How to measure inductance and Q

[Joe Gwinn]

There are two aspects to this issue of allowing people to make comparisons. The first considers the pickup as a circuit, and for this aspect the comparison people can make is between the values assigned to the components of the circuit in different pickups. The second is operational; perhaps the most basic such comparison that could be made is the resonant frequencies that would be obtained when using different cable capacitances. ...

All of these things are true (including the stuff I snipped) but still there is a need for standardized test conditions, and the choice of 100 or 120 Hz for power systems, 1 KHz for audio, 1 MHz for radio, and 1 GHz for microwave is what the respective industries arrived at, because settling on such standards was very useful in practice, despite the imperfections of those standards.

[Tony Bones]

All of these things are true (including the stuff I snipped) but still there is a need for standardized test conditions, and the choice of 100 or 120 Hz for power systems, 1 KHz for audio, 1 MHz for radio, and 1 GHz for microwave is what the respective industries arrived at, because settling on such standards was very useful in practice, despite the imperfections of those standards.

Joe, I must just be dense. I need you to help me understand specifically how not having a standard makes comparison difficult and how establishing a standard will fix that.

[salvarsan]

... help me understand specifically how not having a standard makes comparison difficult and how establishing a standard will fix that.

Okay, it's Friday and I'm hoping that isn't sarcasm.

1) (grabs a strat pickup, hefts it) "Uhm, I guess it's a little over a quarter pound."

2) (puts pickup on a postal scale) "It's 6 ounces."

3) "YO! This one got serious Mojo(tm)!"

#1 is subjective and not objectively reproducible.
#2 is a reproducible measurement in accord with a weight standard.
#3 Don't make me say it.

#2 makes comparison difficult, but only if your audience is subjectivist and/or
prone to magical beliefs such as #3.

[Tony Bones]

Okay, it's Friday and I'm hoping that isn't sarcasm.

1) (grabs a strat pickup, hefts it) "Uhm, I guess it's a little over a quarter pound."

2) (puts pickup on a postal scale) "It's 6 ounces."

3) "YO! This one got serious Mojo(tm)!"

#1 is subjective and not objectively reproducible.
#2 is a reproducible measurement in accord with a weight standard.
#3 Don't make me say it.

#2 makes comparison difficult, but only if your audience is subjectivist and/or
prone to magical beliefs such as #3.

First, I'm not being sarcastic. I realize that to some it might seem like I'm deliberately being argumentative, but that's not true either. I am honestly trying to understand why a standard frequency of measurement is required or even beneficial. (I'll be honest: my opinion is that wouldn't, but I understand that there's lots I don't understand, so I'm ready to listen.)

Anyway.... salvarsan, I appreciate your effort, but all I get from your analogy is that we should reasonably expect that anyone reporting inductance values should get them from some sort of instrument capable of making an accurate measurement and not just be guessing based on the color. I think everyone here agrees about that.

Some requirements that would probably not be helpful:

- everyone perform the measurement at the same time of day
- everyone perform the measurement in the northwest corner of the room
- .... I could go on and on....


OK, some other standards in engineering history have been quoted. Maybe if we start with one (or more) of those and identify how the standard was useful in that case, then try to identify how a standard frequency would be helpful in this case...

Seriously, maybe I'm making too much of this (very likely!) but I'm just trying to make actual sense of it.

[bbsailor]

I have no trouble comparing results so long as I can believe the measurements are reasonably accurate. Suppose I'm comparing pickups of substantially different designs such that each needs to have inductance measured at a different frequency to be accurate. I won't be able to make a useful comparison if we force them both to use the same frequency. At least one of them will have errors, but I'll have no idea which or how much!

I'd rather let everyone measure at whatever frequency they need to to get accurate results. The fact is, for any given DUT and instrumentation there usually will be a broad window of frequencies where accurate measurements can be made. It's conceivable that there might be some common frequency where all the windows overlap for everybody, every pickup, every LCR meter, every day from now until eternity. Or maybe not. I don't know. I do know that forcing everyone to use the same frequency can, best case, buy us no advantage whatsoever. Worst case it will make comparisons difficult or impossible, which is exactly the opposite of what is intended.

Tony,

Let me see if I can answer your question.

