|
|
Post by antigua on May 22, 2017 0:47:55 GMT -5
This is a portion of this post guitarnuts2.proboards.com/thread/7939/donlis-telecaster-pickups-analysis-review , focusing on just an eddy current reduction experiment within that post... Ken Willmott did some research on the effects of eddy current losses in the context of an electric guitar pickup, and found that eddy current losses can be almost entirely eliminated by making cuts in the cover that bisect the circular pattern that the eddy currents take when the electric current moves around in the conductive metal in response to the magnetic field. There are various cut patterns that can be employed, all which would result in similar results. Since the Tele neck pickup came with a brass cover that induced a high degree of attenuation, it was a good time to try this mod out, so using a jeweler's saw, I put one cut over each pole piece, as well as one cut that extends all the way to the bottom of the cover, as seen below. It is necessary for both cuts to exist; the cuts over the pole pieces impede eddy swirls over the pole pieces, while the longer cut impedes swirls that move around the circumference of the cover. Again, thanks to Ken Willmott for making this discovery and documenting it well. Update: it turns out that every slot must extent all the way to the bottom of the cover. See posts below for details. 
I then tested the pickup again, without the cuts, with the cuts, and with no cover at all:
Stock cover, unloaded: dV: -3.9dB f: 9.70 (black)
Stock cover, loaded (200k & 470pF): dV: 0.0dB f: 4.52 (red)
Slotted, unloaded: dV: 12.0dB f: 10.4kHz (green)
Slotted, loaded (200k & 470pF): dV: 7.2dB f: 4.78kHz (gray)
No cover, unloaded: dV: 13.0dB f: 9.26kHz (pink)
No cover, loaded (200k & 470pF): dV: 6.5dB f: 4.41kHz (black) The results show that the slot cuts increase the resonant peak (green line) to nearly the same elevation as if you had no cover at all (pink line). The lower two lines, the black and the red, are the plots of the brass covered pickup, and it's plain to see that they fall far below the other sets, with the cuts and without a cover. This proves Ken Willmott's research correct, that strategic cuts in a pickup cover can nearly eliminated the associated eddy current losses. The 0.8dB difference between the slot cut and no-cover plots is insignificant by itself, but if you look closely, you'll notice that the "slot cut" plot line (green) is different than the coverless plot line (pink) in two ways, 1) the peak frequency is a little higher, because the cover decreases the inductance slightly, and 2) the downward slope of the slotted cover (gray) is not as steep as the drop off of the uncovered plot (black), and this is true for both the loaded and unloaded plot lines. This means that the slotted cover probably makes the pickup sound a bit brighter, compared to having no cover at all. The slotted cut pattern appeals to me, because the cut lines run parallel with the guitar strings, and are therefore less visible. The ironic thing is that these cuts more effectively reduce eddy current losses than the "open" Tele covers, such as that seen on the Lollar Royal T, while being far less visually obstructive. The problem with the "open" style cover is that there is still electrical continuity around the periphery of the cover, meanwhile the top of the cover has removed far more metal than is necessary. |
|
|
|
Post by JohnH on May 22, 2017 16:27:47 GMT -5
This is a nice discovery, and maybe a product opportunity.
What's your impression of any change to the effectiveness of shielding to the pickup? Im thinking particularly of reduction of transient buzz rather than 50/60 hz hum.
Shielding of electrical gear by Faraday cages also depends on allowing circulating continuity, which would be interrupted by the slices. But the PU would still have the benefit of being surrounded by a grounded enclosure and im not sure what effect is most impoertant.
