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previous: Joe Gwinn On 5/17/2006 2:58 PM, moocow said:<... -- 1147953795

Re: Its a damn close to official as possible

5/18/2006 8:59 AM
moocow	Re: Its a damn close to official as possible I found that there was no significant difference between the capacitors. I used several .1uF caps, some old, some new, and one that was known to leak. All tested about the same, but two of the new ones were more 'ideal' at 100kHz than the others. The problem is that the machine tests with a small AC voltage. It can put a DC bias on the component but only a few volts. Guitar amps put a few hundred volts of DC on the caps and the AC signal is much higher. Some versions of the digi-bridge can use an external bias, but I didn't feel it was worth the risk of repeating the experiment with 400V of bias. This also wouldn't solve the probem of the small AC voltage test signal. The machine belonged to the UCLA material science lab and it was used to investigate the properties of various electrochemical cells. The low AC and DC voltages were appropriate for that application since they were interested in linear properties. I wanted to see the nonlinearites caused by DC voltageg and large AC signals so the machine wasn't too helpful. Even with the non-linear inductance of the pickup, the digi-bridge can make a guess as to the inductance value that best fits the data. I suppose that's worth something. I'm not sure how it reacts to eddy currents, though, but again, that's what MATLAB is for. I've posted this before, but here's a lab rig designed to measure impedance of guitar pickups: http://wug.physics.uiuc.edu/courses/phys398emi/398emi_guitar_pickup_results.html The payoff is a graph like this: http://wug.physics.uiuc.edu/courses/phys398emi/Experimental_Results/Pickup_Data/Lindy_Fralin_Hot_Strat_Bridge_Pickup_wCBP_IZI_vs_F.jpg Similar data can be obtained with much simpler equipment but you'll have to take data points by hand for each frequency measurement and input them by hand into your computer to see the plot. Perhaps the plot is enough information to see differences between pickup designs. Anyway, I only wanted to point out that the Extech meter has its limitations, but if you want to go farther than the Extech, you'll need to go through a great deal of time and effort to get there.

5/18/2006 8:59 AM

moocow

Re: Its a damn close to official as possible

I found that there was no significant difference between the capacitors. I used several .1uF caps, some old, some new, and one that was known to leak. All tested about the same, but two of the new ones were more 'ideal' at 100kHz than the others.

The problem is that the machine tests with a small AC voltage. It can put a DC bias on the component but only a few volts. Guitar amps put a few hundred volts of DC on the caps and the AC signal is much higher. Some versions of the digi-bridge can use an external bias, but I didn't feel it was worth the risk of repeating the experiment with 400V of bias. This also wouldn't solve the probem of the small AC voltage test signal.

The machine belonged to the UCLA material science lab and it was used to investigate the properties of various electrochemical cells. The low AC and DC voltages were appropriate for that application since they were interested in linear properties. I wanted to see the nonlinearites caused by DC voltageg and large AC signals so the machine wasn't too helpful.

Even with the non-linear inductance of the pickup, the digi-bridge can make a guess as to the inductance value that best fits the data. I suppose that's worth something. I'm not sure how it reacts to eddy currents, though, but again, that's what MATLAB is for.

I've posted this before, but here's a lab rig designed to measure impedance of guitar pickups:

http://wug.physics.uiuc.edu/courses/phys398emi/398emi_guitar_pickup_results.html

The payoff is a graph like this:

http://wug.physics.uiuc.edu/courses/phys398emi/Experimental_Results/Pickup_Data/Lindy_Fralin_Hot_Strat_Bridge_Pickup_wCBP_IZI_vs_F.jpg

Similar data can be obtained with much simpler equipment but you'll have to take data points by hand for each frequency measurement and input them by hand into your computer to see the plot. Perhaps the plot is enough information to see differences between pickup designs.

Anyway, I only wanted to point out that the Extech meter has its limitations, but if you want to go farther than the Extech, you'll need to go through a great deal of time and effort to get there.

Replies:

David Schwab On 5/18/2006 2:59 PM, moocow said:<... -- 5/18/2006 12:55 PM