Topband: Measuring Inductor Q

[Jim Brown]

On Sun, 06 Jul 2008 13:16:13 -0400, GEORGE WALLNER wrote:

>Is there a better -- cost effective -- method I should be using?

Study the test setup and discussion about it in the two pdfs on my
website about RFI for Hams and Coaxial Chokes. 

http://audiosystemsgroup.com/publish

Two major obstacles to getting accurate measurements of the sort of
things you're trying to measure. First is stray capacitance and input
resistance of the test setup or instrument. Second is that any
instrument that works on the basis of reflections in a transmission
line is only good within about a 3:1 ratio of the characteristic
impedance, and the accuracy degrades rapidly as the ratio gets higher.
Virtually all antenna analyzers work this way. So do network analyzers
when they measure S11. In other words, the instrument is good between
about 17 ohms and 150 ohms, is still sort of in the ballpark at 10 ohms
and 250 ohms, and is pretty useless outside that range. The AIM unit
suffers from both of these limitations.  

My pdfs show a good way to do it with simple equipment. In essence, you
connect the unknown impedance in series between a signal generator and
an RF voltmeter, with a good 50 ohm or 75 ohm load across both the
signal generator and the voltmeter. Then you measure the voltage across
the 50 ohm resistor over a broad range of frequencies that extends from
well below resonance to well above resonance and use the voltage
divider equation to get the magnitude of the impedance. Then plot Z vs
frequency in a spreadsheet.  Now you have the resonance curve of the
unknown inductor, and you can curve-fit it with the equation for a
simple parallel resonant RLC circuit to get R, L, C, and Q.   

One piece of work remains. You need to measure a known resistance over
the same frequency range and find the -3dB point of that resistor with
the stray capacitance of your test setup. Mine worked out to be abou
0.4 pF. 

Note that this setup is only good for Z values that are at least 10X
the 50 or 75 ohm load you're using.  My signal gen is an HP8657A, and
my voltmeter is an HP8590D spectrum analyzer.  Any generator and
voltmeter of decent accuracy will work.  This is the setup I used to
measure all of my coaxial chokes.  

73,

Jim Brown K9YC