Stupid question about resonant frequency test.

[Kindly Killer]

I want to learn some more measurements to quantify pickup design. The following test looks like a good place to start:

http://music-electronics-forum.com/a...-joemethod.jpg

I am using a function generator on my phone, which puts out relatively low voltage. Does that matter?

Can I measure voltage across the resistor with a meter, or do I have to use an oscilloscope? In my dim understanding it seems it should be AC voltage, right? When I set my meter to AC voltage it just goes uniformly down as frequency goes up. If I select DC voltage I get the kind of behavior I kind of expected and a number for resonant frequency that's in the realm of possibility (6.3kHz)...

Anyone care to give me some additional coaching on how to run this test? If I need additional gear, I'm on a very low budget (hobby).

BTW I have the Extech 380193 and Extech 430 DMM (lower end, probably cost $100 new).

Thanks in advance for all help.

Michael

[g1]

Most inexpensive DMM's are only accurate measuring lower frequency AC voltage.
Try measuring the signal coming straight out of the phone, you will probably get the same result.
A scope would probably give the best results.

[Kindly Killer]

Okay next question. Looking at Oscilloscope options, there seem to be a lot of this style for sale in the sub-$100 range.

DSO Nano Oscilloscope v3: Amazon.com: Industrial & Scientific

Are there any obvious drawbacks for using this style with guitar stuff (pickups, pedals, amps)?

The other thing that looks attractive to me is a computer interface to use a notebook as an oscilloscope. My concern about that variety is that computers have a very short lifespan, and backward compatibility is no longer a concern for mgfrs or software companies - at least not for consumers. So generally, what would be a smart option for very low cost and very high longevity?

[charrich56]

Hey there,

If you are looking in this direction (digital scope which connects to laptop via USB) you should really check out the Syscomp CGM-101 Circuit Gear Mini.

CGM-101

It is a combination scope and waveform generator, and digital I/O interface which plugs in via USB to a laptop or desktop.

The scope portion is great for audio work. The software includes a vector network analyzer which allows you to do sweep frequency analysis very easily and plot as well as export the results. Darn near ideal for measuring pickup resonance.

Check the white paper on this very subject.
http://www.syscompdesign.com/assets/...ar-pickups.pdf

The unit is 99.00 but for 129.00 you can get the unit with the scope probes and cables you would basically need to use it.

I have one and it is extremely useful for exactly the thing you are trying to do.

Take care,

Charlie

[the great waldo]

Hi Charlie

Do you have any idea as to what would be usable as a driver coil to measure pickup resonance?

Cheers

Andrew

Hey there,

If you are looking in this direction (digital scope which connects to laptop via USB) you should really check out the Syscomp CGM-101 Circuit Gear Mini.

CGM-101

It is a combination scope and waveform generator, and digital I/O interface which plugs in via USB to a laptop or desktop.

The scope portion is great for audio work. The software includes a vector network analyzer which allows you to do sweep frequency analysis very easily and plot as well as export the results. Darn near ideal for measuring pickup resonance.

Check the white paper on this very subject.
http://www.syscompdesign.com/assets/...ar-pickups.pdf

The unit is 99.00 but for 129.00 you can get the unit with the scope probes and cables you would basically need to use it.

I have one and it is extremely useful for exactly the thing you are trying to do.

Take care,

Charlie

[Kindly Killer]

Okay thanks for the recommendation. That looks perfect.

Hi Charlie

Do you have any idea as to what would be usable as a driver coil to measure pickup resonance?

I think you want anything with a resonant frequency outside the range you're measuring. Years ago I messed with TrueRTA on a windows computer (just using the audio interface) and I made a driver coil on plastic Fender strat bobbin (with no mags). I want to say I used 500 turns of AWG36, but I don't think it matters, so long as it has a really really high peak and is hot enough to get a clean signal out of the pickup you're measuring. HTH

[kayakerca]

Okay thanks for the recommendation. That looks perfect.



I think you want anything with a resonant frequency outside the range you're measuring. Years ago I messed with TrueRTA on a windows computer (just using the audio interface) and I made a driver coil on plastic Fender strat bobbin (with no mags). I want to say I used 500 turns of AWG36, but I don't think it matters, so long as it has a really really high peak and is hot enough to get a clean signal out of the pickup you're measuring. HTH

Any chance of a tutorial on all this to follow for those such as myself that are interested and willing to learn, but lack both the physics knowledge and any hands on experience with the gear?

Addendum. . .

So after reading the paper a few times, my first question would be as above, how do you physically hook all this stuff up to a unit like the CGM-101?

