Emulating a fancy LCR meter

[Joe Gwinn]

Here is an update on my efforts to use to use a computer with soundcard for measuring pickups:

I tried an I-V circuit. This circuit, as discussed above, gives a low voltage at the higher frequencies. There is too much noise pickup, erratic noise probably from the computer hardware, from about 4KHz and above. (I am using a mac book with its internal soundcard.) There are ways around this, no doubt, but a different circuit topology (V-I) puts the higher voltage output at the higher frequencies. This circuit and some comparisons of measurements and modeling are here:
http://www.naic.edu/~sulzer/coreNoCoreComp.png

I'm a little unclear as to what was tried and why it failed. Voltage followers and transimpedance amplifiers are used well into the gigahertz region. Could you also post the failed "I-V circuit"?

As for noise pickup, I built my circuit on a piece of vectorboard mounted in a diecast metal box, with BNC inputs and outputs, and binding posts for the pickup under test, and had no problems. The dark side of sensitivity is sensitivity -- full shielding is required.

I also used OP27 opamps, versus generic ~741s, and had no problem making measurements at 20 KHz (limit of interest, not OP27 capability). The 741s didn't work quite well enough, as I recall, but I don't recall exactly what the problem was, although limited open-loop bandwidth is a likely contender.

http://www.analog.com/en/other/milit...s/product.html

This effort started in May 2003, when I was first trying figure out how to accurately measure pickups. The use of the Maxwell Impedance Bridge and discovery that the Extech LCR meter was suitable arose from this same effort.

[David Schwab]

Mike, do you have access to either a Firewire or USB audio interface? I never used the audio interface in a Mac laptop, and my G4 doesn't have an audio in jack (?!?), but on older Macs I've had, the built in audio wasn't the cleanest I've heard.

Before I put an M-Audio PCI card in the G4 I used a Griffin iMic USB interface which seemed to work OK.

[Mike Sulzer]

I'm a little unclear as to what was tried and why it failed.

Joe, I am sorry about the unclear writing; not the first time nor the last. The analog circuitry did not fail, it was the system as a whole. As David suggests, the problem is likely with the computer. Although I cannot prove exactly where the problem is occurring, I believe it is cross talk between the digital circuitry and the sound card in the computer. A lesser problem is leakage from the outputs of the sound card back to the inputs at high frequencies.

I am using 1/4 of an LM837; it is a good amp for this purpose. 25 MHz GBP, 200KHz power bandwidth, 10 V/microsec slew rate, low noise for pickup type impedances (bipolar input). I have not even thought about using a 741 in an audio circuit for, well, about 35 years. The slew rate is too low to reproduce 20 KHz at full output levels, the gain is too low, and they sound dreadful.

The I-V circuit that I tried is just like the one on the figure except the pickup goes in the input path and a resistor in the feedback path. No followers are required; the computer output is designed to drive headphones and provides 1 V rms into the load used here.

One could use the I-V circuit by increasing the signal in at higher frequencies. But the circuit I am using just does this in a simpler way.

David, yes, I have both those inputs on the computer. If you are suggesting that all the analog circuitry should be outside the computer, yes, that would be better. But that would violate the main goal of this project, which is to measure pickups with a laptop and as little external circuitry as possible. I would do it only with passive components, if possible, but one op amp, either as a buffer or impedance converter is required. The converter has the best performance.

[David Schwab]

David, yes, I have both those inputs on the computer. If you are suggesting that all the analog circuitry should be outside the computer, yes, that would be better. But that would violate the main goal of this project, which is to measure pickups with a laptop and as little external circuitry as possible. I would do it only with passive components, if possible, but one op amp, either as a buffer or impedance converter is required. The converter has the best performance.

I'd just like to have one that works! This looks like a great idea.

UBS audio interfaces are pretty cheap. The iMic looks like a yo-yo (though the new ones are larger) and it's only serving as a sound card external to the computer.

I know it's hard enough to get a noise free recording just being close to my digital mixer.

[Mike Sulzer]

I'd just like to have one that works! This looks like a great idea.

UBS audio interfaces are pretty cheap. The iMic looks like a yo-yo (though the new ones are larger) and it's only serving as a sound card external to the computer.

I know it's hard enough to get a noise free recording just being close to my digital mixer.

A cheap external USB sound card is not necessarily any more immune to the pickup of digital noise than the internal card. RDigital circuitry is required for the interface, and a poor design might not isolate this well enough.

