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Thread: Measuring pickups Capacitance?

  1. #36
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    Someone mentioned the AlNiCo grades varrying in the 50's, I've received some pickups from China that supposedly had AlNiCo 5 pole pieces, but showed a Br value that was about 10% below expected, and yielded a higher Q factor than expected, due to higher resistivity. I don't know what made that Chinese AlNiCo different, be it a composition difference, or a production difference (though I'd think composition on account of the resistivity issue). Maybe the AlNiCo in your 59's has a similar sort of difference. I'd also be curious to know what the flux reading is at the tops of the pole pieces.
    Give me some time to look up my notes. Not going to dissect my old strat at the time, so no frequency plots. But I took resistance, inductance, capacitance and B values. Will send PM eventually.

  2. #37
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    Quote Originally Posted by Helmholtz View Post
    It would be naive, though, to assume the a PU's total transfer reponse could be completely descibed by a simple linear passive low pass filter composed of lumped elements.
    It's not naive, it's a real possibility. I'd even argue it's the more probable possibility. You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.

  3. #38
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    You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.
    How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

    I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.

  4. #39
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    Here are extended impedance plots of four pickups I had made at home, which were intentionally wound with low uniformity and varying degrees of tension, including one which is a loose mess. Each is 8,000 turns with a DC resistance right around 6k ohms.

    eeq6iay.jpg


    3lzlzoh.png

    9olgz0j.png

    c0lwhbb.png

    (the big messy pickup)
    cfqojfm.png

    The earlier theory that a less consistent pickup might exhibit fewer or no anomolous high secondary resonant peaks appears to be squashed, as some of these pickups show several such peaks. The large, bushy Strat pickup which could never fit a cover over the top, has two very prominent secondary resonances, one at 86kHz and another at 153kHz. Two others show a single dominant secondary resonance, with additional peaks that look like ripples. Only one of the four, #3, appears to have no secondary resonance, although there is possibly a subtle knee at 64kHz.

    Based on this information, I wondering if there is some sort of event that occurs randomly in the production of a pickup, one that causes multiple, smaller resonant circuits to appear within the coil.

    Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.

    While it's true that these higher peaks don't manifest in the audible frequency ranges, I think it's important to figure out what causes them, since it is a feature of the pickup all the same. It might even serve as a clue to indicate how a coil was made, without having to disassemble the coil to look at it directly.
    Last edited by Antigua; 04-23-2018 at 12:41 AM.

  5. #40
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    Quote Originally Posted by Helmholtz View Post
    How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

    I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.
    In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

    The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.

  6. #41
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    Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.
    This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
    I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.
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  7. #42
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    In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.
    I don't quite understand what you are talking about. Probably due to my limited command of English. Not sure if I really need to know. Anyway, I prefer technical arguments/discussion.

    But it definitely does not sound like an excuse to me.

  8. #43
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    Great info on the thread. Not that witchcraft or caveman level physics, which is so common when you read web pickup forums.
    Thanks.

  9. #44
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    My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.
    This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
    But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?

  10. #45
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    Quote Originally Posted by Mike Sulzer View Post
    My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.
    Quote Originally Posted by Helmholtz View Post
    This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
    But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?
    This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.

    Another option is to use the Velleman's frequency sweep with persistance, which yields the peak like this:

    uijhs40.png

    I though there was a way to feed it white noise with the function generator, which would reveal a peak in the FFT view even more quickly, but I'm not seeing the option.



    Quote Originally Posted by Helmholtz View Post
    This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
    I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.
    I remember you mentioning this possibility. If this is what is going on, it should be possible to model with LTSpice somehow. I'll work on that more later.

  11. #46
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    Quote Originally Posted by Antigua View Post
    In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

    The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.
    When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

    Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

    Joseph J. Rogowski
    Last edited by bbsailor; 04-23-2018 at 06:17 PM. Reason: correct load

  12. #47
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    This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.
    I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.

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    Quote Originally Posted by Helmholtz View Post
    I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.
    This PDF is a prominent search result when searching for information on how to measure the response of guitar pickups. Any critique you have would be valuable to anyone who finds both that PDF and this thread.

  14. #49
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    Quote Originally Posted by Helmholtz View Post
    This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
    But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?
    The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

    There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

    I use an audio package in Python, incorporated into custom software for the signal processing.

    Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

    alandsteelcores.png
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  15. #50
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    Quote Originally Posted by bbsailor View Post
    When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

    Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

    Joseph J. Rogowski
    I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.