It would be great if your design could tick three boxes:
1. Sparkles like an open single coil
2. Benefit of reduced noise like a standard tele neck PU
3. Still looks like a Tele cover, with some intriguing cuts to discuss with guitar nerds durring the set break.
|
|
|
|
Post by antigua on May 23, 2017 11:06:52 GMT -5
This is a nice discovery, and maybe a product opportunity. What's your impression of any change to the effectiveness of shielding to the pickup? Im thinking particularly of reduction of transient buzz rather than 50/60 hz hum. Shielding of electrical gear by Faraday cages also depends on allowing circulating continuity, which would be interrupted by the slices. But the PU would still have the benefit of being surrounded by a grounded enclosure and im not sure what effect is most impoertant. It would be great if your design could tick three boxes: 1. Sparkles like an open single coil 2. Benefit of reduced noise like a standard tele neck PU 3. Still looks like a Tele cover, with some intriguing cuts to discuss with guitar nerds durring the set break. This is really Ken Willmott's (stratotarts) design. He made a humbucker slot cut version already. A difference with the Tele neck is that you need the extra cut that goes all the way to the bottom, but it's apparently not necessary for humbuckers.  As for the shielding efficacy of a slot-cut cover, I'm not really sure if the shielding is decreased by some percentage proportionate to the decreased surface area, or what. I imagine it's a lot better than an "open top" style cover, but slightly worse than an intact cover. I'm certain the slots due wins back "sparkle", because we know what causes sparkle to be lost, eddy currents, and we see that they've been mostly eliminated after having added the cuts. Normally I'd leave it as is and let the data speak for itself, but I'd like to install this in a Tele and see if I can hear that softer attenuation past the resonance. I did another experiment here guitarnuts2.proboards.com/thread/7914/impact-resonant-peak-tone-pickup where I took a guitar sound sample and applied different slopes of attenuation, and found that more gradual slops have a clearer, more open sounding treble, where as a sharper drop off causes a smothered, nasal high end, like when you cup your hands over your ears. For reasons that are not clear to me, the slotted cover reduces the slope (compare the green and pink lines), and so it's possible that the slotted cover might actually make the neck pickup sound a little nicer than having no cover at all. |
|
|
|
Post by ms on May 23, 2017 13:33:32 GMT -5
This is a nice discovery, and maybe a product opportunity. What's your impression of any change to the effectiveness of shielding to the pickup? Im thinking particularly of reduction of transient buzz rather than 50/60 hz hum. Shielding of electrical gear by Faraday cages also depends on allowing circulating continuity, which would be interrupted by the slices. But the PU would still have the benefit of being surrounded by a grounded enclosure and im not sure what effect is most impoertant. It would be great if your design could tick three boxes: 1. Sparkles like an open single coil 2. Benefit of reduced noise like a standard tele neck PU 3. Still looks like a Tele cover, with some intriguing cuts to discuss with guitar nerds durring the set break. A tele neck pickup has no magnetic hum cancellation, and so I would expect magnetic hum to be the determining factor rather than electric fields. So decreasing the effectives of the metal shield might not make much of a difference. For slotting a hum bucker cover, the side slot is not necessary. (The current all the away around the sides cancels because the coil are out of phase.) For the tele neck cover, I think it is necessary to have just one slide slot; that is no slot around on the other side. In that case the charge can go wherever it wants, and there should be no significant loss in shielding for audio. Not sure if there is a slot on the other side of Antiguas's cover. (Probably not; it would then be a two piece cover.) |
|
|
|
Post by antigua on May 23, 2017 15:07:00 GMT -5
There's just one side cut.
I also tried measuring the peak with only the side cut, and only the top slots (with another brass cover that is not pictured), and in both cases, the peak only increased by about 3dB. Two two cuts together caused it to shoot up 15dB, on par with no cover.
|
|
|
|
Post by stratotarts on May 28, 2017 22:37:34 GMT -5
There's just one side cut. I also tried measuring the peak with only the side cut, and only the top slots (with another brass cover that is not pictured), and in both cases, the peak only increased by about 3dB. Two two cuts together caused it to shoot up 15dB, on par with no cover. That's really interesting and a little unexpected that not every slot needs to go all the way down. It certainly improves the mechanical strength compared with cuts all the way. Thinking about it, I guess it does meet the current path criterion. So I will think about possible changes to the half slot version I made:  |
|
|
|
Post by stratotarts on May 29, 2017 8:20:41 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this?