[Mike Sulzer]

Humbucker impedances

I want to look at the characteristics of some pickups, starting with a humbucker. This pickup was purchased from Allparts some years ago, and is supposed to be something like a PAF replacement. Humbuckers such as this use two coils in series, and so have high inductance, and low capacitance. The resonant frequency is quite high when they are connected to a very high impedance load, but the high frequencies are easily lost in the real world of a volume and tone controls, guitar cable and amp input resistance.

Let's begin by looking at the effect of the cover with the pickup unloaded. The blue line in the plot (http://www.naic.edu/~sulzer/japanHumBuc.png) shows the impedance with no cover; the red line shows the impedance with the cover that came with the pickup. This is a truly dreadful cover. The red line deviates from the blue at less than 2 KHz. Remember, there are two effects from the eddy currents induced in the cover. First, they dissipate energy, and so load the pickup down, equivalent to a resistor connected across the pickup. This effect tends to be important at the higher frequencies where the impedance is high. The other effect is that the currents tend to reduce the inductance. One expects this effect at the higher frequencies as well, but in this case it starts well down in the mid-range.

The green line is the results for a better cover, also purchased from Allparts. It appears to have little effect through the range of a guitar speaker.

The black and light blue lines show the impedances with no cover, but the pickup loaded with some external circuit. The black line shows the effect of 550pf, typical of a guitar cable. The resonance moves down to under 4 KHz with moderate Q.

The additional loading on a humbucker is usually a 500K volume control, a 500K tone control, and a 1 meg input resistance to the amp. When the two controls are on 10, they have a parallel resistance of 250K. (The tone cap is not important with the control on 10; it is like a short.) The amp input brings the impedance down to 200K. So the light blue line is the impedance with no cover, 550pf, and a 200K resistor. Note that the Q is very low, and the impedance begins decreasing at less than 3 KHz.

For some, an explanation of what this impedance means would be useful. The operation of the pickup is described with a voltage source in series with the inductor. The inductor and capacitor form a so-called second order lowpass filter, and the resistance determines the Q, or damping of the filter. (The resistance in series with the L and any equivalent resistance from eddy currents contribute to this resistance.) The low pass filter has a flat response at the low frequencies; it rises near the peak determined by the L, C, and R, and then falls off quickly. The values of these components are determined by the impedances on the plot, and so roughly speaking, one can determine the peak and Q of the filter by reading the values off the plot. The data can be used to calculate these values with good accuracy.

So this pickup, IMO, qualifies a a bit of a dud, especially if the original cover is left on. A good candidate for the dead pickup's society. (I suspect that the base plate is also too conductive, but but I will leave testing and replacing it for another time.)

[Joe Gwinn]

Joe, I am sorry about the unclear writing; not the first time nor the last. The analog circuitry did not fail, it was the system as a whole. As David suggests, the problem is likely with the computer. Although I cannot prove exactly where the problem is occurring, I believe it is cross talk between the digital circuitry and the sound card in the computer. A lesser problem is leakage from the outputs of the sound card back to the inputs at high frequencies.

OK. Low impedance drive of signals going to the sound card can help. As can a better sound card.

Sound card. We need to figure out which sound cards are best. I'd like one for the Mac as well. Any worthwhile soundcard ought to be able to achieve 60 dB isolation, and if the analysis software takes advantage of the fact that we know the drive signal frequency, we ought to be able to achieve profound reductions in the effect of stray (uncorrelated) noise.

I am using 1/4 of an LM837; it is a good amp for this purpose. 25 MHz GBP, 200KHz power bandwidth, 10 V/microsec slew rate, low noise for pickup type impedances (bipolar input). I have not even thought about using a 741 in an audio circuit for, well, about 35 years. The slew rate is too low to reproduce 20 KHz at full output levels, the gain is too low, and they sound dreadful.

The LM837 ought to work OK. With four amps in the package, there is no reason one cannot have full buffering. I usually build such things into a diecast box with two 9-volt batteries to supply +/- 9 volts as Vcc and Vss.

As for the 741, I'll try to keep it clean in the future...

The I-V circuit that I tried is just like the one on the figure except the pickup goes in the input path and a resistor in the feedback path. No followers are required; the computer output is designed to drive headphones and provides 1 V rms into the load used here.

No followers? No transimpedance amps? Therein lies the problem.

One could use the I-V circuit by increasing the signal in at higher frequencies. But the circuit I am using just does this in a simpler way.

But it seems to be too simple to work well.

I'll publish my circuit in a few days. It is not complex. But I will have to draw it up.

