Electronics > Projects, Designs, and Technical Stuff
best way to measure Q
TimFox:
--- Quote from: CatalinaWOW on January 22, 2020, 03:54:01 am ---Yet another lesson in the importance of definitions. Three different definitions for Q in this thread. All correct when used appropriately. All somewhat different. Communication occurs only when the publisher of Q and the user of the information are using the same definition.
--- End quote ---
The different equations for Q are intimately related.
When you connect a coil and a capacitor together, it makes a parallel-resonant circuit with a free-resonant frequency f0. If the coil has a Q value QL and the capacitor has Q = QC at that frequency , then the Q of the resonant circuit is given by
1 / QR = (1 / QL) + (1 / QC)
neglecting the self-capacitance of the coil and the self-inductance of the capacitor. The proof is left as an exercise for the reader. Note that the circuit Q is lower than that of either component.
CatalinaWOW:
I agree to the intimate relation of the various definitions of Q. But in the simplest form, defined in terms of the losses in a resonant circuit with ideal capacitors and inductors it is purely a measure of the resistance in the circuit. There is no frequency dependence in the C or L. When defined as a property of an inductor as a function of frequency there is an implication of non-ideal performance. Parasitic capacitance and internal resistance. Different models for the real inductor will give somewhat different answers. So to interpret the answers you may need to know the model being used. Or maybe for the application you are chasing all that is needed is an ohm meter.
TimFox:
A simple ideal inductor with a constant inductance and constant resistance (independent of frequency) will have a Q proportional to frequency. How is this non-ideal behavior?
CatalinaWOW:
If that was what they were measuring, why not just report the resistance? Why bury it in a graph?
bob91343:
There are always parasitic effects. A wire inductor will have skin effect as frequency rises, slowing the expected increase of Q with frequency. An iron core inductor is a whole other matter.
A Q measurement is only good at the frequency at which it is tested, as well as the amplitude (if nonlinear).
A better test is to plot a curve of Q vs frequency to ascertain suitability of the component for use. You do have an expected use, don't you? That should help you decide how to measure it. If you are just measuring for fun, don't mark the part because the value isn't constant.
Temperature is another factor, since the resistance of a coil varies with temperature, and thus Q.
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