Advanced PI coils

[Bill512]

You're a bit confused, don't you?
Aziz

confused???

[Aziz]

confused???

Yep, confused.

You don't need an ultra low noise op-amp, when you have 1 k resistor at the inputs of it.
Aziz

[Aziz]

Here is an interesting paper regards to AI configurations:
UXO TARGET DETECTION AND DISCRIMINATION WITH ELECTROMAGNETIC DIFFERENTIAL ILLUMINATION
www.serdp.org/content/download/3749/59543/file/UX-1355-FR-01.pdf

Cheers,
Aziz

[Bill512]

Yep, confused.

My point was that ,in general, you can not name an "ultra low noise op-amp" to an "ultra low noise amplifier".

You don't need an ultra low noise op-amp, when you have 1 k resistor at the inputs of it.
Aziz

yes you need, even when you are forced by some reason, to use a relative big and "noisy" resistor.
The 1k resistor with the 1.3uV noise across the 100kHz ,it will give an output of 130uV after the opamp(gain of 100).
Consider an opamp with noise of 5nV/sqrt(Hz), then the total noise output from the opamp only, it will be at 160uV.
You see, is not only comparable to the resistor noise but for a common op amp, simply bigger.
For a opamp of 1nV/sqrt(Hz) the total noise output is 32uV. Between these two op amps there is a noise difference of about 130 uV.
Personally, i will be prefer to have 130uV less noise, for a few bucks more.

[Aziz]

My point was that ,in general, you can not name an "ultra low noise op-amp" to an "ultra low noise amplifier".


yes you need, even when you are forced by some reason, to use a relative big and "noisy" resistor.
The 1k resistor with the 1.3uV noise across the 100kHz ,it will give an output of 130uV after the opamp(gain of 100).
Consider an opamp with noise of 5nV/sqrt(Hz), then the total noise output from the opamp only, it will be at 160uV.
You see, is not only comparable to the resistor noise but for a common op amp, simply bigger.
For a opamp of 1nV/sqrt(Hz) the total noise output is 32uV. Between these two op amps there is a noise difference of about 130 uV.
Personally, i will be prefer to have 130uV less noise, for a few bucks more.

My definition of an ultra-low-noise op-amp/amplifier is:
Input referred noise density <= 1 nV/sqrt(Hz)

But if want to use the 1k resistor at your op-amp input, you're screwing up the op-amp's low noise feature. In this case, you can use the NE5534 further and save your money.

When I mean, you need an ultra low noise amp for some configurations, then it is so.
Aziz

[Ferric Toes]

That's a good EMI noise pickup configuration Eric. I love it.
Aziz

What is noise to a metal detector is music to a radio.

Most manmade noise is vertically polarised, so with the shielded coil oriented as shown in the photo, the e field noise pickup from sources of random noise is minimised, while the magnetic component of the wanted radio signals are maximised. Hence my radio is quieter. With a PI detector and a horizontal coil, luckily the radio pickup is minimised, but not eliminated.
Using a stacked noise cancelling arrangement will reduce r.f. pickup but not eliminate it, even if the coils were perfectly balanced. This is because there is a steep gradient (depending on the frequency) as one approaches the ground surface. Hence the lower coil in the stack will have reduced signal compared to the top one. This will get worse for greater coil spacing. (the trade off effect). Back to back D coils would be better, but there is a trade off there too.

One area I am going to look at is to make various shielded and unshielded AI coils (bifilar centre tapped ones also) and use them as radio antennas to find which gives the least pickup and best HF cancellation. I reckon that the damped and shielded RX coil response ought to roll off sharply somewhere in the region of 200 - 500kHz. The best arrangement would be coil/s plus preamp as used in a PI detector, with the preamp output into my 1941 radio which goes down to 75kHz. Let's see what we can hear!

With regard to conventional monos and DDs I suspect that the shielding used is often woefully inadequate and may even be making the r.f. pickup worse.

Eric.

[Aziz]

Detection Depth Improvement: Gain Compensated AI Coil Comparison vs. Mono Coil (Reference)

Hi all,

provided that, we can eliminate some EMI noise contribution due to anti-interference (AI) coil configuration and compensate for the target response losses with additional gain in the amplifier (due to less EMI noise), we can directly see the detection depth improvement for the specific coil configuration compared against our reference mono loop coil.

You can see, that the detection depth improvement increases at low noise floor levels much more. That's the reason, why you should reduce the noise floor level using an ultra low noise amplifier. There is the real magic buried.

I'll show you the comparison of selected AI coils and additional gain compensation vs. the mono coil.







And this is how to read the detection depth improvement between the mono coil (reference) and the other coil:


Cheers,
Aziz

[Aziz]

Zipped Excel Table of Coil Comparison

I'll add from time to time an updated version.
This analysis hasn't been finished at all. But you can enjoy the latest version of course.

CoilComp5(CoilCMP2)-03.zip

Cheers,
Aziz

PS: Ignore all Copyright (C) infos & rights. It's free for all of course!!!
"MadLabs Inc."(c)(tm)(r) first mad engineer is metal detectorists best friend.

[Aziz]

Hi all,

I'm open to other ideas and wishes now as I have covered most interesting coil architectures. Maybe I did forget something. Please, just remind me.

- I have left over the two-box principle (trivial and causing a lot of work, response is dependent on target orientation, would require a coil software upgrade).

- The tandem coil - what was meant by it? I need a description or a sketch so we know, we are talking about the same thing.

- Spiral coils: trivial but causing a lot of processing time (more wire elements required).

Anything else you would like to know and how a new coil performs?

Or should we invent a new coil together? Yes, we can do it.