Next would related to the resonant peak of a pickup. The basic measurements you see detailed by pickup manufacturers are usually some combination of DCR, Inductance and Resonant Peak. Since inductance is a function of resonant peak and vice-versa, is inductance always quoted at the 1kH of an LC meter, or is it quoted based on the resonant peak of the pickup? The later would somehow seem more appropriate, but obviously out of the grasp (at least initially) of a lot of hobbyists such as myself that don't come to the table with that background.

[charrich56]

Jim,

Dadgummit, I just composed a long reply and fat-fingered, and lost it. Sorry, I will try to answer more in-depth later.

The driver coil I am using is 750 turns of #32 on a one-piece plastic Strat bobbin without any polepieces or magnets. DC resistance is 82 ohms. The CGM-101 waveform generator is 150 ohms internal impedance, so I built the coil to be about the same impedance at 1 KHz.

I don't usually use the Lissajous figure X-Y measurement for inductance like in the paper since I have a DER EE DE-5000 LCR meter. Lots of guys on this forum use the Extech. Both are excellent and I recommend that you have one of these since it will serve you well in doing the pickup winding and testing thing.
If you don't have an LCR meter you can make do with a DMM with capacitance measurement, and the Lissajous display method.

The CGM-101 has three BNC's. One is the waveform generator out and the other two are scope channels A and B. You need three cables - I use a BNC to alligator clips for the generator, and real scope probes for the A and B channels. These are not too expensive and you can get them either with the CGM-101 or independently (got my probes on Ebay and built the other cable.)

Typically to use the CGM in network analyzer mode (for frequency response sweeps, etc.) you need to connect the generator and channel A to the input of the circuit you are measuring, and channel B to the output of the circuit. For a pickup you will use a resistor as the paper describes and connect the signal generator and channel A to one end of the resistor and channel B and the "hot" lead of the pickup to the other end. Then everything else is grounded together.

Hope that helps a bit, can give more details later.

-Charlie

[charrich56]

As for your second question, I don't know that there is a standard for quoting inductance of pickups. Most small audio transformers are specified at 1 KHz. If I were to guess I would say 1 KHz.

That being said, though, there have been several threads on measuring inductance with the Extech on this forum and unless I don't remember correctly, 1 KHz measurements can be suspect because of the winding resistance, stray capacitance, and eddy currents, so 120 Hz seems to be what was regarded as a better measurement frequency. But that is for measurement at a fixed frequency with an LCR meter.

If you are using the Lissajous method, I would recommend that you use a large enough capacitor to put the resulting resonance frequency (with the cap you chose) to be well down in the audio band, preferably below 1 KHz, but not too low like 100 Hz. You are varying the frequency to measure when the coil with the capacitor you chose goes into resonance, so it is a completely different test method than an LCR meter. But in the paper, the author was getting good results with different caps above that band, so maybe it's OK to an extent to use smaller caps and get a pretty high frequency.

The key in this method is being able to accurately measure the value of the capacitance so you can plug it into the formula and get a good number for the inductance.

It's usually pretty easy to see when the Lissajous pattern "flattens out" on the scope, and the waveform frequency at this point, along with knowing what capacitance you are using, lets you find out the inductance.

-Charlie

[Lyrebird Steve]

Any chance of a tutorial on all this to follow for those such as myself that are interested and willing to learn.

So after reading the paper a few times, my first question would be as above, how do you physically hook all this stuff up to a unit like the CGM-101?

Still not sure after reading all this ... for about $100 seems a pretty good diagnostic device, especially if you can set a freq to say 440Hz and analyze the FFT output for harmonic distributions.

Anyone got one of these ?

[kayakerca]

Jim,

Dadgummit, I just composed a long reply and fat-fingered, and lost it. Sorry, I will try to answer more in-depth later.

The driver coil I am using is 750 turns of #32 on a one-piece plastic Strat bobbin without any polepieces or magnets. DC resistance is 82 ohms. The CGM-101 waveform generator is 150 ohms internal impedance, so I built the coil to be about the same impedance at 1 KHz.

That coil I can make

I don't usually use the Lissajous figure X-Y measurement for inductance like in the paper since I have a DER EE DE-5000 LCR meter. Lots of guys on this forum use the Extech. Both are excellent and I recommend that you have one of these since it will serve you well in doing the pickup winding and testing thing.
If you don't have an LCR meter you can make do with a DMM with capacitance measurement, and the Lissajous display method.

I have a Extech LCR200. So far, so good.

The CGM-101 has three BNC's. One is the waveform generator out and the other two are scope channels A and B. You need three cables - I use a BNC to alligator clips for the generator, and real scope probes for the A and B channels. These are not too expensive and you can get them either with the CGM-101 or independently (got my probes on Ebay and built the other cable.)

