Questions for those with LCR Meters

[Possum]

LCR meters...

Just measuring inductance isn't all that useful, the reason is on some pickups they will meausre real low because of added metal like a P90 for instance or a tele bridge. Measure a tele bridge then add the base plate and measure again, the inductance goes DOWN not up. Counterintuitive, you think you're making the pickup brighter because the inductance goes down but you're making the pickup darker in truth. So you need to read the AC resistance and Q as well and take the readings as a whole. The only decent meter that does all this is the Extech LCR meter that most of us use. You also need to take readings at both test frequencies 120hz and 1000hz because whats happening in the bass frequencies isn't the same as whats happening in the higher frequencies. The big bummer in all this is you can't use a 10khz test signal, according to Joe Gwinn, because the readings at that range in pickups becomes nonsensical. Sucks because there is stuff happening up in those frequencies the LCR meter doesn't see, so you might get two identical readings for a pickup yet one will have noticeable treble glassiness and the other won't. I've tried frequency analysis for that range but found it not very useful either, so you end up having to put the pickup in a guitar and listening. The LCR meter also doesn't see the shape of the frequency response so again 2 identical reading pickups again may not sound similar in the real world. ITs a very useful tool but is also very limited for real world. I use it to record changes in variations of design that I can refer back to, mostly.....

[Mike Sulzer]

....both test frequencies 120hz and 1000hz because whats happening in the bass frequencies isn't the same as whats happening in the higher frequencies. The big bummer in all this is you can't use a 10khz test signal, according to Joe Gwinn, because the readings at that range in pickups becomes nonsensical. ...

A pickup has inductance, resistance, and capacitance. It is most nearly inductive at low frequencies because the capacitive reactance is highest and so least significant. So if you want inductance, why would you use anything except the lowest frequency?

But I agree that you want more than inductance. The magnitude of the impedance as a function of frequency, unloaded, or loaded with cable and resistance, in place or on the bench, is what is important. You can use your computer as a noise signal generator and accumulating FFT spectrometer to measure this. Just inject the noise voltage through a large (like 1 Mohm) resistor and then measure the signal across the pickup, accumulating long enough to get a low noise spectrum. You can see the resonant frequency and how wide the resonance is. This measures the relative impedance as a function of frequency unless you carefully calibrate to get absolute. But what is important is the resonance and its width. Why try to measure anything else?

[belwar]

Mike,

Excellent post - Thanks for taking the time to write that. Though some of what you read is a little over my head. I've been trying to figure out how to actually measure resonant peak, do you have any insight into how to properly do that? I'm new to this so the more lamen instructions the better.

[Joe Gwinn]

A pickup has inductance, resistance, and capacitance. It is most nearly inductive at low frequencies because the capacitive reactance is highest and so least significant. So if you want inductance, why would you use anything except the lowest frequency?

Because the Q gets too low for reliable inductance measurements at 1 Hz? And because the inductance of iron-cored inductors like pickups isn't independent of frequency. This is why LCR meters come with two test frequencies, ~100 Hz (for AC power components) and 1 KHz (for everything else, including audio).

But I agree that you want more than inductance. The magnitude of the impedance as a function of frequency, unloaded, or loaded with cable and resistance, in place or on the bench, is what is important. You can use your computer as a noise signal generator and accumulating FFT spectrometer to measure this. Just inject the noise voltage through a large (like 1 Mohm) resistor and then measure the signal across the pickup, accumulating long enough to get a low noise spectrum. You can see the resonant frequency and how wide the resonance is. This measures the relative impedance as a function of frequency unless you carefully calibrate to get absolute. But what is important is the resonance and its width. Why try to measure anything else?

This is a very good method, but simpler methods suffice for pickups. The traditional approach uses generator, 1 Mohm feed resistor, and digital multimeter - look for the peak, read the frequency.

What also works is to measure the impulse response function directly. Excite the pickup with a nearby air coil driven with 10 microsecond wide pulses, digitize the pickup response. Use FFT to transform the impulse response into the pickup response versus frequency function. I've done this, but can't say that it was all that helpful.