I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count.
|
|
|
|
Post by Charlie Honkmeister on May 29, 2017 10:20:55 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this? I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count. If there's any tonality or string amplitude difference because of the slot location, then there might be some interesting experimentation to be done with different number, location, and even depth ( less than full depth top-to-bottom slots ). Bill Lawrence was of the opinion that eddy currents are like seasoning in cooking, and aren't entirely bad. If you're on to something here where you can potentially not just eliminate, but selectively introduce, eddies up to maybe even a string-by-string basis, you just might have something really cool to further investigate. Might be worth experimenting a bit more to find out. -Charlie |
|
|
|
Post by Charlie Honkmeister on May 29, 2017 12:02:02 GMT -5
If you wanted to go this way, one idea would be to use a plastic pickup cover, and adhesive copper tape, to try different slot configurations, instead of all that jeweler's saw/Dremel work.
|
|
|
|
Post by antigua on May 29, 2017 12:46:43 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this? I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count. I had assumed one side cut was enough, because I was under the impression the current needed perfect loops in order to flow, and the one cut was enough to break the loop around the outside of the cover. Another point, which I forget if I mentioned before, is that a tiny little electrical connection at the bottom of the single cut caused all the benefit to be lost, which was significant because it would make the cover stronger if the cut could stop short, at the base of the cover. Maybe you're right that each pole piece can benefit from a full cut, but that this only "cleans up" a smaller portion of the remaining eddy currents. I'll test this again later. |
|
|
|
Post by antigua on May 29, 2017 12:48:41 GMT -5
If you wanted to go this way, one idea would be to use a plastic pickup cover, and adhesive copper tape, to try different slot configurations, instead of all that jeweler's saw/Dremel work. Not a bad idea. I ordered a handful of brass covers from China. They haven't arrived yet, but I'll have an ample supply soon. |
|
|
|
Post by stratotarts on May 29, 2017 15:55:44 GMT -5
If you wanted to go this way, one idea would be to use a plastic pickup cover, and adhesive copper tape, to try different slot configurations, instead of all that jeweler's saw/Dremel work. If only it were that easy. But I have seen in one circumstance that the tape doesn't behave at all like the brass sheet. I examined a humbucker where both coils were wrapped with the copper tape. Surprisingly it only made a few dB difference. From what I've seen with the covers, the same geometry in brass would produce a radical change. I cut the tape in place to prove this. My research was done with thick copper wire, which always conducts reliably. I believe the copper sheet is too thin. |
|
|
|
Post by stratotarts on May 29, 2017 16:05:07 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this? I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count. If there's any tonality or string amplitude difference because of the slot location, then there might be some interesting experimentation to be done with different number, location, and even depth ( less than full depth top-to-bottom slots ). Bill Lawrence was of the opinion that eddy currents are like seasoning in cooking, and aren't entirely bad. If you're on to something here where you can potentially not just eliminate, but selectively introduce, eddies up to maybe even a string-by-string basis, you just might have something really cool to further investigate. Might be worth experimenting a bit more to find out. -Charlie Indeed you can. For the humbucker slots, since they have been tested short and long, I am pretty sure you that you could tune each string individually in that way. Depth has already been ruled out by experimentation. I tried cutting partway through until bumps were visible on the chrome side, but there was no effect that I could see. Location is pretty much guided by the poles. You can find a strong indication of that in my research document - look for the "offset loops". |
|
|
|
Post by antigua on May 29, 2017 17:30:58 GMT -5
I also noticed that even a slight amount of metal bridging the slot caused the cut to fail, even little shavings that were left behind by the saw, so it was important to make sure the slots were completely void of left over debris.
