Measuring AC Impedance with a LCR meter

[Harrysong]

Hi!
Anyone knows how to measure AC Impedance (No to be confused with DC Resistance) on a Guitar pickup with a LCR meter?

[Jazz P Bass]

A bit too complex to state here.
Although I can say it is dependent on frequency.
Here is a good read.http://online.physics.uiuc.edu/cours...asurements.pdf

[Joe Gwinn]

Hi!
Anyone knows how to measure AC Impedance (No to be confused with DC Resistance) on a Guitar pickup with a LCR meter?

Step 1: Acquire a Extech model 380193 LCR Meter.

Step 2: Attach to a pickup.

Step 3: Turn LCR meter on.

Step 4: Set FREQ to 1KHz, L/C/R to R, PAL/SER to SER, and Q/D/R to R.

Step 5: Read and record the displayed AC resistance.

[Mike Sulzer]

......

Step 5: Read and record the displayed AC resistance.

Is the ac resistance measured by the Extech the total measured resistance, or is it the resistance in excess of the dc resistance (that is, total resistance minus the dc resistance). (I looked through a pdf of the manual this morning, but I did not find the definition of ac resistance that it uses.)

[RedHouse]

Step 1: Acquire a Extech model 380193 LCR Meter.

Step 2: Attach to a pickup.

Step 3: Turn LCR meter on.

Step 4: Set FREQ to 1KHz, L/C/R to R, PAL/SER to SER, and Q/D/R to R.

Step 5: Read and record the displayed AC resistance.

(BTW, please don't read my sarcasm as angst)

This question has come up so many times over the last several years, and it's always the same answers. It can be done but blah-blah and you should just buy an Extech.

Even those who in most other threads who posture/argue as "electronic answer men" never quite get there, it's always "buy an Extech".
(no offense intended to anyone here)

So it begs the question, as with all other things, what did the world of electronics do to solve the problem before the Extech? why can't it be done in our case?, why is it got to be a $300 buy-in? not that $300 is beyond the reach of the common man, but as obviously as a winder needn't be a high $ machine, seems odd that a garage-level L meter with AC resistance is not do'able.

We don't need a Huntron Tracker level piece to do pickups, it seems like nearly any other thing hobbyists (or other non-industrialists) can be done with the usual proportionate tradeoff in accuracy-for-dollars, but still a functioning device usable by non-scientific types.

Elektor, ET, and IIRC Nuts and Volts, have all had articles in the past decade on building meters, usually with a PIC or Amtel at the heart. None of which were geared toward the type of coils in a guitar pickup though unfortunately. Others have done it with focus on their end of the electronics universe, but apparently we can't.

There were a couple threads in which Joe and our other resisdent guru's started exploring this (don't have links off-hand) but it died-out when the threads plunged into the usual hyper-esoteric "don't bother isn't perfect, zero error, need infinately pure sinewave's... yadda yadda", sputter sputter, cough cough, stalled out like chitty chitty bang bang on a cold January morning.

Yeah, come to think of it, it's probably better to just buy an Extech, way less trouble, reading, waiting, and you'd be measuring coils long before the thread here would get over the arguements and debates to come to fruition enough to buld something usefull.

Geez, did I get up on the wrong side of the bed today, or what?

Don't mis-read me, I'm not flingin' shite at anybody here, I'm just being appropriately sarcastic about how we have such a combined level of knowlege and abilities here and except for maybe the group-buy of Elepro's gauss meter, we can't seem to pull-together and accomplish anything as an eCommunity.
(except maybe roasting whoever says their pickups are the best)

Recently we have seen some sub forums added, mostly down-level environments, we should have added an up-level sub forum for development work, get a place going on where those who are able can engage in doing something useful for this community can do so. I'm not talking about a place to flog one's latest money making kit, well unless of course one contributes more than just a plan/drawing AND source code, or down loadable programming file like the magazines do when there are microprocessors involved.

Speaking of sub-forums and flogging stuff, why can't we have a Buy-n-Sell? I've been asking that question for 3 years now. How often has anyone/everyone here found they needed like 1 magnet or a bobbin or whatever just to finish up a project or commissioned work? or found they bought too much of an item and wanted to pass on some stock to others, or whatever, as it is we have to sprinkle thet sort of stuff into the mainline threads where they get buried amongst the important discussions about eddy currents and such.

