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Re: Testing


 :
7/9/2004 6:52 PM
moocow Re: Testing
Sorry to take so long. Here is a link to a cartoon showing how to hook up a pickup and signal generator to measure pickup inductance.  
 
http://mysite.verizon.net/res1ns9p/Files/Guitar/Pickup%20Inductance.gif  
 
For best accuracy, you'll need a frequency counter or digital multimeter (DMM) that reads frequency, otherwise, you'll need a very accurately calibrated test tone. The idea is to put a resistor in series with the pickup and to use the voltage divider equation to calculate the reactance of the pickup. The pickup reactance itself is due to the winding resistance of the pickup and the pickup inductance. Deriving the equation is messy so I'm only showing the equation to calculate L based on a few simple measurements. I'm showing a 22K fixed resistor in the cartoon and it seems to work well with the usual inductances found in guitar pickups (3-10 Henries). The actual value of the resistor should be used in the equation, not the value marked by the color bands.  
 
As an example, based on the following measurements,  
 
Rw = 5,980 ohms  
Rt = 21,700 ohms  
Vgenerator = 1.10 volts  
Vresistor = .68 volts  
 
the pickup inductance is 3.44 Henries.  
 
An inductance meter would give the same result and might actually be a tiny bit more accurate. Using a nice LCR meter gives us an answer right away, but we will not have learned anything about guitar circuits in the process. Learning how to use an old-fasioned method such as taking voltage measurements helps us learn more about how our pickups act in a guitar circuit. In fact, a signal generator can be used to find the frequency response of a pickup when it is put in a real guitar circuit. I've seen people using a headphone to do this, but that's a very flawed method. You'd need a calibration curve of the headphone's magnetic field output vs. input voltage for this method to have any sort of accuracy. Also, each of us is going to be using their own headphone so measurements taken by different people will not be directly comparable. But a signal generator is a signal generator, and a volt is a volt, so we all can use this signal generator method and get good, consistent, valid results.
 
7/10/2004 1:00 AM
Dave Stephens
Hi: you are probably going to kill me for asking this, but could you lead us mathmatical idiots through each step of the equation? Most of us never got that far and most of us don't use these kinds of equations in real life at all.  
 
 
 
Also, you mentioned using a formula to calculate resonant peak frequency and you didn't cover that at all, and frankly I am more interested in that aspect of this teting method. I have been trying to figure out a way to measure peak frequency for over a year and you're the only one who seems to have a rational accurate method of doig so. Thanks in advance......Dave
 
7/11/2004 10:59 PM
moocow
Hi Dave,  
 
I can't be too upset about your question since I posted the wrong equation in the first place! I fixed the error and updated the picture to include the numbers from my example being used in the equation. Some of my old textbooks also give examples where numbers are used in the equations, so I really should have included one in the first place:  
 
http://mysite.verizon.net/res1ns9p/Files/Guitar/Inductance%20Measurement.gif  
 
I still need to work on the resonant frequency cartoon, but for now, here's the method I use to measure it directly. Just change that 22K resistor in the inductance measurement test to 1M, then adjust the signal generator frequency until the voltage across the 1M resistor reaches its minimum value. Set your DMM to frequency and measure the frequency of the signal generator. This will be the resonant frequency of the pickup. For a Stratocaster pickup, this should be 10-13kHz or so. The test works because at the resonant frequency, the pickup's impedance becomes very, very high. The pickup and 1M resistor work as a voltage divider, so the greater the pickup impedance, the lower the voltage across the 1M resistor. When the pickup is at the resonant frequency, its impedance is at a maximum so the 1M resistor voltage will be at its minimum value.  
 
You might think that you could measure the voltage across the pickup and adjust the frequency until the pickup voltage reaches its maximum value, but I've found that the presence of the DMM itself will throw off the resonant frequency. The impedance of the DMM is high, but the pickup impedance at resonance is so high that the DMM loads the pickup enough to influence the resonant frequency.
 
7/12/2004 12:46 AM
Dave Stephens
Awesome!!! I have been looking for a simple way to measure resonant frequency for over a year now and this is it! Can you tell me if this signal generator on Ebay would work for this:  
 
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4683&item=3825427458&rd=1  
 
 
 
I know zero about signal/frequency generators so any help is appreciated. Dave
 
7/14/2004 4:12 AM
Dave Stephens moocow where are you???
Are you sure you got this right on measuring resonant frequency? I thought impedance went DOWN when the peak is hit, so it would seem the voltage across the resistor would go up??? Frequency response curves always show the resonant peak on the upper part of the chart and when the pickup chokes on itself the curve drops down off the chart. I did order that frequency generator off ebay for $50 which is probably a real good price so will be trying this out next week. Dave
 
7/17/2004 12:41 AM
moocow Measuring Pickup Resonant Frequency
Sorry for being absent, but I've been trying to get over a case of bronchitis. I'm now on antibiotics and my doctor gave me a bottle of cough syrup that really makes time fly by!  
 
