Measuring the self resonant frequency of an inductor?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 02:47:28 am
content: I'm trying to learn more about inductors and their different electrical parameters. Looking at this [thishttps://www.vishay.com/docs/34104/ihlp-2525cz-01.pdf] datasheet, it appears that *generally* the inductor is most efficient (Q factor) when it's not used near its self resonant frequency (SRF). For this specific question, I'm trying learn how to easily measure the SRF of an inductor.I came across [this video] (https://youtu.be/tjbK4LsOQRk?si=sCQUETSEut7MaGTo)(schematic shown at 16:50) which clearly shows how to easily measure the exact SRF, butI'd love to be able to see the SRF of an inductor on a graph, like [thishttps://images.nagwa.com/figures/explainers/108147821682/2.svg] datasheet, it appears that *generally* the inductor is most efficient (Q factor) when it's not used near its self resonant frequency (SRF). For this specific question, I'm trying learn how to easily measure the SRF of an inductor.Edit:It looks like from (https://youtu.be/C3wYMZ_cjdQ?si=dR9qZnzfRtovJ27m) video, I can use a similar methodology to measure the SRF of a capacitor.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 03:07:12 am
content: For future reference, it looks like  (https://youtu.be/FjLfrrwpjBs?si=w9Z6sLUT7vDWc1D8) video is a good reference on how to measure the saturation current of the inductor.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 03:45:36 am
content: Is this the solution, maybe? To use a frequency sweep using a AWG and the FFT mode of my oscilloscope?https://youtu.be/DqjhTfOy9NY?si=BH0YSjCC2Id0hN7S&t=551

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 03:58:44 am
content: Again, another possible solution?https://youtu.be/uMH2hGvqhlE?si=g7B-Ipr9sbc3_idTI'm not sure whether either of these two solutions will be appropriate for my use-case. Maybe someone smarter can provide me guidance?Thanks!

author: LinuxHata
author_rank: Frequent Contributor
timestamp: October 11, 2023, 07:01:28 am
content: I usually connect inductor to VNA and self-resonance can be clearly seen on frequency sweep.

author: CaptDon
author_rank: Super Contributor
timestamp: October 11, 2023, 01:15:45 pm
content: Take the output of your signal generator through a resistor of perhaps 470 ohms to drive the inductor with the 'lower' end of the inductor at ground return. Connect your oscilloscope across the inductor (low capacitance probe is best) and sweep the inductor. At the parallel self resonant frequency you will see a huge rise in voltage, usually even bigger than the applied voltage. You probably won't see a series self resonant frequency but if you did it would appear as a drop in voltage. The resonance can be very sharp so sweep slowly.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 02:02:52 pm
content: Quote from: LinuxHata on October 11, 2023, 07:01:28 amCan you explain this some more? What is VNA?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 06:05:49 pm
content: Quote from: LooseJunkHater on October 11, 2023, 03:58:44 amI tried following Dave's video and I have absolutely no idea how to replicate his DIY bode plot.I set my function gen to output a 1v sinewave, 1s sweep from 10KHz to 1MHz. Do I have to use all of these exact settings to get an accurate bode plot on my scope? I have no idea.On my Siglent scope, I could not figure out what settings to use either.I tried to follow (https://youtu.be/DqjhTfOy9NY?si=KjPaDzponilCnw4b&t=551) video as well (using the FFT function), but I have no idea how the inductor is wired in the circuit.I think I need a "schematic" essentially showing whether I need any resistors, how to connect the inductor (to ground or only on the positive of the AWG), and where to probe.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: October 11, 2023, 07:00:44 pm
content: VNA is Vector Network Analyzer.Simply saying, you connect your component of interest to it, and run the frequency sweep.On the screen you will see how S-parameters change in Realtime, so you can easily find the self-resonance frequency. Some VNAs can do that for you.

