### Author Topic: Measuring the self resonant frequency of an inductor?  (Read 7100 times)

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#### LooseJunkHater

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #50 on: October 15, 2023, 08:53:10 pm »
Remember that the (lowest frequency) self-resonant mode of a capacitor is a series resonance (parasitic series inductance), while the lowest-frequency SRF of an inductor is a parallel resonance (parasitic parallel capacitance).
For that purpose, using a 50 ohm spectrum analyzer, I merely ran the generator output through 50 ohm resistor in series with the capacitor (to ground) followed by a 50 ohm resistor to the analyzer input.
SRF for the capacitor is the frequency where the minimum signal occurs on the analyzer.

For leaded capacitors, the parasitic inductance is dominated by the total length of wires plus body.

I do not understand anything that was said here.

#### TimFox

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #51 on: October 15, 2023, 08:54:10 pm »
Remember that the (lowest frequency) self-resonant mode of a capacitor is a series resonance (parasitic series inductance), while the lowest-frequency SRF of an inductor is a parallel resonance (parasitic parallel capacitance).
For that purpose, using a 50 ohm spectrum analyzer, I merely ran the generator output through 50 ohm resistor in series with the capacitor (to ground) followed by a 50 ohm resistor to the analyzer input.
SRF for the capacitor is the frequency where the minimum signal occurs on the analyzer.

For leaded capacitors, the parasitic inductance is dominated by the total length of wires plus body.

I do not understand anything that was said here.

I don't mean to insult you, but this can be found in elementary textbooks.

#### LooseJunkHater

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #52 on: October 15, 2023, 09:04:54 pm »
I don't mean to insult you, but this can be found in elementary textbooks.

This is not a helpful response for the questions asked. I work in a completely unrelated field and I'm trying to primarily self-learn some electrical engineering content (as a hobby) but utilizing these forums for questions I cannot find answers to on Google. I have no background in EE, nor does anyone that is close to me. Further, my proficiency in maths (especially the complex maths involved in EE) is low, but I understand that many online calculators exist to aid in my learning. I do not appreciate your response.

#### TimFox

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #53 on: October 15, 2023, 09:20:17 pm »
As I stated, the relevant self-resonant frequency for a capacitor is a series resonance:  capacitor and parasitic inductance in series.
The relevant self-resonant frequency for an inductor is a parallel resonance:  inductor and  parasitic capacitor in parallel.
What can I add to that explanation?

#### LooseJunkHater

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #54 on: October 15, 2023, 09:34:39 pm »
What can I add to that explanation?

So my earlier image as to how I attempted to measure the SRF of a capacitor (attached image 0.) I assume is wrong because as @joeqsmith pointed out, I created an RC-filter.

Instead, should I attempt to measure the SRF of the capacitor in the form of image 5., or how would I measure it (are other components needed in the circuit?)?
« Last Edit: October 15, 2023, 09:48:39 pm by LooseJunkHater »

#### TimFox

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #55 on: October 15, 2023, 09:57:48 pm »
If you use the setup in your image "0", I would add a series resistor (try 1 k$\Omega$ first) from the "terminated" AWG to the top side of the capacitor, very close to the capacitor.
At its SRF, you should see a "null" in the output on the 'scope, assuming you have either a high-impedance input or a scope probe with a high impedance on the right of your diagram.
You want the "almost short circuit" at SRF to be much lower than the total external resistance (source resistor and load resistor in parallel) to see a good null.
(Note that an otherwise unknown 100 nF ceramic capacitor will have substantial parasitic series resistance.  The good NP0/C0G capacitors are normally not available at 100 nF.)
Yes, that circuit is a low-pass filter at frequencies below SRF, but the voltage should start increasing again for frequencies above SRF., where the impedance of the device becomes inductive.

#### TimFox

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #56 on: October 15, 2023, 11:25:51 pm »
Here is an elementary application note from a good capacitor vendor:
https://article.murata.com/en-us/article/impedance-esr-frequency-characteristics-in-capacitors
Note that at the SRF, as seen in their Figure 4, when the parasitic inductance's reactance equals the capacitor's reactance, that is the series-resonant frequency, and the impedance at that point is a resistance.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #57 on: October 16, 2023, 02:24:29 am »
I suggest starting with a tank circuit so you have a known L&C in parallel.  Pick values that you know you can measure independently as well as keeping the resonance in the range of your signal generator and scope.  Work the math first then measure and see if your results match.   Once you understand some of the basics, then build from that.