All pickups are inductor coils and have both a DC resistance that is constant with frequency and a reactance (XL) that varies with frequency. The reactance is calculated by the formula XL = 2 pi F L. The Extech LCR meter measures the coil either at 120 Hz (actually 122 Hz to be exact) or 1 KHz. It also displays the AC resistance at the measured frequency and the coil Q or coil quality at the measured frequency. Any coil Q above 1 means that there is some complex actions occurring other than the simple linear measurements of the XL formula above. A coil that is 1H might have an AC resistance of 301 ohms at 120Hz. ( I am using a CSE-187L current transformer for my example but it works with pickups also) Here is how the Q fits into the picture. XL = 1 X 6.28 X 120 equals 753.6 ohms. Look at the Q of your coil. My Q is 2.50, then divide the calculated XL by 2.50 and you get 301 (back to what the meter says). The Q is the factor that moves the reactance to a higher number due to the coils interaction with: (1) the nature of the metal cores; (2) nearby metal;(3) winding capacitance ;(4) pickup coax capacitance;(5) on board circuit loading including volume pot, tone pot and capacitor interaction with the pickup coil and ultimately (6) the capacitance of the typical guitar cable 300pf to about 500pf.

The only way to isolate the pickup from all the external loading effects is to place a buffer ( JFET Buffer and buffer cable | GuitarNutz 2) right after the pickup to place a load that is at least 10X higher than the XL of the pickup at resonance for less than a 1 db loss. Typical pickups may have an XL of a few hundred thousand ohms at resonance and the typical value of the 500K volume pot is actually 400K when the 1 Meg ohms amp impedance is placed in parallel with the 500K pot. This loading reduces the effect of the coil Q. Using a buffer with a 3 Meg ohm to 10 Meg ohm input impedance will preserve the full passive character of the pickup by putting a minimal load on the pickup. Most 500K pots damp the Q of pickups and some Fender pickup coils would sound very bright if the 250K volume/tone pots were replaced with higher values. Here, the meter tells you the numbers but your ear tells you what you like. Once you get what you like the meter can help you make products that are very similar. If you are into initial design, then you want to make a pickup work in the typical environment that the pickup will see.

Learning how to use the Q reading of the Extech LCR meter will help you understand how sensitive the pickup will be to the typical passive loading of all the items 2 to 6 above and tell you how the use of alternate pickup cores, winding variables and total pickup impedance may cause different results in different pickup environments.

In the early days of guitar pickups, the amp input stage drove the evolution of guitar pickups with 6,000 to 12,000 turns of very fine wire to produce enough output to drive the 1 Meg ohm input impedance of the typical guitar amp of that day, that has stuck with us until today.

I hope this helps?

Joseph J. Rogowski

[salvarsan]

First, I'm not being sarcastic.

Pardon my misunderstanding.

I realize that to some it might seem like I'm deliberately being argumentative, but that's not true either. I am honestly trying to understand why a standard frequency of measurement is required or even beneficial.

Then it's a little simpler.

For pickups, an inductance measurement is good quality control.

Suppose you wind 10 pickups whose Inductances are within 5% of an average,
but #11 is 20% off the average ... that indicates a defect.

Spreadsheets help.

You can do an inductance measurement at any reasonable frequency
as long as it's the same frequency all the time. Consistency is important.

If you want to talk to other people about it, use a standard.
As Joe Gwinn sagely observed, 120 Hz and 1000 Hz are
long- and well- known in audio frequency measurements.

-drh

[Mike Sulzer]

You can do an inductance measurement at any reasonable frequency
as long as it's the same frequency all the time. Consistency is important.

What is the definition of reasonable? I think it means that if the measurement is supposed to be of inductance it should measure that without significant contamination from other circuit quantities. For pickups, a low frequency is easiest because the influence of other parameters is minimized. But here is where what Tony is saying is relevant. It does not matter whether that frequency is 100 Hz, 120 Hz, 200 Hz... There is no need to specify a specific frequency or even use the same frequency all the time. It does not even have to be a low frequency if you measure over a range of frequencies and derive the parameters by fitting. The point is that there are many possible methods to measure the inductance correctly. Any of them can be used and results can be compared.

[salvarsan]

...The point is that there are many possible methods to measure the inductance correctly. Any of them can be used and results can be compared.

Yup.

Now all he has to do is decide
whether measuring inductance is
1) in the service of building pickups for sale
2) or if it's for its own sake.

If #1, buy a meter and get on with it.
If #2, have fun and start a web log about it.

-drh

[Tony Bones]

You can do an inductance measurement at any reasonable frequency
as long as it's the same frequency all the time. Consistency is important.