My generator can also supply noise, so I could also try the noise and FFT approach. It may have a lower noise floor than banging the pickup and listening to it ring. I don't know if it matters, though.

The problem with using LCR meters with a 10 KHz test frequency is that most pickups resonate near 10 KHz, and so one cannot measure the inductance at that frequency. At resonance, the pickup looks like a resistor, being neither inductive nor capacitive. Above resonance, the pickup looks like a capacitor. Only well below resonance does the pickup look like an inductor. This effect is fundamental, and is inherent to the pickup, and cannot be overcome by use of a different or better LCR meter.

[Rosewood]

Isn't there more than one Extech? Which one is best for guitar pickups?

[David Schwab]

Isn't there more than one Extech? Which one is best for guitar pickups?

You want to use the Extech 380193.

[Rosewood]

You want to use the Extech 380193.

Thanks David

[Mike Sulzer]

Because the Q gets too low for reliable inductance measurements at 1 Hz? And because the inductance of iron-cored inductors like pickups isn't independent of frequency.

Right; I meant the lowest frequency that the meter works at. How much difference does the frequency dependence of the iron make in a pickup? The small amount of iron and the short open cores means that I would expect less of an effect than a closed geometry such as used by a transformer.

This is a very good method, but simpler methods suffice for pickups. The traditional approach uses generator, 1 Mohm feed resistor, and digital multimeter - look for the peak, read the frequency.

Depends on what you want to do. In this discussion from July:How covers inpact the inductance, and then some...
I used this method to show how the Q of the resonance is affected by a cover. That is harder to do with the traditional approach. If you want to study the details of pickup performance, you need a sensitive method. And this one has the advantage that if you already own a computer with a sound card, you just need software, and some programs are not very expensive.

[Joe Gwinn]

How much difference does the frequency dependence of the iron make in a pickup? The small amount of iron and the short open cores means that I would expect less of an effect than a closed geometry such as used by a transformer.

It can be a lot, because the iron in pickups is not usually laminated. Nor are the covers for that matter, so we get lots of eddy currents.

Depends on what you want to do. In this discussion from July: How covers impact the inductance, and then some...
I used this method to show how the Q of the resonance is affected by a cover. That is harder to do with the traditional approach. If you want to study the details of pickup performance, you need a sensitive method. And this one has the advantage that if you already own a computer with a sound card, you just need software, and some programs are not very expensive.

The bang it and listen to the ringing method does sense the cover. One can easily see the effect on the scope. I played with this method for a while, but it wasn't that helpful.

My main conclusion was that the impulse response shouldn't be too ragged, or the sound will lack definition, as the ear is sensitive to the beginnings of notes (the attack).

[Joe Gwinn]

Just measuring inductance isn't all that useful, the reason is on some pickups they will measure real low because of added metal like a P90 for instance or a tele bridge. Measure a tele bridge then add the base plate and measure again, the inductance goes DOWN not up. Counterintuitive, you think you're making the pickup brighter because the inductance goes down but you're making the pickup darker in truth.

And the culprit is ... eddy currents! ... But you knew that...

A cute experiment would be to bring the tele bridge near to the core of a big rectangular-frame stacked-lamination transformer (which must be connected to nothing and arranged such that pickup coil wires are mostly perpendicular to the laminations) instead of the baseplate, and measure the inductance and AC resistance. If you have suppressed the eddy currents adequately, the inductance should rise.

[David Schwab]

And the culprit is ... eddy currents! ... But you knew that...

I had a suspicion that was the cause, but not having a meter myself I wasn't sure. That's one of those things that doesn't seem to make sense at first.

So I'd imagine the eddy currents are making the pickup less efficient too.

[Mike Sulzer]

And because the inductance of iron-cored inductors like pickups isn't independent of frequency.

Yes, as a practical matter it is. A meter might indicate a difference but that is because it does not measure the actual inductance.