David, yes, I have both those inputs on the computer. If you are suggesting that all the analog circuitry should be outside the computer, yes, that would be better. But that would violate the main goal of this project, which is to measure pickups with a laptop and as little external circuitry as possible. I would do it only with passive components, if possible, but one op amp, either as a buffer or impedance converter is required. The converter has the best performance.

Change that to one IC (with 4 opamps) and we can do far better. My original circuit uses only two of the opamps, plus a third to buffer the soundcard output.

[Mike Sulzer]

No followers? No transimpedance amps? Therein lies the problem.

But it seems to be too simple to work well.

The circuit with the pickup in the feedback loop seems like the right one given the impedance characteristics of a pickup (rising with frequency up to the resonance). This amp has low output impedance. The input voltage sample has low impedance at high frequencies (where it matters most) because it is taken across a capacitor.

But I will try followers in different places in the circuit, one at a time, and see what the differences are.

[soundmasterg]

I want to look at the characteristics of some pickups, starting with a humbucker. This pickup was purchased from Allparts some years ago, and is supposed to be something like a PAF replacement. Humbuckers such as this use two coils in series, and so have high inductance, and low capacitance. The resonant frequency is quite high when they are connected to a very high impedance load, but the high frequencies are easily lost in the real world of a volume and tone controls, guitar cable and amp input resistance.

Let's begin by looking at the effect of the cover with the pickup unloaded. The blue line in the plot (http://www.naic.edu/~sulzer/japanHumBuc.png) shows the impedance with no cover; the red line shows the impedance with the cover that came with the pickup. This is a truly dreadful cover. The red line deviates from the blue at less than 2 KHz. Remember, there are two effects from the eddy currents induced in the cover. First, they dissipate energy, and so load the pickup down, equivalent to a resistor connected across the pickup. This effect tends to be important at the higher frequencies where the impedance is high. The other effect is that the currents tend to reduce the inductance. One expects this effect at the higher frequencies as well, but in this case it starts well down in the mid-range.

The green line is the results for a better cover, also purchased from Allparts. It appears to have little effect through the range of a guitar speaker.

The black and light blue lines show the impedances with no cover, but the pickup loaded with some external circuit. The black line shows the effect of 550pf, typical of a guitar cable. The resonance moves down to under 4 KHz with moderate Q.

The additional loading on a humbucker is usually a 500K volume control, a 500K tone control, and a 1 meg input resistance to the amp. When the two controls are on 10, they have a parallel resistance of 250K. (The tone cap is not important with the control on 10; it is like a short.) The amp input brings the impedance down to 200K. So the light blue line is the impedance with no cover, 550pf, and a 200K resistor. Note that the Q is very low, and the impedance begins decreasing at less than 3 KHz.

For some, an explanation of what this impedance means would be useful. The operation of the pickup is described with a voltage source in series with the inductor. The inductor and capacitor form a so-called second order lowpass filter, and the resistance determines the Q, or damping of the filter. (The resistance in series with the L and any equivalent resistance from eddy currents contribute to this resistance.) The low pass filter has a flat response at the low frequencies; it rises near the peak determined by the L, C, and R, and then falls off quickly. The values of these components are determined by the impedances on the plot, and so roughly speaking, one can determine the peak and Q of the filter by reading the values off the plot. The data can be used to calculate these values with good accuracy.

So this pickup, IMO, qualifies a a bit of a dud, especially if the original cover is left on. A good candidate for the dead pickup's society. (I suspect that the base plate is also too conductive, but but I will leave testing and replacing it for another time.)

That was very interesting!

So was the original cover that came with the pickup made out of brass? I've noticed most of the pickups with a brass cover tend to sound like crap, with high end harshness, and the one with nickel silver covers usually sound better.

Greg

[Mike Sulzer]

That was very interesting!

So was the original cover that came with the pickup made out of brass? I've noticed most of the pickups with a brass cover tend to sound like crap, with high end harshness, and the one with nickel silver covers usually sound better.

Greg

I do not know, but when I get home sometime tonight, I will scrape off some of the plating and take a look.