Ooops, I guess I went off topic there, now, back to your regularly scheduled program.

[Joe Gwinn]

The Extech is about $200.

Very few pickup makers are going to build their own LCR meter. As a practical matter, if one does not already have a solid electronics background, this project is out of reach.

The cheap solution is already published, the Maxwell-Wein Impedance Bridge, but the schematics of the Extech are also available.

Joe Gwinn's website homepage

[Joe Gwinn]

Is the ac resistance measured by the Extech the total measured resistance, or is it the resistance in excess of the dc resistance (that is, total resistance minus the dc resistance). (I looked through a pdf of the manual this morning, but I did not find the definition of ac resistance that it uses.)

It's the total AC resistance, including the DC resistance. The only way to separate the AC and DC resistances is to also make a DC resistance measurement using a multimeter.

This is why I also talk of the "excess AC resistance", being the measured AC resistance minus the measured DC resistance. It is the excess AC resistance that is caused by eddy current loading.

As a practical matter, for guitar pickups, one can approximate the excess AC resistance using only an Extech LCR Meter by subtracting the 120 Hz AC resistance from the 1 KHz AC resistance.

[FunkyKikuchiyo]

Joe, would that mean that at low enough of a frequency, with dead on-accurate equipment, DCR and ACR will read the same?

[salvarsan]

The quest for meaning

I do not understand what the Extech LCR meter reports as AC resistance or how it relates to reactance or impedance or wire skin effects.

Bulk impedance calculations put the AC resistance much higher while skin effect resistances are negligible at 1kHz.

Method and calculations follow.

Sanity Check:

Using a 50,000 count Chauvin-Arnoux DMM and the Extech 380193,
a 10k Cfilm resistor's DCR and ACR read as 9905 and 9965, respectively.
The ACR is v.slightly outside the Extech's specified 0.5%+3digit accuracy,
but good enough for Rock'n'Roll.

My SK P90 pickup reads as follows:
DCR = 8202 ohms DCR at 72 F
ACR = 8328 @120Hz (7.441 Henries)
ACR = 13590 @1kHz (7.072 Henries)

7.4 Henries -> 2 * Pi * 120Hz * L = ~5600 ohms reactance.
7.07 H @1kHz -> 44400 ohms reactance

The bulk impedance would be DCR+reactance, or ~13800 and 52600 ohms at 120 and 1000Hz, respectively.

Skin effect resistance seems either negligible or ~60 ohms at 1kHz.

Anyone got any clues as to what the Extech AC resistance describes in a guitar pickup?
Should we be modeling it as some kind of lumped RLC network?

[Joe Gwinn]

Joe, would that mean that at low enough of a frequency, with dead on-accurate equipment, DCR and ACR will read the same?

Exactly. In the low-frequency limit, AC and DC resistance are the same.

[Steve Conner]

I'm clutching at straws here, but I'll guess the LCR meter measures the magnitude and phase angle of the impedance, does a polar to Cartesian conversion, and reports the imaginary part as inductance, and the real part as AC resistance.

A terrible oversimplification for any real inductor, as they are all RLC networks. For instance if you happened to measure at the pickup's self-resonant frequency, you might get an inductance of zero.

[Joe Gwinn]

I do not understand what the Extech LCR meter reports as AC resistance or how it relates to reactance or impedance or wire skin effects.

Bulk impedance calculations put the AC resistance much higher while skin effect resistances are negligible at 1kHz.

Skin effect in #42 or finer wire is negligible at audio frequencies, as you compute. The eddy current effects are in nearby masses of metal, like magnets, baseplates, covers, and the like.

Method and calculations follow.

Sanity Check:

Using a 50,000 count Chauvin-Arnoux DMM and the Extech 380193,
a 10k Cfilm resistor's DCR and ACR read as 9905 and 9965, respectively.
The ACR is v.slightly outside the Extech's specified 0.5%+3digit accuracy,
but good enough for Rock'n'Roll.

My SK P90 pickup reads as follows:
DCR = 8202 ohms DCR at 72 F
ACR = 8328 @120Hz (7.441 Henries)
ACR = 13590 @1kHz (7.072 Henries)

7.4 Henries -> 2 * Pi * 120Hz * L = ~5600 ohms reactance.
7.07 H @1kHz -> 44400 ohms reactance

By Lenz's Law, the eddy currents reduce the flux through the ferromagnetic core, and so reduce the inductance.