To understand this, consider the results of the following measurement:
http://www.naic.edu/~sulzer/humbWWoSlugs.png

This measurement compares the same coil with and without standard humbucker slugs. The coil has a resistance of 3.9 Kohms; so it is a bit underwound by some standards. This measurement is of the kind described above: a 1 Mohm resistor in series with a noise source feeds a pickup and the voltage as a function of frequency is measured. The result is proportional to the magnitude of the impedance as a function of frequency.

Consider first the blue line, the measurement without the cores (slugs). It shows that the impedance rises with increasing frequency throughout the useful frequency range of a guitar pickup. This is what an inductor does. At the highest frequencies the impedance rises more quickly because of the resonance caused by the pickup's capacitance. This resonance is not displayed accurately because the anti-aliasing filter of the instrument starts rolling off at about 18 KHz.

The red line shows the same coil and measurement setup with the slugs inserted. The behavior in the frequency range of a guitar pickup (say, up to six KHz) is the same, except that the impedance rises more quickly with increasing frequency. This indicates that the inductance is larger with the slugs in place. It is somewhat less than a factor of two greater; short open cores do not have nearly as large an effect on the inductance as their relative permeability implies. If the inductance changed in this frequency range, the slope of the response would also change. It does not, and so the inductance is essentially constant.

The behavior at high frequencies is very different. The resonance frequency is lower, as expected when the inductance is greater. Also the response is more heavily damped because of eddy currents flowing in the slugs.

[chevalij]

How is the vertical index measured in this example? Units of voltage? It looks more like an output in milivolts than inductance. Which if that was the case, would indicate more output with cores until at least 16k. This would lean towards there being less impedance with cores. Again, if this is milivolts and measured as a voltage drop across two series loads, is the drop measured across the coil or the 1M resistor? If across the coil it would show higher impedance yes.

[Mike Sulzer]

How is the vertical index measured in this example? Units of voltage? It looks more like an output in milivolts than inductance. Which if that was the case, would indicate more output with cores until at least 16k. This would lean towards there being less impedance with cores. Again, if this is milivolts and measured as a voltage drop across two series loads, is the drop measured across the coil or the 1M resistor? If across the coil it would show higher impedance yes.

The voltage is measured across the coil, so yes, we are looking at inductors at low frequencies. You can measure the inductance if necessary by putting a capacitor across the coil to lower the resonant frequency (like 2200 pf). Measure the frequency peak and use the appropriate equation to calculate L. You can include the Q if you want a really accurate value, but that really is not necessary.

[Possum]

...

well am getting back late in this thread. Why measure anything except the lowest frequency? Because seeing only one part of the elephant tells you nothing about the whole elephant. It really doesn't matter if the meter is "accurate" at either test frequency, what matters is that you use the same meter in all your measurements that you do with your pickups. You establish a baseline based on one measuring instrument. Measuring only at the lowest frequency doesn't tell you what is happening at 1khz, the two aren't related to eachother, in some meausrements they may be real close to eachother in others, far apart. I have a suspiscion that the meters that go to 10khz even though they are not accurate in pickups up there it still might be useful in relative measurements. the biggest disappointment of the Extech is that you have no clue what is happening in the higher frequencies, the meter can give you nearly identical readings on two different pickups yet one pickup may be doing something completely different in the high frequencies that you can hear but the meter can't. Doing a frequency analysis chart I found interesting but not very useful either, maybe I didn't go far enough with it, I probably should have "pegged" certain higher frequencies to get actual digital read numbers at those spots for side by side comparisons against other variants in pickup design. there is a point though where all this geek stuff becomes useless. A Charlie Christian pickup isn't going to sound like a strat or a humbucker and numbers on a chart will never tell you what a pickups character will be like. Numbers are good when comparing doing changes in pickup design, its a tool and is helpful but ears and playing tests always should be the final test and this should be a studio recording test, a home bedroom scenario and playing loud at a live gig, which is probably the most important. Owning alot of expensive test gear isn't going to make your pickups sound better :-)