FKK coils, free beach movement :)

[Davor]

I was told once that dfbowers successfully uses the unshielded coils, but apparently it is only a half way correct. I found the topic that goes deep into the wet grass and grounding/non-grounding the coil, and in fact shields are there: http://www.geotech1.com/forums/showt...ounded-RX-coil

The point that was established there was that the coil supplied to a differential amplifier performed better in wet grass conditions, confirming the famous wet grass as a common mode phenomenon. Nothing more.

There is also a point that differential amplifiers are noisy. If an op amp is considered a preamp choice, the least noisy would be a non inverting configuration with carefully chosen low impedances in a feedback path. But what is the use of (laboratory) low noise if asymmetry promotes common mode noise to enter a rig freely?

Instrumentation amplifiers are considered bad because of two noise sources (the inputs) in a signal path, but in fact it is true only if these noise sources are bigger than the source resistance noise. Otherwise instrumentation amps are just fine because of their superior common mode rejection performance. Given all the possibilities of coil configurations, an instrumentation amplifier approach seem a most flexible one - the one that can accommodate just about any polarity/grounding/shielding/centre tapping you can think about. I'll stick to the instrumentation amp solution.

[simonbaker]

You forgot the coil resistance, so there is always some residual common mode with a center tap. In my case it works well enough. It can be much better with some real balanced preamp.

Yes, some resistance, but not frequency-dependent, which was my concern about center-tap grounding. I was worried that high-frequency, high-voltage common-mode signals might not be well grounded, but if they actually "cancel" the inductance, then that is less worry. I thought it was an interesting point you made in favor of center-tap grounding.

-SB

[Jerry]

You do not need to twist the strands because of very long length of the wires, so you can't make a significant error in length this way. The best possible configuration I can think of would be a real litz weave because of the lower capacitances achieved this way, but in that case you'd have to make it quadrifilarly and connect wires accordingly ... too much to do with not too much to gain.
Anyway, to get a center tapped coil you need to connect b. and c. to form a tap. Coil ends are a. and d.
The easiest way to picture it is like this:
a. >-------------------------------------------< c. tap b. >-------------------------------------------< d.

That makes sense. Thanks

Jerry

[Davor]

... perhaps the mutual inductance coupling again creates a low inductance path to PCB ground.

You forgot the coil resistance, so there is always some residual common mode with a center tap. In my case it works well enough. It can be much better with some real balanced preamp.

I picture two wires wound parallel and end c. and d. connected to make the center tap and ends a. and b. being connected to the cable to the detector.

a. >-------------------------------------------< c.
b. >-------------------------------------------< d.

You do not need to twist the strands because of very long length of the wires, so you can't make a significant error in length this way. The best possible configuration I can think of would be a real litz weave because of the lower capacitances achieved this way, but in that case you'd have to make it quadrifilarly and connect wires accordingly ... too much to do with not too much to gain.
Anyway, to get a center tapped coil you need to connect b. and c. to form a tap. Coil ends are a. and d.
The easiest way to picture it is like this:
a. >-------------------------------------------< c. tap b. >-------------------------------------------< d.

[Jerry]

Well, mine doesn't, and it is not quite optimal at the moment - my preamp I currently use sucks. Winding coils bifilarly with a center tap reduces the most of such effects significantly.

Hi Simon,

Could you elaborate a little on the actual construction & winding of your coil I understand what bifilar winding are but do not understand the arrangement of the center tap or if the individual wire strands are twisted together or not prior to winding.

I picture two wires wound parallel and end c. and d. connected to make the center tap and ends a. and b. being connected to the cable to the detector.

a. >-------------------------------------------< c.
b. >-------------------------------------------< d.

Jerry

[simonbaker]

It is interesting to see how mutual inductance in case of a center tap actually cancels the coil inductance for a common mode signal.
With good coupling you are left with much less than 1% of the normal coil inductance, and a half of the coil resistance. In effect the stray capacitances with that half resistance form a high pass filter, so your observation of slow E field contributions is correct.

That's an interesting point I hadn't considered, so actually you're saying that high frequency common mode signals will be effectively grounded also (in addition to low freq signals) because there is no net inductance blocking the path to ground. You gave me credit where I didn't deserve it.

However, I could imagine a case where one side of the coil has a static discharge (high freq), and might not be effectively grounded. Perhaps that never happens, or perhaps the mutual inductance coupling again creates a low inductance path to PCB ground.