Cheers,
Aziz

[authere]

Hi Aziz,

I played with coils a while back and came up with an anti interference coil of sorts,but i never really tested it fully and never had a dc ohm and inductance meter,but directly under power lines it was as quiet as a mouse and could detect nuggets

The coil bundle( cross section )was wound in clockwork fashion 12-2-4-6-8-10 0'c then alongside each one of those until all the gaps were filled so that the coil bundle was hollow and in a full circle

The current would flow around the outside of the coil bundle 3-4 times before exiting and the inductance comes from the induced inductance between each wire(damn thats hard to explain)

I don't know if your programme can handle that or if in fact you understand what i was trying to explain

I'll leave you's a while to try and take it in!!!...or not!!!

Ron

[Aziz]

Hi Aziz,

I played with coils a while back and came up with an anti interference coil of sorts,but i never really tested it fully and never had a dc ohm and inductance meter,but directly under power lines it was as quiet as a mouse and could detect nuggets

The coil bundle( cross section )was wound in clockwork fashion 12-2-4-6-8-10 0'c then alongside each one of those until all the gaps were filled so that the coil bundle was hollow and in a full circle

The current would flow around the outside of the coil bundle 3-4 times before exiting and the inductance comes from the induced inductance between each wire(damn thats hard to explain)

I don't know if your programme can handle that or if in fact you understand what i was trying to explain

I'll leave you's a while to try and take it in!!!...or not!!!

Ron

Hi Ron,

you surely mean the low capacitance coil weave technique (see examples below):
http://www3.telus.net/chemelec/Proje...l/New-Coil.jpg
http://www.vk2zay.net/article/file/865
http://img172.imageshack.us/img172/2733/silk1ir1.jpg

Yes, I can do such coil simulations as well. But they won't perform better than the mono coil. The inductance law remain the same of course. It's just a means of reducing the coil capacitance.

The benefit I see is, that the magnetic field strength is getting diluted over a wider region range due to a wider coil bundle, which is causing less disturbing ground magnetising effects.

Cheers,
Aziz

[Ferric Toes]

Hi Aziz,

Something I have thought about but never tried is the use of a soft ferrite cored inductor as RX2 in a AI coil system. This inductor would be a small diameter (less than 25mm) and more easily mounted above and on axis than another air cored coil. I have used ferrite cored coils for the main TX/RX mono type arrangement and they work well, provided you get the right grade of ferrite. What do you think?

Also, another unusual IB coil arrangement I saw in a book many years ago, was a ferrite rod with the RX coil mounted horizontally inside the outer TX coil. As the axes are at right angles, there is a point where coupling is zero. The book was about building your own detector which was a simple VLF design.

Eric.

[Tinkerer]

Hi all,

I'm open to other ideas and wishes now as I have covered most interesting coil architectures. Maybe I did forget something. Please, just remind me.

- I have left over the two-box principle (trivial and causing a lot of work, response is dependent on target orientation, would require a coil software upgrade).

- The tandem coil - what was meant by it? I need a description or a sketch so we know, we are talking about the same thing.

- Spiral coils: trivial but causing a lot of processing time (more wire elements required).

Anything else you would like to know and how a new coil performs?

Or should we invent a new coil together? Yes, we can do it.

Cheers,
Aziz

Tandem coil, what is meant by that:

Place 2 coils side by side.
TX in counter phase, so that the coil fields are aiding, the field vectors are north-south in one coil, south-north in the other coil. This causes the field vectors to go more horizontal at a certain depth, enhancing detection of vertical coins.

If we do this with 2 concentric, coplanar IB coils, the RX+ and RX- signals can be added to cancel the interference and the ground.

[Aziz]

Hi Eric,

Hi Aziz,

Something I have thought about but never tried is the use of a soft ferrite cored inductor as RX2 in a AI coil system. This inductor would be a small diameter (less than 25mm) and more easily mounted above and on axis than another air cored coil. I have used ferrite cored coils for the main TX/RX mono type arrangement and they work well, provided you get the right grade of ferrite. What do you think?

Also, another unusual IB coil arrangement I saw in a book many years ago, was a ferrite rod with the RX coil mounted horizontally inside the outer TX coil. As the axes are at right angles, there is a point where coupling is zero. The book was about building your own detector which was a simple VLF design.

Eric.

unfortunately, my coil software is limitted to air core coils. Ferrite cored coils would make the task very non-trivial.

Why not. Provided that, it picks up the same amount of EMI noise sure a winner due to its small size. But due to the different coil characteristics, I would suggest to use a matching network (of RX1 and RX2) to maximize the EMI noise rejection ratio. A big pitfall is, that the EMI rejection ratio is frequency dependent (due to different coil characteristics).

The second proposal:
The GB is going to be challenge. Ground noise could be high. Could be a hotrock detector.
I don't know. It should simply be tested.

Cheers,
Aziz

[authere]

Hi Ron,

you surely mean the low capacitance coil weave technique (see examples below):
http://www3.telus.net/chemelec/Proje...l/New-Coil.jpg
http://www.vk2zay.net/article/file/865
http://img172.imageshack.us/img172/2733/silk1ir1.jpg

Yes, I can do such coil simulations as well. But they won't perform better than the mono coil. The inductance law remain the same of course. It's just a means of reducing the coil capacitance.

The benefit I see is, that the magnetic field strength is getting diluted over a wider region range due to a wider coil bundle, which is causing less disturbing ground magnetising effects.

Cheers,
Aziz

Hi Aziz,

No, no,no, consider this, try thinking of using ribon wire where the wires are laid side by side, use this to wrap around the outside of a tube.

Then you should understand what i explained before

Ron