Emulating a fancy LCR meter

[Mike Sulzer]

Joseph,

Thank you for the suggestions. The pickup is done, but I am still working on the electronics. This pickupMeter is a lot of fun, so I have been working on it instead.

The design has gone in the opposite direction. Each core is two .4 " long ferrite beads end to end. The diameter is similar to the usual alnico magnet, and the relative permeability of the material is 5000. A small neo mag disk (1/8" dia 1/32" thick) is on top of each pole. That is all you need with the ferrite behind it. 4000 turns #43; could have fit more on the ~.6" long spools. Output is about 300 mv p-to-p, which gives very good SNR using a quiet bi-polar amp (LM837). Each coil is loaded with a resistor and capacitor to bring down the resonant frequency and Q to reasonable values. A seventh coil serves as a hum field sensor; it is preamped and subtracted from the others. There is about 20 db isolation between string outputs from the coils, which is increased to about 30 db with a resistor summing network after the preamps.

I will start a discussion on this sometime after after Thanksgiving (have a lot of work that week). Thanks for your interest; I look forward to discussing it with you and all others interested.

Mike

[David Schwab]

Mike, that sounds really interesting. I'm looking forward to reading about it.

Here's the parts my prototype Wal clone. They aren't assembled, just sitting against the keeper bars and magnets. I'm going to wind them next week.

It's not going to have a poly-phonic output, but it could.

[Mike Sulzer]

Here is the schematic of the version of this pickup meter, in use for a couple of weeks: http://www.naic.edu/~sulzer/pumSchem.png.

The noise generator feeds a low pass filter; this improves the accuracy of the device at low frequencies where the main amplifier has low gain due to the low impedance of a pickup at low frequencies. When the follower on the main amp is omitted, the signal level at a peak above 12KHz drops by about 1 or 2%. So there is some loading effect, probably due to cable capacitance. The follower on the low pass amp assures that the output impedances of the two channels are very nearly equal.

Attempts last weekend to make an I-V design, using a high pass filter on the noise source to help overcome leakage in the sound card, failed. It is still not good enough. Even with the high pass filter, the fall off of a peak above 12KHz is obscured by the leakage.

It is important to measure the accuracy of the device, more on that in the next week or so.

[Joe Gwinn]

Here is the schematic of the version of this pickup meter, in use for a couple of weeks: http://www.naic.edu/~sulzer/pumSchem.png.

The noise generator feeds a low pass filter; this improves the accuracy of the device at low frequencies where the main amplifier has low gain due to the low impedance of a pickup at low frequencies. When the follower on the main amp is omitted, the signal level at a peak above 12KHz drops by about 1 or 2%. So there is some loading effect, probably due to cable capacitance. The follower on the low pass amp assures that the output impedances of the two channels are very nearly equal.

If cable capacitance is the problem, one solution is to have matched impedances at both ends of the cable. While this cuts the the available voltage in half, it will flatten things out, and so long as noise sources are shielded out, amplification will save the day. The buffer amplifiers can easily be arranged to provide some gain.

Attempts last weekend to make an I-V design, using a high pass filter on the noise source to help overcome leakage in the sound card, failed. It is still not good enough. Even with the high pass filter, the fall off of a peak above 12KHz is obscured by the leakage.

Leakage from where to where? And of what magnitude? If from output to input, one solution is dual sound cards. But most cards have very good isolation between sections, because the ear is very good at detecting such things. And because the marketing dept needs something to talk about.

What make and model of sound card are you using? I'd like to read up on it.


More generally, it will be widely useful if we can identify which sound cards are suitable, just as we did with LCR meters. Actually, what will be really useful is to identify makes and models that are not suitable, and why they fail to be suitable.

[Mike Sulzer]

Leakage from where to where? And of what magnitude? .

Joe, thanks for keeping me honest here. The problem was that on my test card the film bypass caps were not connected. (The electrolytics were.) Since I have been using the same amps for both circuits and for checking leakage, everything was affected. The i-v works much better now. I will try to get some valid comparisons tonight.

It looks as though the i-v does require a resistor in series with the signal source as you suggested for certain cases. This is for when one connects a cap across the pickup to act as the cable cap.

[Mike Sulzer]

Here is the IV circuit (http://www.naic.edu/~sulzer/ivSchem.png) now in use as a pickupMeter. Why 100K in series with the noise generator? A resistor is needed to limit the gain of the iv amp when the impedance under test has a very low value, such as a capacitance at high frequencies. But selecting the right value for this resistor can also help alleviate two problems: 1. A wide variation of the level of the output of the iv amp; 2. the need to adjust the level of the noise source with different test impedances.

The pickupMeter will be used with impedances that typically can vary from somewhat less than 10K to about 1M. With 100K in the feedback loop of the iv amp, its gain varies from .1 to 10, that is, 100 to 1. We can use the resistor in series with the noise source to reduce the variation at the amp output, transferring some of it to the buffer amp that drives the line to channel 2 with voltage across to the test impedance.

This plot (http://www.naic.edu/~sulzer/iv01rs.png) illustrates a test of the performance of the circuit using three different values of resistors. The values plotted are those measured by the pickupMeter as a function of frequency divided by the dc value measured by my DMM. The 100K resistor in the iv circuit was measured with the same meter, and it is this reference that is used in the software. (This is not a good absolute reference, but is adequate for testing.) Ideally all the values on the plot should be one. Note that the scale goes from .995 to 1.003; so the errors are small.

Obtaining good performance over the whole frequency range requires careful wiring to keep the noise source away from the summing junction of the iv amp and a single point ground system. It also requires good power supply bypassing (10 microf electrolytic, .047 microf polypropylene, and .047 microf ceramic in parallel from each power supply terminal to the ground). The problems at low frequencies with the 1.005M resistor are the result of hum pickup. This would not be a problem with a normal pickup that has low impedance at low frequencies.

This plot (http://www.naic.edu/~sulzer/iv01c1000pf.png) shows the measurement of a nominal 1000pf capacitor. The measured impedance is converted to a value of capacitance versus frequency. Hum affects the measurement at low frequencies, but I think there is another source of inaccuracy as well. The measured value is consistent at high frequencies but increases at low frequencies. It is up 1% of the high frequency value at somewhat under 2 KHz.

Finally, here (http://www.naic.edu/~sulzer/iv01jpnhb.png) is a plot of the impedance of the same japanese humbucker shown in an earlier earlier (with the cover removed). The peak has a somewhat higher impedance, showing the higher accuracy of this circuit at high frequencies and high impedance.