Here's where I really begin to step out of my wheelhouse, but I'm a pretty quick study. Would the CircuitGear Mini Kit CGM-102 bundle include all the cables/connectors you are referring to/need?

Typically to use the CGM in network analyzer mode (for frequency response sweeps, etc.) you need to connect the generator and channel A to the input of the circuit you are measuring, and channel B to the output of the circuit. For a pickup you will use a resistor as the paper describes and connect the signal generator and channel A to one end of the resistor and channel B and the "hot" lead of the pickup to the other end. Then everything else is grounded together.

I'd need some more help at this point, maybe even a picture of the setup if possible.

Hope that helps a bit, can give more details later.

-Charlie

So, with this setup you could measure the resonant peak of a pickup and the impact on the resonant peak of a pickup for a particular type of wind at a given DCR (e.g. a Strat at 200 tpl vs. 156 tpl vs 100 tpl vs 50 tpl) for a predetermined DCR and tension? And, for a wound pickup to a specified DCR, you could alter the Inductance to measure the impact on the resonant peak? I'd REALLY like to be able to try this as I find it extremely difficult to stay away from any tech aspects that may be within my grasp as it relates to something I am interested in, not to mention that I find it very cool!

I am extremely interested in learning more on how to set this up.

[Kindly Killer]

I ordered one yesterday - the kit that comes with the cables/probes. I will share what I learn, but I'm not very science-adept. I had to look up BNC to figure out that's the connector type, which should have been obvious from context LOL. I've never used a real hardware oscilloscope.

I guess I can report on how easy it is to use for a novice.

Jim, this is the kit that includes the cables. CircuitGear Mini Kit
AFAIK that has everything you need to do the tests in the PDF that Charlie linked to. I figured I'd make a real primitive fixture to hold the probes, pickup, etc to make it easy.

[kayakerca]

I ordered one yesterday - the kit that comes with the cables/probes.

From a Canadian company. Wow. Looks like finally the wait and border crossing issues will be in the south bound direction. The shoe will be on the other foot for a change!

I will share what I learn, but I'm not very science-adept. I had to look up BNC to figure out that's the connector type, which should have been obvious from context LOL. I've never used a real hardware oscilloscope. I guess I can report on how easy it is to use for a novice.

I took a course in university called "The Physics of Hi-Fidelity". I referred to it as "Stereo Watching for Arts Students". Seems I still have the text.



We did get to use oscilloscopes on a wide variety of stereo equipment as part of the course, but that was back in 1977 and I was an accounting and finance guy, so I think I may want to crack that text open again for a bit.

Jim, this is the kit that includes the cables. CircuitGear Mini Kit
AFAIK that has everything you need to do the tests in the PDF that Charlie linked to. I figured I'd make a real primitive fixture to hold the probes, pickup, etc to make it easy.

As I was saying in previous posts, I'm really interested in seeing how this setup goes. Please post on your progress (maybe even some pics). I also hope Charlie does some more tutoring on all this.

[charrich56]

Still not sure after reading all this ... for about $100 seems a pretty good diagnostic device, especially if you can set a freq to say 440Hz and analyze the FFT output for harmonic distributions.

Anyone got one of these ?

I have one . I think it's a great piece of gear.

I haven't used the FFT function yet. Probably will at some point.

Hey, as a gear-crazy techie by inclination, I do just want to point out that these tools are fantastic for getting repeatability and some sort of handle on some of the electrical characteristics of what you are winding/building, but there are also some very subtle effects going on here (e.g. magnetic circuit, eddy currents, winding technique) also that affect the final tone of the pickup. It can't be reduced to an exact science yet. Which is one of the reasons it is so fun to fool with it.

If some more folks would like some more information on measuring pickups in the form of a tutorial, pictures, etc. I might be able to oblige, but we have a new cold front rolling into north TX and it is pretty freaking cold in the garage, so it might have to wait a little bit.

Yes, the kit for 129.00 would include all the cables you would need to do all that is mentioned in the white paper. I cheaped out and built one BNC cable for the signal generator, and ordered the two scope probes from Ebay. Only saved about ten bucks, though.

-Charlie

[kayakerca]

If some more folks would like some more information on measuring pickups in the form of a tutorial, pictures, etc. I might be able to oblige, but we have a new cold front rolling into north TX and it is pretty freaking cold in the garage, so it might have to wait a little bit.

-Charlie

Hi Charlie:

As you know, specific focuses and skill sets are all over the map around here, so I can't speak for any others, but I would appreciate some form of tutorial (with pic's) if you have the time and inclination. It would be much appreicated.

As for the cold, it's headed down to -8°F here tonight. And the low tonight in North Texas?