#### T3sl4co1l

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #58 on: October 16, 2023, 02:51:04 am »
The FFT shows gibberish because it's way down in the noise floor at the upper end.  Oscilloscopes are rather poor instruments for this task as they have low dynamic range; typically an 8-bit ADC.  If you want to plot the impedance over a 30,000:1 range, you need at least 15 bits, with the waveform just filling up the screen at its highest peak.

You could still take spot measurements along the sweep, by using a delayed trigger and zooming in, or just setting it to a narrower sweep range (a decade at a time, say), but you still have the problem that you're measuring the ESL of the cable plus capacitor.  Notice this part:

We, can at least assume, the generator is 50 ohms (it's in Hi-Z setting, but that just means the amplitude read off the display is the open-circuit ("Hi-Z") value, or twice the terminated value, or the short-circuit current is that value over 50Ω), and the cable probably is as well, until the point indicated.  Beyond that point, it has a variable transmission line impedance around 100-150Ω, with some impedance dropped between grounds resulting in common mode error increasing with frequency.  That is, the twin-lead section can be treated as a transmission line, but it's mismatched to the coax it comes from, which means inductance.  Inductance that counts into the capacitor SRF measurement.  You aren't really measuring the capacitor per se in this setup, but an equivalent circuit with the tapped inductances around it.

You can perhaps still use a difference measurement (i.e. measure a short, calculate its inductance, then measure the capacitor, and subtract), but better would be to use BNC tee and binding post connectors, with coax going between generator and scope the whole way; the scope should be set to 50Ω termination then.  And then you will see the normal notch response around the SRF, shifted down by the fixture ESL, but the ESL can be calibrated and subtracted out effectively in this way, without being sensitive to wire lengths and positioning.

I don't mean to insult you, but this can be found in elementary textbooks.

This is not a helpful response for the questions asked. I work in a completely unrelated field and I'm trying to primarily self-learn some electrical engineering content (as a hobby) but utilizing these forums for questions I cannot find answers to on Google. I have no background in EE, nor does anyone that is close to me. Further, my proficiency in maths (especially the complex maths involved in EE) is low, but I understand that many online calculators exist to aid in my learning. I do not appreciate your response.

I'm afraid there aren't many good ways to put this.  You can choose to react negatively to it, or to take it as a direction -- it's another gap in your knowledge to explore.  That's your choice.

Consider what it really means, though:

Most respondents here have taken at least four years of college courses, and no, not all of those are on strictly this topic, but at least, like, 2-4 classes minimum to go from a basic off-the-street to a functional level to approach this specific topic.  Probably circuit analysis, AC steady state, and preferably EM wave theory for transmission lines, and network theory for their lumped equivalent circuits.

We cannot possibly convey all the required information in this thread.  Nor are we responsible for your lesson plan; that would be a ridiculous expectation of course.

This is the challenge of self study; you will find road blocks, obstacles, gaps in knowledge that you just have to stick a pin in, and come back to later -- potentially much later, months, years, decades even, depending on depth of the topic, and how much time you have to commit to its study.  Along the way, you will have half-formed ideas of how things interact and connect, and you'll be wrong a lot of the time, and it's a part of that to just be humble about your likely-ill conclusions until you've discovered the missing pieces, put them in place, and then you can make accurate inferences.  That's just learning, it's messy, it's frustrating, it's insightful and rewarding.

And it's no accident that learning is big business; well, aside the issue of how stupidly big its business is nowadays, or whether it should really be a business at all, but just to say, at its heart: it takes real effort from many people, authors, instructors, assistants, students, etc., to implement a curriculum as accelerated as four years even.

So, you aren't going to get your answers in any one place, certainly not here.  And I'm sure you don't expect to (and, frankly, neither should you [get it all in once place] anyway!).  I get it, it's frustrating, and I get it, there are at least actual people to lash out at here, versus the endless impersonal void that is the internet at large; and Google and other search engines are not particularly good at finding basic topics, especially these days.  Good searching is a skill all unto itself, but a skill beside the immediate challenge; an enabler, but not a prerequisite, to it.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!

#### p.larner

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #59 on: October 18, 2023, 11:58:08 pm »
could a gdo not do this?.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #60 on: October 19, 2023, 11:09:03 pm »
I'm thinking the tank circuit was too much so let's make things easier to understand.  Take out a ruler and lay it flat on a table with the end hanging off the table.  Hold the end still on the table down with one hand and strike the other end with your other hand.   Note how it vibrates.   Now slowly apply the same some force to the same end.  No vibration.   That vibration is the natural resonance of the ruler (for the length you chose).  Kid science stuff.

