Electronics > Projects, Designs, and Technical Stuff
best way to measure Q
ricko_uk:
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
Benta:
There's no such thing as an "inductor's Q", unless you're talking about it's self-resonance frequency.
Loss factors in inductors are winding resistance, and if cored, core losses. At higher frequencies, skin effect is also to be considered.
ricko_uk:
Thank you Benta,
I'm not sure I understand your comment. Could you please elaborate.
Almost all of Coilcraft's inductors include a chart of their Q vs Frequency. This link is just an example: https://www.coilcraft.com/1812ls.cfm
The Q is always specified at 1MHz for all values and that is nowhere near the inductor's SRF which clearly also varies for every inductor's value.
Could you please explain what you mean.
Many thanks :)
TimFox:
Yes, inductors have a Q factor. At a fixed frequency, the Q is the ratio
Q = (reactance of series inductance) / (series resistance).
Things get messy when you are close to the self-resonant frequency of a physical coil, since the measured series inductance will increase due to the parallel self-capacitance of the winding.
The tabulated values are ordinarily measured at a frequency well below self resonance.
Even after correcting for self capacitance, neither the Q nor the equivalent series resistance is independent of frequency.
chris_leyson:
Q = wL/R = 2*pi*f*L/R has nothing to do with the SRF. You can measure Q at any frequency you like depending on the inductor and/or the circuit application. Q of a parallel resonant circuit is defined as q = fr/BW where fr is the resonance frequency and BW is the 3dB bandwidth. You could make a parallel resonant circuit with a good quality capacitor and couple into and out of the tuned circuit with very small capacitors and tune a sig gen either side of resonance to find the 3dB bandwidth. Another method called the ring down method applies a step or pulse to start the circuit oscillating and then you measure the time for the oscillation to decay. See: https://www.giangrandi.ch/electronics/ringdownq/ringdownq.shtml
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