On the matter of depth, here's something I can try when I get my 12 brass covers in; take a belt sander to some of them and see if the attenuation decreases with thinner layer of brass. I'm not expecting that it will make a huge difference, but conventional wisdom is that thickness makes a difference. Maybe thickness is more important when dealing with low conductivity metals, such as nickel silver.
|
|
|
|
Post by ms on May 30, 2017 12:32:57 GMT -5
I also noticed that even a slight amount of metal bridging the slot caused the cut to fail, even little shavings that were left behind by the saw, so it was important to make sure the slots were completely void of left over debris. On the matter of depth, here's something I can try when I get my 12 brass covers in; take a belt sander to some of them and see if the attenuation decreases with thinner layer of brass. I'm not expecting that it will make a huge difference, but conventional wisdom is that thickness makes a difference. Maybe thickness is more important when dealing with low conductivity metals, such as nickel silver. A very thin conducting layer reduces eddy currents because of the higher resistance. On the other hand, making it thicker will have no effect past a certain thickness because the skin effect keeps the current near the surface. Skin depth is a function of frequency, decreasing with increasing frequency. Adhesive copper tape has a very thin layer of copper, and so it makes sense that eddy currents would be reduced. |
|
|
|
Post by antigua on May 30, 2017 21:29:53 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this? I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count. You're right, the cuts need to be present for every single pole piece. There are much higher eddy current attenuation over the slots that don't extend through the base of the cover. I'll mention this in the TDPRI thread also. |
|
|
|
Post by ms on May 31, 2017 16:06:39 GMT -5
I'm wondering about the string balance on your implementation. It's clear that the bulk response is transparent, but I'm worried about individual poles. The G string slot clearly has no conductive path, but from an individual pole point of view, the A string for example, does. It's possible for currents to travel in the side wall and circumvent the slot. I wonder if you could compare the G with the A using your small test coil to confirm or deny this? I have one more brass cover to butcher, and it will be a while before I can get more. I want to make the next experiment count. We have, I think, two effects: 1. Attenuation of the signal from the string by currents induced in conductive parts of the pickup. 2. Changes in the electrical circuit, primary damping of the coil and capacitance resonance. As I recall, in your document you show measurements from a tele neck pickup, and the effect was mostly the second. The second effect is caused by current flowing in the coil, and I do not think it matters which pole piece is responsible for the time varying flux that induces the voltage in the coil that drives it. Thus I think an overall measurement is the right one to make. But this needs some more thought and maybe measurement. |
|
|
|
Post by antigua on May 31, 2017 17:20:18 GMT -5
With driver coil over a slot that didn't extend to the base, the eddy current losses were very great. There was more resonant amplitude than having no slot, but only a rise of about 2 or 3dB. A rise of 12dB is seen then the slot extends to the bottom of the cover, which is close the having no cover at all. Therefore, I suspect that "string" related eddy currents are mostly what is at play, as opposed to eddy currents that work against the secondary magnetic field created by the current carrying coil. It stands to reason, too, that the magnemotive force of the moving guitar string is a lot greater than the force of the coil's own magnetic field, and so it interacts with eddy causing metals more prominently. This also serves to explain why eddy currents caused by base plates are trivial compared to covers, on top of the fact that this is easily demonstrated through experimentation: guitarnuts2.proboards.com/thread/7861/humbucker-base-plates-eddy-currents |
|
|
|
Post by ms on May 31, 2017 19:48:25 GMT -5
With driver coil over a slot that didn't extend to the base, the eddy current losses were very great. There was more resonant amplitude than having no slot, but only a rise of about 2 or 3dB. A rise of 12dB is seen then the slot extends to the bottom of the cover, which is close the having no cover at all. Therefore, I suspect that "string" related eddy currents are mostly what is at play, as opposed to eddy currents that work against the secondary magnetic field created by the current carrying coil. It stands to reason, too, that the magnemotive force of the moving guitar string is a lot greater than the force of the coil's own magnetic field, and so it interacts with eddy causing metals more prominently. This also serves to explain why eddy currents caused by base plates are trivial compared to covers, on top of the fact that this is easily demonstrated through experimentation: guitarnuts2.proboards.com/thread/7861/humbucker-base-plates-eddy-currentsIf the db loss is greater at the peak than elsewhere, the loss should involve the electrical circuit. Perhaps when you have alnico pole pieces, not as lossy as steel, then the cover causes a significant loss in the electrical circuit. Maybe in such a situation, a metal baseplate would also cause a similar loss. |
