Stupid question about resonant frequency test.

[charrich56]

19 degrees with wind chill below zero tomorrow AM. We were walking around in short-sleeved shirts this time last year.

I am into winding low impedance right now, but have a motley assortment of "normal" pickups I can go through the test on.

Just in case you guys haven't seen this, here's a chart of some common pickups, inductances, and resonant frequencies with different capacitances.

Pickup Resonance Chart

And for those who want to make it a graduate-level project and play with $50K worth of gear, there is

http://www.google.com/url?sa=t&rct=j...S5F-WOZBQX5X4w

[Kindly Killer]

I would also love to see a tutorial using this gear.

I can do a tutorial on shoveling snow. I shoveled about a foot of it yesterday before dawn to get my wife on the road. Dipped into the +- single digits for a few days here in Kansas City, though our winters are usually much milder.

[Kindly Killer]

FWIW my package arrived today (ordered Tues from Canada, arrived Sat). Looks nice and solid. No time to mess with it this weekend.

[kayakerca]

FWIW my package arrived today (ordered Tues from Canada, arrived Sat). Looks nice and solid. No time to mess with it this weekend.

CGM-101 arrived Thursday. Loaded up the app, connected the hardware and ran a few signals through it from the generator. Sine wave looked like a sign wave. Square looked like a square wave. The measurements lined up with the output from the signal generator. Triggers cursors and measurement cursors worked. Everything seemed to work just fine. Of course, how could I possibly know if that were really the case as this is just not part of my skill set.



I'll wind up a coil to Charlie's spec this week as I have a spool of AWG 32.5 that Mark Hammer generously loaned me a while back, but I'll have to pull out the old sewing machine winder as the 32.5 just won't fit in the small ID wire guide on the CNC winder.

[charrich56]

Dude! A CNC winder?

Anyway, glad you have things in hand. The CGM-101 is a fine piece of gear.

-Charlie

[kayakerca]

Dude! A CNC winder?

Charlie, I found building CAMs took me a REAL long time to get one that worked even close to reasonably well for me and since each type/size of pickup would need its own CAM, I went down a different path fairly early in the process. Designing and building a CNC Winder turned out to be a bit easier than I thought at the outset.

My Frist Winder: My CNC Winder:

Anyway, glad you have things in hand. The CGM-101 is a fine piece of gear.

-Charlie

Not even close to having things in hand with the scope setup and implementation for pickup measurement. Definitely going to need the tutorial as I'd pretty much be the textbook definition of the Peter Principle on this one.

[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

-Charlie

Well, hopefully I'm getting a little closer on this. I wound a driver coil with 680 turns of AWG 32.5 on a garbage Strat plastic coil to 56Ω. Here is the first pickup plot on the CGM-101.



Going to head down to wind a little hotter driver coil and see how it affects things on the plotting

[charrich56]

Jim,

If you're handy enough to build both of the machines in your pics (thanks by the way for posting), you should have no problem whatsoever with the Circuitgear box.
I will try to do the tutorial type thing as soon as I can. Lots of weather, work, and kids things going on now. But I can answer specific questions pretty quickly.

It wouldn't be a bad idea to have at least one set of alligator test/jumper clips handy - usually a set of 10 in different colors, with mini alligator clips on each end. Radio Shack sells them if you have a Rat Shack up there in Ottawa or you can get them from Ebay or a number of other suppliers.

Are you using a microcontroller to control the steppers? If so, which one are you using? It looks like you have one stepper for the bobbin rotation and a linear actuator stepper
for the guide traverse. Nice. Did you roll your own stepper driver board or get one ready-made?

Actually both winders are nice pieces of work. I do see your approach with the cam in the first version. My winder has a gear-operated guide with stops and a lever so I just move the lever back and forth to wind at the appropriate place. But I am winding low impedance pickups right now so those only need 1/10 the turns, so my Fred Flintstone setup works quite well for now. 8-)

-Charlie

[kayakerca]

Jim,

If you're handy enough to build both of the machines in your pics (thanks by the way for posting), you should have no problem whatsoever with the Circuitgear box.
I will try to do the tutorial type thing as soon as I can. Lots of weather, work, and kids things going on now. But I can answer specific questions pretty quickly.

It wouldn't be a bad idea to have at least one set of alligator test/jumper clips handy - usually a set of 10 in different colors, with mini alligator clips on each end. Radio Shack sells them if you have a Rat Shack up there in Ottawa or you can get them from Ebay or a number of other suppliers.