The pickup's own impedance becomes very high at resonance, which may not make sense, but it is true. Typically, the voltage measurement is taken across the pickup which gives a spike in the frequency response. However, that method introduces error into the measurement, but the 1M resistor method does not. In fact, the exact value of the 1M resistor isn't even important. It only functions as part of a voltage divider and it needs to be large, otherwise, the minimum voltage will be essentially zero which will be difficult to read with your meter.  
 
It is true that measuring across the pickup is more realistic since that's how a pickup is connected to an amplifier, but the response of the pickup changes when it is loaded by volume and tone pots, guitar cable capacitance, and the amplifier input resistance. The overall frequency response is quite different and it is a completely different topic than 'pickup resonance'. For now, I will focus on the pickup characteristics only and save the overall pickup/guitar/cable/amplifier response for later.  
 
Here's the cartoon showing the test setup and equations for the resonant frequency test:  
 
http://mysite.verizon.net/res1ns9p/Files/Guitar/Resonant%20Frequency%20Measurement.gif  
 
It shows how to calculate the Q and capacitance of the guitar pickup. I'm not sure if Q tell us anything important, but the winding capacitance C is something pickup makers think is important to talk about, but apparently not important enough to measure. I hope that some of you pickup makers take the time to measure the C of your pickups and see what kind of values you get.  
 
Here's a link to a university site where a more elaborate setup was used to determine pickup impedance:  
 
http://wug.physics.uiuc.edu/courses/phys398emi/398emi_guitar_pickup_results.html  
 
It may seem confusing, but their method is similar to mine. There are some pictures to click on, but the most important one is the Magnitude of Impedance,
 
7/23/2004 7:54 AM
Dave Stephens
Moocow: damn I wish I had your email address, you have got me started on this quest and I mean to make this thing work somehow with what I have. So here goes again:  
 
 
 
Well......the method for finding resonant frequency according to your method (or my stupidity) just plain doesn't work in real life. I got a nice audio signal generator on Ebay that goes from 1hz-1mhz, nice deal for $50. Anyway, so I take my strat pickup, connected a 1megohm resistor across the leads. Then I connected the frequency generator to either side of the resistor, and also across the resistor connected my digital DVM (Wavetek Meterman, which also measure frequency and inductance, very cool...). I put the meter on volts AC and start sweeping the generator through the various ranges. Problem is the lower the frequency the lower the voltage, the higher the frequency the higher the voltage, there is no peak and no drop anywhere. Am I doing something wrong? Needless to say I was VERY disappointed.  
 
 
 
So ok, I thought about it a bit and got a bit pissed off. So I start thinking ok, a resonant peak (in my warped theoretical brain) should be putting out the most voltage from the pickup right? So (ok don't wretch here....) I hooked up a cheap pair of headphones to the signal generator and taped my pickup to the ear piece. Hook the DVM to the pickup and started sweeping the frequencies again. OK, this time there IS a definite high voltage peak. Samples I got are: a Fender strat coil (one that really sucks FYI) peaked at 8,2KHZ, one of my Twister neck strat pickups at 6.7KHZ, and one of my P90 style pickups in a humbucker casing at 6KHZ.  
 
 
 
Now of course I realize that the headphone piece has a magnet in it and that the headphone itself has its own peak resonance and maybe these readings are kinda OFF. But this goes back to windng a coil just to drive the pickup in such a test. I have gotten various opinions on how that coil should be wound and what gauge wire etc. but this should be a cleaner method of driving a signal into the pickup. But, is it true that the resonant peak will produce the most voltage coming from the pickup in such a test?  
 
 
 
Sorry for being obsess here but thats how I am......OK, so what if you want to put a signal into the pickup and want to use a cheap oscilloscope (they are cheap on Ebay for old ones), would this or some other method make the peak more readily apparent than all this math or whatever? I have thought that (I think I read this somewhere) that if you put white or pink noise into a pickup and put it on an oscilloscope you will see where the peak is visually since supposedly white or pink noise is a combination of all frequencies, is this right?  
 
 
 
Another subject covered awhile back was the effect of capacitance in a pickup, but it seems that the capacitance is so awfully low that its probably not really something that you can use to design a coil right? We also discussed whether a normal DVM can actually read the capacitance of a coil, a very confused thread if I remember right....  
 
 
 
OK, Moocow you got me into this now get me out of it and thanks for your input too......Dave
 

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