|
|
|
Post by ms on Jun 1, 2017 7:15:11 GMT -5
It stands to reason, too, that the magnemotive force of the moving guitar string is a lot greater than the force of the coil's own magnetic field, and so it interacts with eddy causing metals more prominently. The loss induced in the electrical circuit by eddy currents is a function of the frequency dependent impedance of the circuit element they introduce relative to the other impedances in the circuit. It is not a direct function of the relative magnitude of the two fields you mention. |
|
|
|
Post by Charlie Honkmeister on Jun 1, 2017 8:50:43 GMT -5
It stands to reason, too, that the magnemotive force of the moving guitar string is a lot greater than the force of the coil's own magnetic field, and so it interacts with eddy causing metals more prominently. The loss induced in the electrical circuit by eddy currents is a function of the frequency dependent impedance of the circuit element they introduce relative to the other impedances in the circuit. It is not a direct function of the relative magnitude of the two fields you mention. Static magnetic fields don't count at all, which I'm assuming is what Antigua is saying about the coil. There's no such thing as a DC eddy current. The string is magnetized by the static magnetic field of the PU magnetic structure, and is the only source of time-varying magnetic field changes. Dr. Scott Lawing (maker of Zexcoil pickups) recently had a good clear post with an experiment which showed this. link |
|
|
|
Post by ms on Jun 1, 2017 16:09:00 GMT -5
The loss induced in the electrical circuit by eddy currents is a function of the frequency dependent impedance of the circuit element they introduce relative to the other impedances in the circuit. It is not a direct function of the relative magnitude of the two fields you mention. Static magnetic fields don't count at all, which I'm assuming is what Antigua is saying about the coil. There's no such thing as a DC eddy current. The string is magnetized by the static magnetic field of the PU magnetic structure, and is the only source of time-varying magnetic field changes. Dr. Scott Lawing (maker of Zexcoil pickups) recently had a good clear post with an experiment which showed this. linkI, too, have spent quite a bit of time in explaining how pickups work and how they do not work, and Sl's writeup that you linked to is great.. But I do not think you are interpreting what Antigua wrote in the way he meant it, but maybe he will comment on that. |
|
|
|
Post by Charlie Honkmeister on Jun 1, 2017 17:01:33 GMT -5
Static magnetic fields don't count at all, which I'm assuming is what Antigua is saying about the coil. There's no such thing as a DC eddy current. The string is magnetized by the static magnetic field of the PU magnetic structure, and is the only source of time-varying magnetic field changes. Dr. Scott Lawing (maker of Zexcoil pickups) recently had a good clear post with an experiment which showed this. linkI, too, have spent quite a bit of time in explaining how pickups work and how they do not work, and Sl's writeup that you linked to is great.. But I do not think you are interpreting what Antigua wrote in the way he meant it, but maybe he will comment on that. Yep, Mike, I looked at it again and I think you're right. Sorry for a slight misunderstanding, Antigua, but I was maybe thinking you were using the coil-centric paradigm of how the darn thing works. That's really the most prevalent view that's in most of the articles that folks can find when they are researching magnetic pickups, including for example Hartley Peavey's pickup article. The string-centric theory fits pretty well with stratotarts' research into eddies in which he found that baseplate eddies aren't a significant factor. |
|
|
|
Post by antigua on Jun 3, 2017 18:23:54 GMT -5
Here is how much worse the losses are with a partial cut that doesn't extend to the bottom:  For reference, the black line in the plot below shows the result with no cuts at all: So you can see that the partial cut increased the resonant amplitude by about about +dB, but the full cut brings it up to nearly +15dB, just shy of the no-cover plot. So it was a pretty big mistake on my part to present the single slot as being effective, it's hardly effective at all, except for one particular string. I didn't think to test it further because I didn't realize it would be an issue. |
|
|
|
Post by ms on Jun 4, 2017 15:44:03 GMT -5