Now, when you hook up your scope and Arb as you did and excite the inductor with a sinewave you simulate a nice slow steady motion.  But there are waveforms that will simulate a striking motion.   Can you guess which?

To repeat your test using an Arb and scope to measure the self resonance of an inductor, I made a simple fixture on a breadboard with two resistors and the inductor I want to test.  I marked up your schematic.   I am using a J.W.Miller (Bourns now) PM54-470L 47uH, +/-15% with an SRF of 10MHz.

Note now as I set my Arb to a squarewave the inductor rings.   The ring slowly dampens out, just like the ruler test.   Magically the frequency is constant and is close to the specified 10MHz.   My breadboard and the path down the two resistors adds some error.  Ignore that for now.

Try repeating this test with a known inductor.  Because we are dealing with very small parasitics, you may want to avoid using a breadboard.  You also need to use a 10X scope probe and use resistors with as high a value as possible where you can still measure the frequency to reduce the error.

Hopefully this makes some sense.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #61 on: October 20, 2023, 03:54:35 pm »
If the signal is too small to measure, you can just bump up the signal from your Arb using a jelly bean transistor.    In my example I am using a 2N3904 and a 40VDC power supply.  If your scope supports averaging, you may want to use that to lower the noise even further.   Plot showing an average of 10 sweeps.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #62 on: October 20, 2023, 04:19:07 pm »
So why does my measurement fall outside of the datasheet.  I've seen two published values for SRF, one at 10MHz min, the other at 12MHz min.  The datasheet on Bournes site shows 12 and 9.5 is a long way from that.

https://www.bourns.com/docs/Product-Datasheets/pm54_series.pdf

Remember I mentioned the errors in my breadboard fixture.  That thing has all sorts of parasitics.  There are people on this site that will argue their use but I'm not one of them.  They have their place.  An easy fix for our problem,  use air.    In other words,  lift the common node of the two resistors and test inductor into the air.   A quick blob of solder and instantly we reduce all those parasitics.    Our measured SRF goes from 9.5 to 12.0MHz.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #63 on: October 20, 2023, 05:44:35 pm »
Of course the leads add a fair amount of parasitics as well and we can certainly improve our measurements further by shortening them. Moving the fixture to some PCB, note the common node is still in air and there is a wide low inductance strap for the other leg of the device I am testing. Also note the three stacked layers on the scope probe. Didn't want to cut up some thicker board so staked them to help reduce the parasitic capacitance.

If we use the Coilcraft 15% derate this is going to give us a typical value around 13.8MHz. With the new fixture we now measure and SRF of 13.3MHz. I suspect we are now in the ballpark of the typical numbers for this device.

I pretty simple experiment for the beginner to conduct.  Give it a try.

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#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #64 on: October 20, 2023, 07:00:10 pm »
Just for a sanity check,  shown is a Vishay Dale IMS05RU331K 330uH.  The following link is to the datasheet.

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/525/IMS-5_Series.pdf

SRFmin for this part is 6.5MHz, measured with full length lead.  Scope showing both channels zoomed out.  Note I am triggering on the rising edge of the input to the base.   There is an inversion and we are actually looking at the falling edge.  It's easier to look at the lower voltage levels near ground than riding on top of the 40VDC.   We measure an SRF of 7.0MHz.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #65 on: October 24, 2023, 11:58:38 pm »
Here is a Sumida power inductor, part number CDR125NP-100M, 10uH +/-20%.  What is nice about this part is the manufacture provides typical values for SRF rather than a minimum.  This part has a typical SFR of 24.2MHz.   See link for datasheet:

https://products.sumida.com/products/pdf/CDR125.pdf

Even with 40V excitation, the signal is very small.  The scope directly measures 24.15MHz.   Using the manual cursors to the center of 4 peaks, it measures 24.48MHz.

This is pretty much the limit of what I can measure using this setup.   If you wanted to measure smaller parts, you may need to smack it harder or add a gain stage for your scope.

#### joeqsmith

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##### Re: Measuring the self resonant frequency of an inductor?
« Reply #66 on: October 26, 2023, 02:02:52 am »
Looking at larger inductors, here is an old Dale 4.7mH part.  Sadly I was unable to find a datasheet that provides the SRF.   Scope measures about 1.3MHz.

Smf