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    I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.
    After extensive testing I have settled with guitar cables having 1000pF to 1200pF, both for my Strats and my Les Pauls. The wiring harness in a typical vintage LP adds 300 to 500pF. A tube amplifier input adds another 150pF typically. I use vintage style PUs.
    A realistic load resistance for strats is 100k to 200K(bridge PU) and 200k for LPs. This includes typical amplifier/pedals input resistance.

    I don't think any pro player would be comfortable with a stage cable of 10ft or less.
    Last edited by Helmholtz; 04-23-2018 at 09:55 PM.

  17. #52
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    Quote Originally Posted by Helmholtz View Post
    As a guitar player I have settled (after extensive testing) with guitar cables having 1000pF to 1200pF, for my Strats as well as my Les Pauls. The wiring harness in a typical LP adds 400+pF.
    400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.

    Fender guitars usually feature non shielded hookups and wiring, so the capacitance there is really low, well under 50pF I'd recon.

    Quote Originally Posted by Helmholtz View Post
    A tube amplifier input adds another 150pF typically. I use vintage style PUs.
    A realistic load resistance for strats is 100K..200K and 200k for LPs. This includes typical amplifier/pedals input resistance.
    Since peak freq. will vary from rig to rig, I think it's best to settle on a "center", and then let people shift the frequency up or down mentally, depending on their own rig. So if you know you like 1000pF cables, you know that you will have a peak that is somewhat lower than the standard loaded data points. If the intrinsic L and C are known, then any loaded peak freq. can be solved for, and because inductance factors more prominently, L is is the more valuable metric to have on hand.

    Quote Originally Posted by Helmholtz View Post
    I don't think any pro player would be comfortable with a stage cable of 10ft or less.
    Pros often use wireless units too, which sometimes have selectable capacitance, or a fixed values. I analyzed a Line 6 G10 and found that it imparted about 120pF capacitance http://www.strat-talk.com/threads/th...ay-g10.467237/

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    400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.
    The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
    The self-capacitance of humbuckers with cloth-insulated coax cable is often dominated by the cable attached .
    The capacitance of the guitar cable is the strongest influencer of PU frequency response besides inductivity.
    Last edited by Helmholtz; 04-23-2018 at 11:37 PM.

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    The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

    There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

    I use an audio package in Python, incorporated into custom software for the signal processing.

    Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

    alandsteelcores.png
    Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.

  20. #55
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    Quote Originally Posted by Helmholtz View Post
    Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.
    Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.

  21. #56
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    Quote Originally Posted by Helmholtz View Post
    My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
    I first read about the cloth shield hookup varying in capacitance by humidity in Manfred Zollner's book Physik der Elektrogitarre, and it's a rather significant claim, because people make hay over the littlest things, but never talk about how their pickups sound different between dry and wet climates. I tried a little experiment where I left shielded cloth hookup wire outside, measured the capacitance, the put it in the oven to dry it out, then measured again, but the capacitance didn't change much. I might give it another try, though, ensuring that it's very damp and then very dry.

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    I think this model jives somewhat closely with the notion that having the winding fall into the edges puts a capacitance across two distant sections of the coil:

    4udgvpu.png

    In this model, the series inductance and resistance are broken into three sets, with the middle "RL" set having a capacitance in parallel with it, as would happen if, say, the 3,000th winding somehow managed to come side by side with the 2,500th winding, by falling into a gap at the edge of the coil, or something or that sort. The result of the model is a side by side impedance dip and spike, which isn't 100% like what is seen in the practical plots, which appears to have only an impedance spike, but it has the similar characteristic of a second high frequency resonance. Maybe with a little more refinement the model can duplicate the practical plot completely.
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    Quote Originally Posted by Helmholtz View Post
    The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
    Even humidity aside, my experience agrees with your statements.

    I've measured 268pF per meter in normal conditions on the braided shielded that I use.

    And there's a serious lenght of cable in a LP... This site recommends to have 5ft at disposal for such a wiring: Six String Supplies ? How to Wire a Les Paul (50s Wiring)

    Let's add to it the cables coming from the pickups themselves and the stray capacitance of other components: it forms a highly capacitive inner wiring. Its tonal effect is especially obvious IME when both pickups are selected.

