Electronics > Projects, Designs, and Technical Stuff
best way to measure Q
TimFox:
The original poster referred to a table of manufactured coils from a given vendor that tabulated Q at discrete frequencies because that’s how the maker verified the quality of the coils rolling off the line. He may have graphs of typical Q vs frequency.
Stray Electron:
--- 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)?
--- End quote ---
Depending on the frequency, my favorite is one of the Gen Rad (General Radio) 165x Digibridges! But the fact is that almost no two companies measure Q at the same frequencies! Depending on the model of Digibridge they can measure Q at several, more or less, standard frequencies and you can also use an external signal source for other frequencies. There are plenty of old LCR bridges made by companies like General Radio, Leader, etc that are manually tuned and can be used to measure an external Q by comparing it to an internal (or sometimes external) known resistance using a bridge. I would suggest going on E-bay and seeing what kinds of LCR meters are available to you and then looking up their specs and/or their operating manuals and reading those. GR for one published a LOT of material about how to do tests like this and the all of theory behind it. In the end you might not want to use one of those old LCR bridges due to the weight, size and the fact that they are slow to take measurements with since they are manually adjusted but you will at least understand what's involved in the process.
A GOOD place to start would be to look up the manual for a GenRad 1657 or 1659 Digibridge and read it.
uer166:
Okay so was about to make a separate thread but will hijack this one instead since it is relevant. I am trying to measure inductor's AC resistance (losses due to skin effect and copper losses) at specific frequencies. I tried:
* Injecting an AC voltage and measuring current. With some complex plane math can determine real part of the impedance (what I'm after). Unfortunately the results are wildly inaccurate. Probably since the phase shift due to AC resistance is so tiny.
* VNA, same result, unable to measure phase accurately enough
* Ring-down method previously mentioned
Trying to make ring-down method work but have unexpected results. The test jig picture attached. The tank circuit is excited with a one-turn winding attached to a function gen outputting a square wave.
The ringdown voltage waveform across cap is attached, white is the Litz, yellow 14AWG. The capacitor is made from 20 47pF C0G 0.5% caps (0.94nF total) in an attempt to reduce its' AC resistance to much lower values than the inductor is. Foil tape would hopefully help with skin effect. For the experiment I made two ~130uH inductors, one with Litz, other with regular 14AWG. What I expected: litz wire version should have much higher Q since at ~450KHz skin effect losses are substantial. Reality: both have a Q of around ~16. And not only is this much lower than I expected (I thought it'd be something like 100 with Litz), the difference between regular and Litz wire is marginal.
The core is a High Flux material: https://www.mag-inc.com/Media/Magnetics/Datasheets/0058726A2.pdf. Both have 27 windings.
Question that I have is same as OP, what is the best way to measure Q, and if this is the best way, why am I getting unexpected (much much worse) values?
Stray Electron:
--- 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
--- End 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.
Weston:
--- Quote from: uer166 on January 23, 2020, 12:43:41 am ---Okay so was about to make a separate thread but will hijack this one instead since it is relevant. I am trying to measure inductor's AC resistance (losses due to skin effect and copper losses) at specific frequencies. I tried:
* Injecting an AC voltage and measuring current. With some complex plane math can determine real part of the impedance (what I'm after). Unfortunately the results are wildly inaccurate. Probably since the phase shift due to AC resistance is so tiny.
* VNA, same result, unable to measure phase accurately enough
* Ring-down method previously mentioned
Trying to make ring-down method work but have unexpected results. The test jig picture attached. The tank circuit is excited with a one-turn winding attached to a function gen outputting a square wave.
The ringdown voltage waveform across cap is attached, white is the Litz, yellow 14AWG. The capacitor is made from 20 47pF C0G 0.5% caps (0.94nF total) in an attempt to reduce its' AC resistance to much lower values than the inductor is. Foil tape would hopefully help with skin effect. For the experiment I made two ~130uH inductors, one with Litz, other with regular 14AWG. What I expected: litz wire version should have much higher Q since at ~450KHz skin effect losses are substantial. Reality: both have a Q of around ~16. And not only is this much lower than I expected (I thought it'd be something like 100 with Litz), the difference between regular and Litz wire is marginal.
The core is a High Flux material: https://www.mag-inc.com/Media/Magnetics/Datasheets/0058726A2.pdf. Both have 27 windings.
Question that I have is same as OP, what is the best way to measure Q, and if this is the best way, why am I getting unexpected (much much worse) values?
--- End quote ---
The output impedance of the function generator is in parallel with the LC tank and is going to damp the resonance during ringdown and reduce the measured Q.
For linear (air core inductors) you can create a series / parallel resonant circuit and measure the 3db points and calculate Q from that. Given the accuracy range of most VNA's, series resonance is probably more accurate.
For cored inductors, which have loss that changes non-linearly with drive level, the previous method will only give you the small signal Q level, under higher power it is going to be less. For a more accurate test you can drive the resonant circuit with a function generator / power amplifier and measure the voltage / current gain at resonance. By changing the drive level you can measure how Q changes with flux density. This paper details a possible test setup: https://www.rle.mit.edu/per/wp-content/uploads/2016/01/Hanson-Measurements.pdf (Figure 13)
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