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Inductor Q & self resonance freq.?

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I want to find the Q factor of an inductor according to Q=ωL/R where ωL is the reactance in resonance and R is internal resistance. So in order to solve the equation I need to find the self resonance freq. of the coil (for ω), inductance ( as I measure it in DC?) and internal resistance (also DC). How do I measure the resonant freq. of the coil? My best guess is to take a signal generator and observe the voltage of the coil as I change the freq. When the amplitude is maximal that's the resonant freq.
 

It can be a tricky measurement to make. Coilcraft has some tips, see the attached.
 

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  • Coilcraft - Testing inductors.pdf
    76.9 KB · Views: 102

Inductor Q is usually specified for a frequency considerably below self resonance frequency.

Calculating Q based on DC series resistance can be a first guess, at least for air coils. The skin effect will however increase the effective AC resistance and additional loss meachanisms add, particularly for inductors with ferromagnetic core.

So an actual Q measurement should be performed at the specified frequency. Measuring the impedance minimum in series resonance is a possible method if you have only limited measurement capabilities. A network or impedance analyzer would be the preferred tool if available.
 

FvM said:
Inductor Q is usually specified for a frequency considerably below self resonance frequency.

Calculating Q based on DC series resistance can be a first guess, at least for air coils. The skin effect will however increase the effective AC resistance and additional loss meachanisms add, particularly for inductors with ferromagnetic core.

So an actual Q measurement should be performed at the specified frequency. Measuring the impedance minimum in series resonance is a possible method if you have only limited measurement capabilities. A network or impedance analyzer would be the preferred tool if available.
Click to expand...

I have an inductor that is unbranded. I know it's DC resistance and inductance value from my DMM. I don't have means to directly measure inductance at different frequencies (no LCR meter) so how do I approach finding the self resonance freq. and the Q (at different freq. as well)? I am not going to freq. over 50KHz so "RF effects" should not be very noticeable.
 

Accurate measurement of Q is one of the most difficult measurements of all.

R cannot be measured with the necessary accuracy & precision with a multimeter.

A good method is to resonate the inductor with a capacitor of known Q. For a good paper, go to

**broken link removed**

and find "The Two Faces Of Q".

Note that not just any capacitor can be used; high-quality devices with well-specified Q are required.

The top capacitor makers make such devices; I use either CDE or ATC porcelain types.
 

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