    I don't think any pro player would be comfortable with a stage cable of 10ft or less.
    BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.
    Of course, this observation hasn't much interest for you since you use high capacitance cables. But it would be a pity not to share with all potential readers a possibly useful info, while Sommer cables are so unequally known by musicians around the World... :-)
    Last edited by freefrog; 04-24-2018 at 03:46 PM.
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    BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.
    Yes, this Sommer cable is fine, as is e.g. Klotz GY 107 ("La Grange"). I use both types with different lenghts. But generally I have no need for extra low specific capacitance, as this forces me to buy and use cables measuring 15m or more in length.
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    Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.

    Thanks. Seems like a powerful and useful tool. Unfortunately I don't have the time to dig any deeper and make myself familiar with your method at present.

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    Quote Originally Posted by Antigua View Post
    This PDF is a prominent search result when searching for information on how to measure the response of guitar pickups. Any critique you have would be valuable to anyone who finds both that PDF and this thread.
    I edited my first post and changed it completely as I noticed, that it was not the author's intend to plot the PU's impedance accurately. For the Lissajous method a series resistor of 56k seems fine.

    Here are my comments on this http://www.syscompdesign.com/assets/...ar-pickups.pdf paper.

    First of all I acknowledge that it gives some useful information regarding measurement methods of PUs' parameters. The description of a magnetic PU as a Variable Reluctance Sensor is perfect. But:

    Measuring PU inductance via Lissajous figure
    This is an excellent and accurate method to determine the inductance of a parallel resonant circuit like a PU. But it requires the capacitance to be known exactly.

    There are two problems associated with measuring the inductance with an LCR meter at a fixed frequency:

    1) The meter can only measure apparent inductance. The apparent L of a PU (parallel resonant circuit) increases steadily with increasing frequency below and up to resonance, caused by the effect of capacitance. Apparent L has no practical meaning for PUs and is only a theoretical way to descibe the systematic measuring error of LCR meters. As this error increases with frequency, the value at the lowest measuring frequency is the most meaningful.

    2) Eddy current effects in conductive parts (especially in ferromagnetic cores ->magnetic skin effect) reduce the effective L with increasing frequency. Inductors with conductive, ferromagnetic cores do not have a single true inductance. Instead L is a function of frequency. This means that the L value at 100Hz is not per se better or truer than the value at a higher frequency.

    What we actually want to know is the (effective) L at or close to the resonant frequency in real life operation. This is where the Lissajous method comes in. Done carefully, it can deliver the correct effective L at the chosen resonant frequency of interest.

    As mentioned before, for accurate results the total capacitance Ctot= Cpu+Cadd needs be known exactly. If Ctot is too low by 10%, your calculated L will be too large by 10%.
    Cadd can be easily measured with an LCR but also Cpu should be determined beforehand at least approximately.
    The method indicated in the article, namely "overpowering" an unknown Cpu by a huge Cadd of several nFs, will give the effective L at a much too low frequency. The result will only be useful for PUs where L does not depend on frequency. But in these cases you may as well use your LCR meter at 100Hz.


    And here is the more important part of my comments, dealing with measuring the PU's transfer response:

    Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
    The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
    The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
    Last edited by Helmholtz; 04-25-2018 at 06:45 PM.

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    Quote Originally Posted by Helmholtz View Post
    1) The meter can only measure apparent inductance. The apparent L of a PU (parallel resonant circuit) increases steadily with increasing frequency below and up to resonance, caused by the effect of capacitance. Apparent L has no practical meaning for PUs and is only a theoretical way to descibe the systematic measuring error of LCR meters. As this error increases with frequency, the value at the lowest measuring frequency is the most meaningful.

    2) Eddy current effects in conductive parts (especially in ferromagnetic cores ->magnetic skin effect) reduce the effective L with increasing frequency. Inductors with conductive, ferromagnetic cores do not have a single true inductance. Instead L is a function of frequency. This means that the L value at 100Hz is not per se better or truer than the value at a higher frequency.

    What we actually want to know is the (effective) L at or close to the resonant frequency in real life operation. This is where the Lissajous method comes in. Done carefully, it can deliver the correct effective L at the chosen resonant frequency of interest.

    As mentioned before, for accurate results the total capacitance Ctot= Cpu+Cadd needs be known exactly. If Ctot is too low by 10%, your calculated L will be too large by 10%.
    Cadd can be easily measured with an LCR but also Cpu should be determined beforehand at least approximately.
    The method indicated in the article, namely "overpowering" an unknown Cpu by a huge Cadd of several nFs, will give the effective L at a much too low frequency. The result will only be useful for PUs where L does not depend on frequency. But in these cases you may as well use your LCR meter at 100Hz.
    Thanks for the write up. For some reason this forum truncates this URL http://www.syscompdesign.com/assets/...ar-pickups.pdf on your posts.

