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best way to measure Q
bob91343:
As I said above, the Q meter does a nice job over a wide range of frequency and inductance. It works by measuring the resonant rise of a simple circuit.
The VNA has a different method, equally valid. Using a bridge is misleading because of the many variables involved, especially operator skill. My GR bridges indicate Q but only for rather low frequencies. The Q meter takes it from there up to a few hundred MHz. The VNA indicates R + jX or, in the case of the nanoVNA, R and L. This, over a very wide frequency range from 50 kHz.
The important thing is to compare apples with apples. Standardize on a method and stick with it. Comparison with other methods can be interesting. The lower the Q, the less important is the frequency. If the Q is low enough, it becomes a resistor. A perfect resistor has a Q of zero; a perfect inductor has infinite Q.
You can use a generator and oscilloscope but the 'scope input impedance becomes important for high Q.
There is, as noted above, a wealth of material available on the subject. One could start with Terman's Radio Engineering. The old highly competent and respected companies that made inductors are pretty much gone. They were spearheaded/masterminded by some of the most important people, and their work will never become obsolete.
TimFox:
--- Quote from: Stray Electron on January 23, 2020, 12:56:50 am ---
--- Quote from: ricko_uk on January 21, 2020, 08:55:23 pm ---Hi,
what is the best (most accurate?) way to measure and inductor's Q at a specific frequency using scope, signal generator and standard lab equipment (but not RF type equipment such as antenna or network analysers)?
Most manufacturers tend to specify it at 1MHz (for most coils) and often also provide a Q vs Frequency chart. But when I use the formula Q = 2 * pi * f * L / R the result I get is nowhere near what is displayed in the charts for that frequency.
Thank you
[/quote
You do realize that in that formula f is in radians per second and not cycles per second, don't you? 1 cycle = 360 degrees = 2 pi radians. So multiply CPS times 2 pi to get "f" in radians per second.
--- End quote ---
His formula shows 2 pi correctly.
--- End quote ---
Stray Electron:
--- Quote from: TimFox on January 23, 2020, 03:14:27 pm ---
His formula shows 2 pi correctly.
--- End quote ---
Agggg! So he did. I don't know why I didn't see that at the time. I guess that that's what I get for trying to read stuff when I'm too tired.
OP, take a look at this page http://www.prc68.com/I/GR1657Digibridge.shtml#Description He has a ton of information about LCR bridges and many of the related patents.
Tim is right, it gets tricky to read Q or L, C, R of D at higher frequencies or with great precision. The problem is that every R has some L and C, every C has some R and L and every L has some amount of R and C! Not only in the Device Under Test but in every component in the test equipment as well. And to add to the problems the output level of every AC signal source varies somewhat with frequency and the sensitivity of every measurement system also varies with frequency.
ricko_uk:
I have a LCR45 meter (https://www.peakelec.co.uk/downloads/lcr45-datasheet-en.pdf)
it reads the complex impedance/admittance both real and imaginary part and phase & magnitude of the impedance measurement. And it does so at the frequency of interest 200KHz. Would that help in any way to measure/compute the Q factor of the inductor?
Thank you
Kleinstein:
For relatively high Q values the ring down method can work well. It does not work well anymore if Q is much below 5. The measurement of the resonance curve with a VNA or similar and thus quasi settled measurements works well for a low Q and gets more an more demanding with a high Q , especially at lower frequency. The non resonant measurements with a VNA or bridge is kind of limited by the calibration / reference for the phase shift. Especially a high Q (low loss) is not so easy to measure off resonance. So it depends on the Q range (and frequency) which method works best.
A parallel resonance can get pretty high impedance, so the scope probe can cause damping. Another point is the excitation - ideally relatively weak coupling is used for the resonant methods.
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