Here is how much worse the losses are with a partial cut that doesn't extend to the bottom: For reference, the black line in the plot below shows the result with no cuts at all: So you can see that the partial cut increased the resonant amplitude by about about +dB, but the full cut brings it up to nearly +15dB, just shy of the no-cover plot. So it was a pretty big mistake on my part to present the single slot as being effective, it's hardly effective at all, except for one particular string. I didn't think to test it further because I didn't realize it would be an issue. I do not understand this result. Are you sure that you are driving the coil with a current source, or in practice, with a high enough impedance to act as a current source to a good enough approximation? |
|
|
|
Post by antigua on Jun 4, 2017 22:18:00 GMT -5
Here is how much worse the losses are with a partial cut that doesn't extend to the bottom: For reference, the black line in the plot below shows the result with no cuts at all: So you can see that the partial cut increased the resonant amplitude by about about +dB, but the full cut brings it up to nearly +15dB, just shy of the no-cover plot. So it was a pretty big mistake on my part to present the single slot as being effective, it's hardly effective at all, except for one particular string. I didn't think to test it further because I didn't realize it would be an issue. I do not understand this result. Are you sure that you are driving the coil with a current source, or in practice, with a high enough impedance to act as a current source to a good enough approximation? How does the result differ from your expectations? |
|
|
|
Post by stratotarts on Jun 4, 2017 23:52:49 GMT -5
My 50 turn coil should have about 210uH inductance, according to an online calculation. So the inductive reactance at 20kHz is about 25 ohms. I'm using a 100 ohm current limiting resistor in series with the test coil, so I figure it's doing a reasonable job of providing a current source at lower frequencies. Even at 20 kHz, that's not too bad. A larger resistor and greater drive voltage could be used, but it never seemed necessary.
|
|
|
|
Post by ms on Jun 5, 2017 6:43:47 GMT -5
I do not understand this result. Are you sure that you are driving the coil with a current source, or in practice, with a high enough impedance to act as a current source to a good enough approximation? How does the result differ from your expectations? The strong effect on the resonant peak indicates that it is the electrical circuit, coil, capacitance, and resistive loss that is being excited differently when the exciting coil is moved. In this case, the eddy current loss should involve the entire cover. If moving the source of the exciting field around affects this resonant loss, then I suspect that more is going on. In particular, I suspect that there is coupling between the exciting coil and the pickup coil. In the absence of coupling, you might still see some differences depending on the location of the coil, but I would expect this difference to have a broader frequency characteristic, rather than the resonant response of the pickup electrical circuit. |
|
|
|
Post by ms on Jun 5, 2017 6:57:44 GMT -5
My 50 turn coil should have about 210uH inductance, according to an online calculation. So the inductive reactance at 20kHz is about 25 ohms. I'm using a 100 ohm current limiting resistor in series with the test coil, so I figure it's doing a reasonable job of providing a current source at lower frequencies. Even at 20 kHz, that's not too bad. A larger resistor and greater drive voltage could be used, but it never seemed necessary. The driving coil plus pickup is a poorly coupled transformer; that is, it has a lot of leakage flux. Ignore the leakage to get an approximate analysis. You have roughly a 100:1 turns ratio or a 10,000:1 impedance ratio. At resonance the pickup impedance is high, usually in the range 100,000 ohms to 1,000,000 ohms, and so the transformed impedance is probably somewhere between 10 and 100 ohms. Based on that, the interaction is small, but maybe not negligible. The poor coupling, with its added series inductance should help a bit. But the real question is what is Antigua using with his small coil. |
|
|
|
Post by antigua on Jun 5, 2017 17:29:51 GMT -5
How does the result differ from your expectations? The strong effect on the resonant peak indicates that it is the electrical circuit, coil, capacitance, and resistive loss that is being excited differently when the exciting coil is moved. In this case, the eddy current loss should involve the entire cover. If moving the source of the exciting field around affects this resonant loss, then I suspect that more is going on. In particular, I suspect that there is coupling between the exciting coil and the pickup coil. In the absence of coupling, you might still see some differences depending on the location of the coil, but I would expect this difference to have a broader frequency characteristic, rather than the resonant response of the pickup electrical circuit. So ideally, you're expecting the resonant amplitude to be constant, regardless of where the coil is positioned? |
|