Mike

[Mike Sulzer]

Joe,

I put a follower between the voltage input, that is, the top of the capacitor (V2), and the output to the sound card. This does indeed make a difference. Typically a bit more than 1% of the impedance value, but more like 2% near the peak (13 -14 kHz) and decreasing to near zero at about 20KHz. So the resistor in the input path of the op amp and the sampler see different voltages. I think the explanation for this is that the the sampling process occurs over a short time interval and loads the input significantly just during this time. This means that one should have an identical follower on the output of the main op amp so that the voltage ratio is as accurate as possible. This makes three op amps. So I might as well use the fourth op amp to buffer the input noise signal and use its feedback loop for the frequency gain adjustment. One should give this amp some gain (about two or three) so that the input resistor in the main op amp can be increased, decreasing its gain and so further increasing its accuracy. All this might be a bit more than necessary for looking at pickups, but as you said, one might as well use the whole chip, especially once one sees that there are problems with using just one amp. (I had hoped to eventually have a really low power circuit using batteries that would last a very long time.)

[Joe Gwinn]

Joe,

I put a follower between the voltage input, that is, the top of the capacitor (V2), and the output to the sound card. This does indeed make a difference. Typically a bit more than 1% of the impedance value, but more like 2% near the peak (13 -14 kHz) and decreasing to near zero at about 20KHz. So the resistor in the input path of the op amp and the sampler see different voltages. I think the explanation for this is that the the sampling process occurs over a short time interval and loads the input significantly just during this time.

I would worry about transmission-line effects and cable capacitance first, before worrying about sampling offsets. Specifically, the output impedance of the V or I buffer amps needs to match the impedance of the coax line to the soundcard, and the soundcard impedance must match (or be made to match) the coax line as well. At the opamp end, the traditional solution is a series resistor of value equal to the transmission line impedance. At the soundcard end, one may require a T-adapter with terminator on one port.

Also, the lower the drive impedance, the less the effect of capacitance, especially at the higher frequencies.

This means that one should have an identical follower on the output of the main op amp so that the voltage ratio is as accurate as possible.

It's sufficient that all ratios are accurately known. I usually measure all components to 1% before assembly into a circuit, and then use the actual measured values in the analysis software.

This makes three op amps. So I might as well use the fourth op amp to buffer the input noise signal and use its feedback loop for the frequency gain adjustment. One should give this amp some gain (about two or three) so that the input resistor in the main op amp can be increased, decreasing its gain and so further increasing its accuracy. All this might be a bit more than necessary for looking at pickups, but as you said, one might as well use the whole chip, especially once one sees that there are problems with using just one amp. (I had hoped to eventually have a really low power circuit using batteries that would last a very long time.)

I must say that putting an uncontrolled (from unit to unit) and complex thing like a pickup in the feedback loop bothers me. There has to be a reason all those $15K LCR instruments don't do it that way. They cannot be scrimping on opamps for sure.

But you are getting plausible curves. I would try the circuit out on some components of known properties, like a RC network, to see how accurately these known properties are measured.

One practical disadvantage of the feedback-path approach is there must be a DC path through the component under test, so things like the bifilar-wound pickup that couples through the capacitance between the windings could not be handled. One could bridge the component under test with a 1 Mohm resistor, but this loads the component, leading to a discussion about the significance of this loading.

[Mike Sulzer]

I would worry about transmission-line effects and cable capacitance first, before worrying about sampling offsets.

I do not think that one need worry about transmission line effects when using a cable a few feet long at 20 kHz and below. Cable capacitance, maybe. But 250 pf at 20 KHz is a bit over 3e4 ohms. It does not seem likely that this would bother an amp that can operate into 600 ohms. I will try a very short cable tonight and see if that makes any difference.

[If matching did matter it could be handled at the load end only. A match there kills any reflection. If matching were required at both ends, rf transmitters could not necessarily be made efficient, but they can.]

It's sufficient that all ratios are accurately known....

Again, I am not being clear in my writing. The use of identical followers would be to make the sampling issues identical for the two signals, and so the problem would drop out when the ratio is computed.

I must say that putting an uncontrolled (from unit to unit) and complex thing like a pickup in the feedback loop bothers me. There has to be a reason all those $15K LCR instruments don't do it that way.

The issue with capacitors you mentioned would be one reason. The possibility that someone would come up with an impedance that would make it oscillate is another. I think the high resistance and capacitance of a pickup coil imply that there are no stability issues.

[Mike Sulzer]

That was very interesting!

So was the original cover that came with the pickup made out of brass? I've noticed most of the pickups with a brass cover tend to sound like crap, with high end harshness, and the one with nickel silver covers usually sound better.

Greg

Yes, the original cover is brass, as is the base plate.

[Mike Sulzer]

The results of comparing a short cable (less than one foot) to a longer one (about five feet) from the output of the op amp to the sound card input (V1): There is a small difference (less than 1%) above 14 KHz. If the designed is changed so that both outputs use similar op amps, this effect should tend to cancel out.