And, my point is that I see no need to use the same frequency all the time. You can get consistent measurements at different frequencies, provided, as Mike pointed out, the frequencies are all low enough.

Let me give examples of some standards that do make sense, Suppose we want to measure the frequency response of an amplifier. Typically, gain is flat over some broad range and falls off at both high and low frequencies. The drop-off is not sudden. If fact, at first the fall off can be very gradual. So what's the useful frequency range of the amp? Ten people might give you ten different answers. So, the standard was established that the amp stopped being flat when it reached 3 dB below the flat section. If you use 6 dB you'll get a different measurement. So everybody uses the same 3 dB and we can compare different amps measured by different people. That's a useful standard.

But wait, the frequency response also depends on the level of the input signal; a tiny input might give better results. So, the standard was established of using an input signal that resulted in 1 watt output. That standard is still a little under-defined, but it helps a lot when comparing measurements made by different people. Of course that's an old standard. I honestly don't know but they probably use a fixed input level now...

While we're talking about old amplifier measuring standards, what about maximum output power? We all know that when you turn up the volume control it gets louder and louder but at some point it starts to distort. You could turn it up past that, but you really don't want to. (I'm talking about stereos, not guitar amps!) The standard was established that maximum power would be measured when the output had 1% harmonic distortion. As long as everybody measures at 1% we can make comparisons.

Notice that the frequency response of an amplifier actually depends on the signal level, so some common standard is absolutely required if we want to compare different measurements. But the inductance of a pickup coil does not depend on frequency. Yes, the value that your LCR meter reports changes as you go to frequencies that are too high, but that's because there's error in the measurement, not because the inductance actually changes.

What the hell, go ahead and declare that everyone performs the measurement at, say, 120 Hz. It won't do any harm and it might prevent someone from trying to measure inductance at the resonant frequency or something else silly, possibly because they just don't know any better.

[Tony Bones]

Yup.

Now all he has to do is decide
whether measuring inductance is
1) in the service of building pickups for sale
2) or if it's for its own sake.

If #1, buy a meter and get on with it.
If #2, have fun and start a web log about it.

-drh

Dig it. I made a mountain out of a mole hill. I'm not sure why. Maybe pressures at worked needed an outlet? I am sorry if I upset anyone.

[kayakerca]

Dig it. I made a mountain out of a mole hill. I'm not sure why. Maybe pressures at worked needed an outlet? I am sorry if I upset anyone.

For me, inductance comes down to this. I measure the inductance of, say, a Stat style single coil before and after I charge the magnets. I find that the inductance value is +/- 90% wire and +/- 10% magnets, as in (an example) 2.330 henries before the A5's are magnetized and 2.556 henries after. Always comparable for the same wind and charge on the magnets.

No magic, all mojo. . .

[J M Fahey]

I am honestly trying to understand why a standard frequency of measurement is required or even beneficial.

For proper and useful comparisons which do not degenerate into a brawl, a pissing match or similar, standards mark the field and set some rules which make them possible.

There must be some reason that Science and even everyday life (such as Commerce or Law) has defined standards as to weight, length, temperature, volume, brightness, colour, driving and drinking age, and 1000 other parameters, including salt o sugar or alcohol or whatever content in food, dairy products acceptable shelf life, and probably even condom rubber thickness.

Arguing about such an important reality in our Civilization really does *sound* like being contentious.

Just to add to the discussion, I think inductance is an important parameter, but resonant peak frequency is a much more important one, and is *very* audible, if inside the Audio band (which it is, if we consider cable capacitance).

EDIT: just checked, out of curiosity, and there are ISO standards even for that:
ISO 4074:2014 - Natural rubber latex male condoms -- Requirements and test methods
Australian Government
Medical Device Standards Order (Standards for Natural Latex Rubber Condoms) 2008
- F2008L04335
"ISO 4074: 2002 Natural latex rubber condoms – Requirements and test methods, including Technical Corrigendum"

[pee-tr]

What is the definition of reasonable? I think it means that if the measurement is supposed to be of inductance it should measure that without significant contamination from other circuit quantities. For pickups, a low frequency is easiest because the influence of other parameters is minimized. But here is where what Tony is saying is relevant. It does not matter whether that frequency is 100 Hz, 120 Hz, 200 Hz... There is no need to specify a specific frequency or even use the same frequency all the time. It does not even have to be a low frequency if you measure over a range of frequencies and derive the parameters by fitting. The point is that there are many possible methods to measure the inductance correctly. Any of them can be used and results can be compared.