    As for inductance varying with frequency, I've noticed that Fender pickups, with AlNiCo pole pieces and little to no other metal parts, show about the same inductance at 1kHz test freq as they do at 100 or 120 Hz. It's only pickups with steel cores that show incorrect readings. One reason I prefer taking down the loaded and unloaded resonant peaks of pickups is because 1) it's a value that relates more closely to audible performance, and 2) it overcomes errors that might arise from trying to solve for peak freq. from incorrect values for L and C.

    Quote Originally Posted by Helmholtz View Post

    Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
    Can the field coil still load down the pickup even if the coupling factor is very small compared to a traditional transfomer?

    Quote Originally Posted by Helmholtz View Post
    The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
    The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
    The PCSU200 shows "output impedance: 50ohm" https://www.velleman.eu/products/view/?id=407512 , so the field coil's impedance would need to be well below 50 ohms, otherwise series resistance must be added?

  28. #63
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    Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

    v2adfer.png

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    Quote Originally Posted by Antigua View Post
    Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

    Click image for larger version. 

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    Without checking all the details of what you are doing, I would guess that it is the effect of the coil resistance.

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    Quote Originally Posted by Mike Sulzer View Post
    Without checking all the details of what you are doing, I would guess that it is the effect of the coil resistance.
    This plot is with a 1meg resistor in series with the pickup, then comparing the voltage across the resistor and the pickup, but the same thing happens when using an external inducer coil in a transformer configuration.

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    Quote Originally Posted by Helmholtz View Post
    And here is the more important part of my comments, dealing with measuring the PU's transfer response:

    Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
    The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
    The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
    I use coils with diameter equal to or smaller than a pole piece radius with 3 to 6 turns, driven from an audio amp through an 8 ohm resistor, with a current of about 1 amp. Coupling is very small. Driving a pickup with a pickup size coil, as some otherwise clever people do, seems like asking for trouble.

    You can make a response model from the parameters derived from an impedance measurement that works well. I think the only reason for using a driving coil is to include the eddy current loss encountered in passing through an extra thick cover.

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    Quote Originally Posted by Antigua View Post
    This plot is with a 1meg resistor in series with the pickup, then comparing the voltage across the resistor and the pickup, but the same thing happens when using an external inducer coil in a transformer configuration.
    In either case, the coil resistance places a lower limit on the magnitude of the coil impedance.

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    Can the field coil still load down the pickup even if the coupling factor is very small compared to a traditional transfomer?
    I have no values for the coupling factor. Generally a coupling factor below 100% introduces additional series inductance in the equivalent circuit. But why don't you just measure and compare? Call it loading down or not, in result the (high) frequency response will change.

    The PCSU200 shows "output impedance: 50ohm" https://www.velleman.eu/products/view/?id=407512 , so the field coil's impedance would need to be well below 50 ohms, otherwise series resistance must be added?
    You have to add the field coil's resistance to the series resistance for total circuit resistance Rtot. What matters is the Rtot/L ratio. It should be well above 150kOhm/H. In other words the corner frequency should lie well above the frequency range analysed and is given by f=Rtot/(2pi*L). This can be achieved by increasing series and/or coil resistance as well as by decreasing field coil inductance.

    Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

    v2adfer.png
    My impedance plots of PUs without anomalies show almost perfect -6dB/octave (i.e. capacitive) behaviour above ca. 100kHz. To stay ahead of noise floor I recommend max. generator voltage and automatic voltage scale.
    Anomalies indicate that the PU's behaviour is not purely capactive (but disturbed by the interaction with a smaller separated part of the inductance) in the corresponding frequency range, thus no clear -6dB/octave slope.

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    Quote Originally Posted by Mike Sulzer View Post
    In either case, the coil resistance places a lower limit on the magnitude of the coil impedance.
    But only below resonance. As the whole thing is shunted by the distributed capacitance, impedance tends to 0 for very high frequencies.

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    Quote Originally Posted by Helmholtz View Post
    But only below resonance. As the whole thing is shunted by the distributed capacitance, impedance tends to 0 for very high frequencies.
    I believe Antigua's question was for below 200 Hz where the pickup coil resistance sets a lower limit on the magnitude of the pickup coil impedance.
    okabass likes this.

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