-SB

[Davor]

I have no idea about Tesoro. I've seen a short video showing their facilities and coil production. Their coil winding machine was making a monofilar coil using a self-adhesive wire so I'm pretty sure that particular coil was not center tapped.
It is interesting to see how mutual inductance in case of a center tap actually cancels the coil inductance for a common mode signal. With good coupling you are left with much less than 1% of the normal coil inductance, and a half of the coil resistance. In effect the stray capacitances with that half resistance form a high pass filter, so your observation of slow E field contributions is correct.

I'd say that a center tap is a cornerstone of any serious FKK Rx coil. It may not be perfect due to the coil resistance, but it works, and it is not prone to components tolerance problems.

[simonbaker]

You glow blue for a very short period, and after that your hair style becomes entirely different
There is a large static tension at about 10 000V/m on nice weather, but then again, air resistance is also very high, so don't worry. Problems with static electricity are related to the phenomena where that natural tension powers up effects similar to the condenser microphone against the surface, e.g. grass. So you swing your coil, and by virtue of distributed capacitance and a large electric field, the roughness of grass cover provides your coil with some common mode voltage.
It is not much of a question what to do IF something like that happens, because it just does. Much better question is how (else) we can deal with that, and why is such approach good, and even better - what other choices we may apply.

Regarding EMI, true, shielding will reduce influence of the E field component, but not the H component. For a vertically polarised signal our coils are just perfectly aligned with the axis perpendicular to the ground. Hence, for far field sources of interference the metal detector coil is a perfect antenna - shield or no shield. That kind of EMI can be cancelled by using differential coils, just like humbucker pickups do, however, such coils would require a 4 quadrant discrimination to work properly ... it is another story.

If the common mode voltage due to E-field is not varying too fast, then I'd agree the center tap ground of the coil might be all you need for that. Even the lopsided RX grounding of the original TGSL coil should keep the coil at PCB ground for slowly varying E-fields.

And I agree totally with your antenna description -- noise with that polarization is received just like the target signal, shield irrelevant.

But is there more to shields than that? What kind of shields do the latest Tesoro coils use, anyone? Wouldn't MD makers abandon shields if they added nothing? Or have they?

-SB

[Davor]

You glow blue for a very short period, and after that your hair style becomes entirely different
There is a large static tension at about 10 000V/m on nice weather, but then again, air resistance is also very high, so don't worry. Problems with static electricity are related to the phenomena where that natural tension powers up effects similar to the condenser microphone against the surface, e.g. grass. So you swing your coil, and by virtue of distributed capacitance and a large electric field, the roughness of grass cover provides your coil with some common mode voltage.
It is not much of a question what to do IF something like that happens, because it just does. Much better question is how (else) we can deal with that, and why is such approach good, and even better - what other choices we may apply.

Regarding EMI, true, shielding will reduce influence of the E field component, but not the H component. For a vertically polarised signal our coils are just perfectly aligned with the axis perpendicular to the ground. Hence, for far field sources of interference the metal detector coil is a perfect antenna - shield or no shield. That kind of EMI can be cancelled by using differential coils, just like humbucker pickups do, however, such coils would require a 4 quadrant discrimination to work properly ... it is another story.

[simonbaker]

But such solutions do provide a capacitive screen against any common mode electric field.

Exactly -- but not EMI, so serves as some confirmation that shields are not for EMI, along with Qiaohzi's statements.

What do you think about possibility of large static discharge to coil?

-SB

[simonbaker]

The coil shield is also known as a Faraday Shield. Although it can block electromagnetic interference to some extent, this is not the main purpose. It is there to block external electric fields (both static and non-static). However, it does not block relatively slowly changing magnetic fields; which is lucky because that's what a metal detector needs to be able to do its job. Any electric field that builds up around the coil, as it is moved over the ground, will be distributed throughout the shield material, and this cancels the effect of the field within the shield's interior. Hence, since the coil is within the shield, it is not affected by the external electric field. The reference to a capacitor has to do with the fact that the shield acts like one of the capacitor plates, whereas the ground is the other plate. Without a shield, the coil may also react when you bring your hand near the coil. In this case, the hand and the coil itself form a capacitor.

Right -- I just don't see how electric charge slowly building up on the coil being a problem unless there is a sudden discharge -- which then would seem a little hazardous to the circuitry, although people have claimed that op amps are protected against a big common mode spike. So if big discharges are possible in some circumstances, I'd think a shield would be recommended, because even a center-tap ground probably wouldn't help due to the high frequency of the discharge.

-SB

[Davor]

This amp has a gain of around 200 with a real bandwidth of at least 1 MHz and fairly good phase response. Noise is around 1nvrootHz with matched, thermally coupled Jfets for J1 and J3.