Are you using a microcontroller to control the steppers? If so, which one are you using? It looks like you have one stepper for the bobbin rotation and a linear actuator stepper
for the guide traverse. Nice. Did you roll your own stepper driver board or get one ready-made?

Actually both winders are nice pieces of work. I do see your approach with the cam in the first version. My winder has a gear-operated guide with stops and a lever so I just move the lever back and forth to wind at the appropriate place. But I am winding low impedance pickups right now so those only need 1/10 the turns, so my Fred Flintstone setup works quite well for now. 8-)

-Charlie

Hi Charlie:

Yes, I made both the winders. On the first sewing machine motor based winder, I added a tachometer and a pair of rheostats so I could set the speed of the traverser and winder to a relationship that gave me the tpl I was winding to. Worked very well. That part of the setup is the box in the lower left with the 2 controls and digital tach display.

The controllers on the CNC winder are from Phidgets. I just bought them and coded up an app in VB.NET

rotary winder controller board: Phidgets Inc. - 1067_0 - PhidgetStepper Bipolar HC

traverser controller board: Phidget Stepper Bipolar 1-Motor Stepper Motor Controller - RobotShop

There are dll's from Phidgets that are specific to the controllers that you install on the computer and contain the connectivity and programming functions to interface with the control boards that run the steppers. They work pretty slick. You just code up your winder app in pretty much any language you like and use the commands to interface with the steppers. The boards have ramp up and ramp down variables as well, which gives you very nice control over stops and starts. Steve answered the few questions I had about coding the app. As I was saying, it ended up being a lot easier to do than I had envisioned at the outset. I was a finance and accounting guy in my career and had no work related technical background, so it had to be pretty easy to do.

But back to what I don't know how to do for a bit. . .

If I am getting it correct, the upper graph is for the driver coil and reflects the strength of the frequency for the specified range for the pickup. And, you just move the cursor over the plot after the sweep is compete to see the frequency of the high point which is the resonant peak of the pickup? I got between 5K and 7K peaks on a sample of pickups I tested (assuming this is how it is done).

Something along those lines?

[charrich56]

Jim,

Yes, you got it 100%. The sweep display shows the amplitude and phase of the voltage out (as read on Channel B of the scope) relative to the waveform generator signal voltage in (as read on Channel A of the scope) as the waveform generator sweeps its frequency from low to high frequencies, all at a constant generator amplitude. The "sweep" is really a series of steps in frequency in which each step is a certain increase in frequency over the step before it. So if the step number were 1, for example, and the last frequency that the generator was set to was 1000 Hz, the next step (new frequency) would be 1001 Hz, and so on. The software will keep stepping up the frequency until it hits or exceeds the upper frequency limit.

You will notice in this scheme that the there are more steps, thus more resolution, the higher you go in frequency, for a given step size. So you can (actually, need to) control the tradeoff between step size, upper and lower limits, and time to analyze, to get the information you need.

You can "zero in" on a better resolution on the resonant frequency by setting narrower limits and smaller steps. So if you did a wideband sweep and found an approximate peak at,say, about 5 KHz, you could re-run the sweep with the lower limit at 4.9 KHz, and an upper limit of 5.2 KHz, and a step of 10 Hz. The time to analyze of course depends on the number of steps. Then you could more easily get a better number for the resonant peak since the display will be "zoomed" horizontally.

The actual self-resonant frequency (or fairly close to it) is what you are measuring, because you are adding very little capacitance to the capacitance already existing in the pickup itself, and about 1 Megohm resistance. So 5 to 7 KHz is a good range and consistent with what I have seen in measuring commercial pickups. If you add any other capacitance and resistance, such as the guitar's volume and tone controls, and the capacitance of the connecting cable (typically 25-40 pf/foot of cable), and the amp's input impedance, the resonant peak will change in both frequency (capacitance effect) and in peak amplitude (resistance effect.)

Helmuth Lemme has a good writeup (and a book, for that matter) on this; if you haven't seen it, take a look at his web page:

BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

Of course you don't need his $1000.00 pickup tester since you will be able to do as well or better with your Circuitgear box and your laptop.

You are definitely on the right track with those numbers. Be careful comparing actual resonant peak numbers with published ones for the same pickup you are measuring since the external circuit isn't usually specified and as you will see, can drastically change the numbers you are getting.

