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Old 24th January 2006, 10:25 PM   #1
DaBit is offline DaBit  Netherlands
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Default Weird idea: measuring inductance with a gyrator?

I have a weird idea. Many multimeters can measure capacitance, but only some can measure inductance.

Now what about using a gyrator (opamp circuit to 'reverse' impedance) to transform the inductance to a capacitance, and measuring that?

Anyone ever tried something like that?
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Old 24th January 2006, 11:12 PM   #2
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If you have in mind the two op-amp gyrator for "the art of electronics", then I would say it is not doable.
As far as I remember the impedance 'to be inverted' is in negative feedback loop of one of op-amps. Inductor in feedback loop means instability because of total phase shift of 180 by inductor together with frequency compensation.
Other ways, with one op-amp assume series resistance I think, but I'm no sure about this.
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Old 25th January 2006, 02:37 AM   #3
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I just cut and paste this every time I see someone trying to measure impedance, capacitance etc., the hard way:

The sound card impedance tester is here Post #1
discussed in the most recent edition of the ham radio magazine QST (with a more scientific discussion in QEX it's companion mag.)

The LMS test set uses a dual opamp to compare the voltage drop across an unknown impedance and an acurately known resistor. It is remarkably simple.

You can test inductors, capacitors, transformers, negative impedances, etc.

A PDF of the QEX article is available at www.arrl.org/qex/2005/Steber.pdf

the code can be downloaded at
http://www.arrl.org/files/qst-binaries/
just look about 1/3 down the page

you may need to run
VBRun60sp5.exe (availbable at Microsoft Downloads) if you get the error message "Required DLL file MSVBVM60.DLL was not found"

Jack

edit: I should have credited the author of the article -- Dr. George Steber, WB9LVI, Prof. Emeritus of EE at the University of Wisconsin, Milwaukee.
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Old 25th January 2006, 06:52 AM   #4
DaBit is offline DaBit  Netherlands
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Nice article. Worth a try. Thanks for pointing it out.
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