrated samples could rise above the noise level rather than pumping more energy into the TX mode. This design concept had consequences of needing a fast PPS TX rate and controlling sweep speed to get enough samples to integrate while the coil is over the target.

Once you get a coil to operate at lower sampling times, the eddy currents in the wire itself, the coil shield, the coax cable solder joint to the coil wire may become unexpected targets and need a new design strategy. The more you tinker, the more you see how different and even unexpected pieces come together. If any measuring method produces repeatable results by anyone using the stated method, that is good science.

Joseph J. Rogowski

[green]

If you can explain another method and someone else can get the same results, this is not magic. Look at how Eric Foster designed detectors more sensitive to small low TC targets by using the concepts of a "Lock-in" amplifier where multiple RX signals were integrated while the coil was over the target and many integrated samples could rise above the noise level rather than pumping more energy into the TX mode. This design concept had consequences of needing a fast PPS TX rate and controlling sweep speed to get enough samples to integrate while the coil is over the target.

Once you get a coil to operate at lower sampling times, the eddy currents in the wire itself, the coil shield, the coax cable solder joint to the coil wire may become unexpected targets and need a new design strategy. The more you tinker, the more you see how different and even unexpected pieces come together. If any measuring method produces repeatable results by anyone using the stated method, that is good science.

Joseph J. Rogowski

Don't have a signal generator to compare my method with yours. Thinking of using a TLC555 to make an adjustable frequency square wave oscillator to excite coil. Tried with spice first. My method in spice, coil calculates 1MHz, circuit oscillates at 995kHz. Tried exciting coil with signal generator, coil calculates 1MHz, circuit peaks at 995kHz. What am I missing?

[godigit1]

Don't have a signal generator to compare my method with yours. Thinking of using a TLC555 to make an adjustable frequency square wave oscillator to excite coil. Tried with spice first. My method in spice, coil calculates 1MHz, circuit oscillates at 995kHz. Tried exciting coil with signal generator, coil calculates 1MHz, circuit peaks at 995kHz. What am I missing?

Hi Green,

Just last night started to get stuff together for a Signal Genny.
https://www.rmcybernetics.com/scienc...gnal-generator

This one is comparitor based you can even run two comparators at the same time.
I currently have mine set up as Pi front end Im adding fets and damping resistor locations so it can also be used as a dual pulsing front end.

You have engineering skills I cant begin to touch Im wondering what you think of this circut as a Signal gennerator and or front end used for coil testing?
Thanks Again

[green]

I recently built a new 3dss coil for my minipulse and I'm unsure if I measured the self resonant frequency correctly. from https://www.geotech1.com/forums/show...685#post260685 ____Appears measuring SRF isn't always easy.

I apologize for taking This is how science is done. It is not a magic belief system but something that can be replicated over and over once you understand what is going on. Accurate measurements don't lie! personal from reply #172 above. I had just suggested a different method than suggested in reply #172 a couple replies before and was thinking maybe it was magic belief. I'm learning metal detectors so I'm sure I make some measurement errors. Tried another spice simulation, closer to what bbsailor suggested. With C2 and R2 disconnected(scope probe)signal peaks at 195kHz not 988kHz with the probe. The difference errors don't matter for the SRF measurement for a PI detector. But if we want to make an accurate measurement it would be nice to know what can cause errors. Does anyone know or have a guess why SRF calculates 1MHz and measures 995kHz? Is there a reason spice could show an error when the actual measurement wouldn't have an error? Do the spice simulations need correcting?

[green]

Hi Green,

Just last night started to get stuff together for a Signal Genny.
https://www.rmcybernetics.com/scienc...gnal-generator

This one is comparitor based you can even run two comparators at the same time.
I currently have mine set up as Pi front end Im adding fets and damping resistor locations so it can also be used as a dual pulsing front end.

You have engineering skills I cant begin to touch Im wondering what you think of this circut as a Signal gennerator and or front end used for coil testing?
Thanks Again

I'm still wondering if my method, laying the probe loop on the test coil and positioning the test coil near a operating PI to excite resonance isn't just as accurate as exciting test coil with a signal generator. For me it is a lot easier even if I did had a signal generator. I usually have a coil connected to my TRT circuit with the scope connected to amplifier out. All I have to do is turn TRT on and hold test coil near TRT coil and read frequency on scope. Test coil isn't connected to anything. Think getting Q measurement is ok at amplifier out but not sure. Have been using probe loop for Q measurement. I have some coils without lead or shield that measure close to 4MHz SRF that I think would have more error using the generator method.

[godigit1]

I'm still wondering if my method, laying the probe loop on the test coil and positioning the test coil near a operating PI to excite resonance isn't just as accurate as exciting test coil with a signal generator. For me it is a lot easier even if I did had a signal generator. I usually have a coil connected to my TRT circuit with the scope connected to amplifier out. All I have to do is turn TRT on and hold test coil near TRT coil and read frequency on scope. Test coil isn't connected to anything. Think getting Q measurement is ok at amplifier out but not sure. Have been using probe loop for Q measurement. I have some coils without lead or shield that measure close to 4MHz SRF that I think would have more error using the generator method.

Im right with you there I think it is way easier to just excite the coil with a live pi.
I think there has been a misunderstanding, All of my Q calculations and SRF have been done by using a working pi and a dual field coil. Im actually taping the coil to the center of the dual field. Thats why you see the high voltage on my screen shots. I dont have a signal genny that goes over 1 meg . I want to get the rigol as it pairs with my scope but my mad money keeps getting sucked up by things like work truck repairs. I hate being my own mechanic
Ive been thinking of making one to double check The #s but I think the way we are doing it seems ok.
I have not tried any more testing since I did the first coil and had the no damping resistor in it.
I will try the loop method you have provided and see what I get. I think we will be close enough for the girls I go out with .

Im going to be moving back home to NV in three days to start my fall mountain job so ill be buried for a while I may not be able to respond in a timely manner so Im sorry for that.
I will try to get a couple more tests done before I have to tear down my workstation and pack up.

Mr Green Thank you I know Ive said that but you have helped me out emensly giving me things to think about, test procedures and evaluation and sent me off on study tangents more than once.

I must have missed the magic post I thouht it was the post where you teasingly said maybe I would find that magic coil.
So much goes over my head.
Thank you all I finally feel like im learning what I need to learn!!!!!

P.s this is all still magic to me.
and here goes the corny thing. Electricity does not make things work its the magic smoke. As soon as you let out the magic smoke no amount of electricity will get it going again. Where do they hide that smoke anyway?

[6666]

Where do they hide that smoke anyway?

Inside the LBT's

Im going to be moving back home to NV in three days to start my fall mountain job

Hurry back your contribution has been welcome
also I forgot to say thanks for your reply #165