The bulk impedance would be DCR+reactance, or ~13800 and 52600 ohms at 120 and 1000Hz, respectively.

Skin effect resistance [of the wire] seems either negligible or ~60 ohms at 1kHz.

Anyone got any clues as to what the Extech AC resistance describes in a guitar pickup?

Total eddy current (and coil resistance) losses, no matter the complexity of the various specific causes.

Should we be modeling it as some kind of lumped RLC network?

It's actually worse than that -- lumped parameter circuits aren't quite right, and one ought to be using partial differential equations (PDEs). But PDEs are more trouble than they are worth for guitar pickups, so we use lumped-parameter circuits and live with the imprecision.

[FunkyKikuchiyo]

Exactly. In the low-frequency limit, AC and DC resistance are the same.

Extech LCR meter = $199. Joe Gwinn telling me how to use it = priceless.

Thanks!

[Mike Sulzer]

It's actually worse than that -- lumped parameter circuits aren't quite right, and one ought to be using partial differential equations (PDEs). But PDEs are more trouble than they are worth for guitar pickups, so we use lumped-parameter circuits and live with the imprecision.

Well I am glad that someone (Salvarsan) besides me has finally realized that there is no simple interpretation of what the Extech does.

Here is the crux of the problem:
1. All guitar pickups have series resistance.
2. Many, in particular humbuckers, have their impedances affected significantly by currents in metal parts (eddy currents).
3. These effects show up as an impedance in parallel with the coil. The simplest model has an inductor and a resistor in series connected in parallel with the coil.
4. The Extech measures amplitude and phase, or real and imaginary part if you like.
5. At low frequencies you might get away with a model with the coil, a series resistor and a parallel resistor. But the two numbers that the Extech measures at a single frequency cannot be interpreted as three parameters: the coil inductance, the series resistance, and the parallel resistance. It is impossible.

Therefore, the measured values are wrong. They have significance relative to other similar measurements, and they might be close enough for some purposes, but that has to be demonstrated, not assumed. (I am working on this, but it is slow work.)

An accurate description of pickup impedance, even at low frequencies, requires measurements at multiple frequencies, and a mathematical interpretation of the resulting impedances.

Joe, you are wrong about this lumped impedance stuff. You can always make a model of an impedance using simpler elements (Rs, Ls, and Cs). It might take a lot in some cases, but it is not that hard for pickups.

It is also absurd to claim that we have a variable frequency inductance due to eddy currents: that is a useless model. The point of a model is to allow pickup characteristics (such as frequency response) to be computed. For that we need to figure out what simple impedances need to go into the model. Now, there is nothing wrong with parameters which vary with frequency if that is what the simplest description of the physics requires. For example, if the parallel resistance is dominated by a current in a conductor that is also ferromagnetic, the skin depth is very small, and so, for example, the currents around the cores of a pickup have a definite skin effect, and you must use a variable frequency resistor in the model.

In any case, the differences between the 120 Hz and 1000 Hz Extech inductance measurements are explained by the attempt to use the meter to make an impossible measurement.

[Mike Sulzer]

Here is a bit more on why a frequency variable inductor is the incorrect model of a pickup.

The physical model suggests that an R and L in parallel. Could this be equivalent to a frequency variable L?

At first thought, this might make sense. Consider the L and R in series that go in parallel with the main coil. At low frequencies the impedance of the L is low and the R dominates. The L has little effect. At high frequencies the impedance of the L is large and it dominates, appearing in parallel with the main coil. Parallel inductors are equivalent to a smaller value inductor. So you might think of this as an inductance that that decreases with frequency.

However, this falls apart if you look at the effects on the frequency response of the circuit. The R and L in series decrease the output from the pickup in a frequency selective manner: they act as a shunt impedance loading the series impedance of the main coil inductance and resistance. If you try to include the series R and L in the main coil as a frequency variable inductance, you lose this effect: you would predict the incorrect frequency response. You could get it back by making the voltage source in the model frequency dependent in just the right way. But in order to figure out how to do this, you would need to make the better model first. Why not stop with the one that works?