You can do some of that experimentation yourself to see these effects. When you are using the driver coil, you will be able to measure the pickup response in the entire guitar circuit all the way through the tone and volume controls and connecting cable, since you don't actually need to connect directly to the pickup itself (since you are coupling your generator signal magnetically.)

Oh- one important trivia bit. For single coil measurements (Strat, Tele, P-90, etc.) just place the driver coil " right on top" of the pickup you are measuring, on the same axis relative to the pole pieces. For a humbucker, you need to turn the driver coil 90 degrees and center it between the coils (see Lemme's web page I linked to earlier for a picture.) For a stacked or side by side humbucker in a single coil size (e.g. Hot Rails, JB Junior, etc.) , you might have to get creative. But the majority of things folks are winding fit the first two categories and should work fine with the Strat-sized driver coil.

Hope that helps for now!

-Charlie

[kayakerca]

Jim,

Yes, you got it 100%. The sweep display shows the amplitude and phase of the voltage out (as read on Channel B of the scope) relative to the waveform generator signal voltage in (as read on Channel A of the scope) as the waveform generator sweeps its frequency from low to high frequencies, all at a constant generator amplitude. The "sweep" is really a series of steps in frequency in which each step is a certain increase in frequency over the step before it.

Thanks Charlie! Yes, I think I have got it now. I'm one of those non-technical "tech" junkies and you've given me a very cool new measurement technique for the arsenal. You a teacher by any chance?

[Lyrebird Steve]

Well, hopefully I'm getting a little closer on this. I wound a driver coil with 680 turns of AWG 32.5 on a garbage Strat plastic coil to 56Ω. Here is the first pickup plot on the CGM-101.

[ATTACH=CONFIG]27441[/ATTACH]

Going to head down to wind a little hotter driver coil and see how it affects things on the plotting

Jim,

just got my kit in the mail today

I've got some driver coils ... but not 100% certain as to the hookup required.

Can you post of pic of the connection between the CGM-101 and the driver coil & the CGM-101 and the pickup

Thanks
Steve

[kayakerca]

Jim,

just got my kit in the mail today

I've got some driver coils ... but not 100% certain as to the hookup required.

Can you post of pic of the connection between the CGM-101 and the driver coil & the CGM-101 and the pickup

Thanks
Steve

Hi Steve:

I am out of town for a bit and will post a pic next week. But it is pretty straight forward. Connect the hot leads of the signal generator and input 1 probes to the hot lead of the driver coil and the ground leads of the signal generator and input 1 probes to the ground lead of the driver coil. Then connect the hot lead of input 2 probe to the hot lead of the pickup and the ground lead of probe to the ground lead of the pickup. That's about it.

I test with the 2 single coils lying parallel to each other with a ~ 3/4 spacer between them. I used one of those Stadler white pencil erasers as the spacer. The spacer was so the strength of the field generated the driver coil would be equal (in a non precise kinda way) when I was comparing 2 different pickups. You can save one plot, then test another coil and have it map the plot over the saved plot to visually compare the frequency plots of the 2 pickups being tested. Pretty cool actually. You can start to theorize why the pickups sound different based on the differences in the plots. As an example, when I compared one of single coils to one I removed from a Squier, you could see that the Squier had a much flatter plot. My theory there was that the flatter plot showed that the secondary harmonics of the Squier pickup were much less prominent in the tonal mix which could explain why that pickup sound had such a dull tonal character. Well, that was my speculation anyway. Probably way off on that but who knows.

You can also see the relative shape and magnitude of two coil plots especially where something like the inductance of the two pickups are quite different and also see the difference in their resonant peaks. Still trying to figure how some single coil pickups wound with AWG 42 to a moderate output level can achieve resonant peaks in the 10 kHz range.

Anyway, all that to say that it is a very cool little tool for ~ $100. Post some of your results if you don't mind.

[Lyrebird Steve]

Thanks Jim,

got it going ... thanks


Duncan 1980s 5R (13.37k DCR, 6.75H)

[charrich56]

Jim and Steve,

Glad you have worked this out. I don't know if you have started testing with typical volume and tone controls, and real or simulated cable capacitance into an amp input load, but the CGM-101 can do it.

Jim, if you aren't loading the pickup with more capacitance, the resonant frequency will be a lot higher. Check out Lemme's discussion on this in:

BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

-Charlie