- - -
hi,
I'm just a late visitor in the relative future (2022), and having read this debate, I'd like to sum up how this read out to me...
(otherwise, this is the MOST elucidating thread I can even imagine!! 3 of you guys honestly confesses what the actual use of recording measured pickup data is or could be!!
I'm all amazed! thank you for your bright sincerity )

but as to this end-debate about whether or not standardizing the measurement methods is mandatory,

Mike Sulzer's post was the turning point I think...

Mike starts off (as I see) in a self confident manner, cause having just read Tony Bones' train of though he / you must have thought he / you saw the point...
which is a great thing... cause he / You understood what Tony was saying, without originally being on Tony's side, so to speak...
so, You, Mike, made a journey over to the other side, which is really a cool thing....

BUT...
Your series of statements (in the quote) break after a point.... and only natural enthusiasm keeps it going on...

#1 doesn't matter which frequency, but it must be low... // TRUE, but already contradicting, cause the "lowness" is a criterion... it should be low, this is declared, if it was not low, the measurement would not be credible, or measurements would not be compatible*... of if you mean it like "it doesn't matter which frequency of 100Hz, 120Hz, 200Hz... etc" then etc is not defined... or can we put 2000Hz there too? And if not, then this "100Hz, 120Hz, 200Hz" basically reads like "100Hz to 200Hz"... which is a pretty firm definition, as a matter of fact...

#2 There is no need to specify a specific frequency or even use the same frequency all the time. // TRUE, but contradicting... cause it has to be low... typically below 1KHz, but 100/120Hz is even better... and anyway, since the accuracy changes with frequency, frequency is a key parameter, so it has to be "attached" to the result...

#3 It does not even have to be a low frequency if you measure over a range of frequencies and derive the parameters by fitting. // FALSE, completely, as it contradicts the already achieved "consensus" and this measurement method is already different, and therefore it's NOT compatible*

#4 The point is that there are many possible methods to measure the inductance correctly. // ERR -- this statement cannot be rendered logically within this context otherwise: other established measurement methods also have standard parameters, how-tos, cause methods only exist since they are described and defined

a raw example: I have a temperature! How much? 36.9°C / 98.42°F ... now what would anybody ask? (not that I'm writing this in 2022 when infrared touchless temperature scanners have become established norm along normal thermometers)

#5 Any of them can be used and results can be compared. // FALSE ... 2x ...

--------------------------------------

the point, in short:

*COMPATIBLE
measurements must be compatible...
compatible means: comparable...
which means...

let's say you have two measurements, one that you made last year (A), and one that you've just made (B)...

the results go: 3.91H, 3.92H

now, if you know (cause you had recorded the data ) that last year you used your DE 5000, in series mode,
and now you make the measurement using another DE 5000, in series mode,
you can say that the 2 pickups are identical, or that their inductance values are identical...

and vice versa, if the values are 3.91 and 4.27 you can take it for granted that it's not the same pickup, nor are the inductance values the same or even close...

Having these two differing measurement values, if in both cases it was a DE 5000, you can tell for sure, that it's two different pickups...
but if you know that one was measured like #4 while the other with a DE 5000, you cannot have a conclusion...


............

that's it on my part, thank you very much for the great conversation, it was an illuminating read!!
I hope that everyone is fine just like 7 years ago...

Peter

[Mike Sulzer]

When I measure the complex impedance of a pickup at hundreds of frequencies at once, I can identify a region at low frequencies where the imaginary part of the impedance increases linearly (passing through zero at zero frequency) closely enough so that for practical purposes the use of any frequency to predict the inductance is essentially equivalent. (Of course, I prefer to use multiple frequencies by fitting to a line.) At higher frequencies we have eddy current losses when using conductive cores and other conductive parts. Their effect on the complex impedance can be modeled and included in the model used in a fit. Of course I would not bother to do this for measuring the inductance because it is simpler just to use the low frequency range where the effect of eddy currents is too small to matter.

[the great waldo]

To the "how" question.

An LCR meter can be pickup up pretty cheap on ebay that will provide an inductance value. Q can be calculated by formula

Backlight Control LCR RCL Inductance Capacitance Resistance Meter Tester 4070L | eBay

Or you can go the distance and get an LCR meter that calculates both.

LCR200 - Passive Component LCR Meter

http://www.extech.com/products/LCR200