In reality it would not have that little noise with any realistic coil because of the coil resistance, and that is the ultimate limit of low noise design here. Otherwise, to reach the coil noise, you must match it to an equally low noise preamp, and that would be it. To reach any better you'd have to use thicker wire for your Rx coils.
This way or another, such low noise design will give you extra 10dB or so of system gain, and you'll reach maybe 25% deeper. Not bad at all for 3 bucks worth electronic parts.

...some commercial designs seem to use "paint-on" shields where are fairly high resistance and wouldn't shield EMI...

But such solutions do provide a capacitive screen against any common mode electric field. Air resistance is measured in teraohms, so every paint-on shield with several hundred kohms will do, and on the plus it would not produce any measurable eddy currents at any meaningful angle, while tin foil produces a lot of eddy current responses in ... tin foil and small gold angle range. Not quite good news for gold prospectors.

Without a shield, the coil may also react when you bring your hand near the coil. In this case, the hand and the coil itself form a capacitor.

Well, mine doesn't, and it is not quite optimal at the moment - my preamp I currently use sucks. Winding coils bifilarly with a center tap reduces the most of such effects significantly. With proper preamp that provides proper loading for both common mode and differential signal, and with considerable CMMR, will surely outperform any shielded coil for a simple reason of having no parasitic matters in close proximity.


Yet another thought - Tx coil is usually not balanced, yet it provides some swing against the system ground. That may account for the residual imbalance that you simply can't overcome by geometry alone. Besides, a second harmonic generated by asymmetric oscillator is a door for entering various problems of the PWM kind to your detector. I'll give this some thoughts.

[Qiaozhi]

Sounds fine to try, see how it works.

I've asked in several threads about exactly what shield is doing. Some people mention EMI suppression but I don't understand that because seems you'd also shield target signal same and not improve S/N -- also, some commercial designs seem to use "paint-on" shields where are fairly high resistance and wouldn't shield EMI. Wet grass mentioned -- perhaps center-tapped coil will handle that. Some people mention ground "capacitance" or something, not sure what that effect is. People swear by experience shield is necessary, and I'm sure many of those coil designs were center-tapped. But let's find out more by trying.

-SB

The coil shield is also known as a Faraday Shield. Although it can block electromagnetic interference to some extent, this is not the main purpose. It is there to block external electric fields (both static and non-static). However, it does not block relatively slowly changing magnetic fields; which is lucky because that's what a metal detector needs to be able to do its job. Any electric field that builds up around the coil, as it is moved over the ground, will be distributed throughout the shield material, and this cancels the effect of the field within the shield's interior. Hence, since the coil is within the shield, it is not affected by the external electric field. The reference to a capacitor has to do with the fact that the shield acts like one of the capacitor plates, whereas the ground is the other plate. Without a shield, the coil may also react when you bring your hand near the coil. In this case, the hand and the coil itself form a capacitor.

[simonbaker]

First off, I'm not joking.

I've gathered some initial experience with an unshielded coil and there is nothing wrong with it. You might have seen my coil in action, links to youtube are on IGSL page, and so far I did not notice any annoyance that would convince me to dress it.

My coil is center tapped and supplied to not so perfect frontend, hence all common mode troubles are tackled with by center tap alone. I have some goals on my mind that I wish to share here with intention to gather other enthusiasts with same level of optimism. Let me summarise what I wish to achieve, so far with VLF:
- unshielded coil;
- aperiodic operation - to promote less than perfect coils made by amateurs;
- low noise - of course;
- differential operation with good CMMR.

My "true differential op amp" was a failure due to the common mode asymmetry, and in meantime I reminded myself on some real differential solutions that take care of common mode as well.

So, shall we go FKK?

Sounds fine to try, see how it works.

I've asked in several threads about exactly what shield is doing. Some people mention EMI suppression but I don't understand that because seems you'd also shield target signal same and not improve S/N -- also, some commercial designs seem to use "paint-on" shields where are fairly high resistance and wouldn't shield EMI. Wet grass mentioned -- perhaps center-tapped coil will handle that. Some people mention ground "capacitance" or something, not sure what that effect is. People swear by experience shield is necessary, and I'm sure many of those coil designs were center-tapped. But let's find out more by trying.

-SB

[moodz]

.... no one listens on this forum when you try to tell them the advantage of differential coils / amps / EMI rejection ... you name it LOL




This amp has a gain of around 200 with a real bandwidth of at least 1 MHz and fairly good phase response. Noise is around 1nvrootHz with matched, thermally coupled Jfets for J1 and J3.

moodz.