author: rteodor
author_rank: Frequent Contributor
timestamp: October 11, 2023, 08:27:06 pm
content: Quote from: LooseJunkHater on October 11, 2023, 02:02:52 pmJust a very quick exampleLater edit: On NanoVNA Saver I got a peak of S11 Z at ~2.5MHz but directly on the display I get the peak at ~7.3MHz. I don't know what's going on.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2023, 08:35:55 pm
content: Here's a very simple circuit that I used with a spectrum analyzer:The two capacitors C1 and C2 are chosen to have reactance much less than 50 ohms over the frequency range of interest.Ltest and C_Parasitic form the self-resonant circuit.I have used this with a known capacitor between C1 and the inductor to measure the inductance and resonant Q of coils.J1 and J2 connect to 50 ohm source and load, respectively.If you sweep (manually or automatically) the frequency, you will see a minimum output at the SRF, where it parallel resonates.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2023, 08:57:02 pm
content: Most efficient -- in what sense?For example, the impedance of an inductor is maximum at its (lowest frequency, parallel) self resonance. It never gets "more efficient" at choking an AC signal!An alternate definition of Q can be used to express its "efficiency" near resonance. Note we must distinguish between the Q factor of the terminal impedance itself (that is, the resistance and reactance of the component at a given frequency), and some kind of inference about what's inside it (e.g. an RLC parallel equivalent that describes the impedance peak). In the latter case we can define a Q factor at resonance, and it can be measured for example by peak bandwidth, or some reasonable assumptions about the values of L and C approaching the SRF.You may find playing with / thinking about this model, to be of interest:https://www.seventransistorlabs.com/Calc/Coilcraft1.htmlGranted, the skin effect / core loss elements aren't so straightforward, but they can be set to zero and the basic RLC circuit still used.Tim

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 09:22:40 pm
content: Quote from: rteodor on October 11, 2023, 08:27:06 pmHow that is such a cool tool... But likely something I won't purchase for a very long time lol

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2023, 09:26:10 pm
content: Quote from: TimFox on October 11, 2023, 08:35:55 pmCan you explain the purpose of C1/C2/R1/R2 and choosing the right components? Further, what would the expected waveform look like? From the videos I've watched on YouTube, I haven't seen anyone use the same type of circuit that you are using.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2023, 09:53:40 pm
content: R1 = R2 = 50 ohms are just to give reasonable terminations for the input and output.In parallel with each of C1 and C2 are 100 ohms (series combination of 50 ohm source and R1, and 50 ohm load and R2).Large values of C1 and C2 in parallel with 100 ohms are approximately equal to similar values of C in series with much smaller resistances.(See series-parallel transformation for impedance/admittance in elementary textbooks.)Therefore, you end up with a large capacitance and small resistance in series with the inductor under test.You drive the left side (R1-C1) from a variable frequency sine wave: this could be an AWG, the tracking generator of a spectrum analyzer, the source of a VNA, or other AC generator.You can see the output at the right side (R2-C2) on any measuring device you want, with a 50 ohm termination: voltmeter, oscilloscope, spectrum analyzer input, VNA, etc.When the inductor self-resonates (parallel resonance), the total impedance across it is much larger than the impedance at R2-C2 and the signal will be a minimum.I have never posted anything on YouTube, but I have used this circuit to measure inductors.I just mounted two BNC connectors on a single-sided circuit board, with the two capacitors mounted on appropriate stand-off insulators to fit the inductor.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 12, 2023, 12:10:09 am
content: Based on my understanding of that circuit, it doesn't seem like it would be possible to see a graphing of the SRF of an inductor with that circuit. I'm hoping to be able to see a graph, like [thishttps://images.nagwa.com/figures/explainers/108147821682/2.svg] and be able to create it using my oscilloscope + AWG.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 12:33:20 am
content: No, the resulting graph will be inverted from that example, since the self-resonant frequency will have a high series impedance and a low output.Worked for me.You need to make sure that there is minimal capacitance in parallel with the inductor to do SRF.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 12, 2023, 02:06:03 pm
content: Quote from: TimFox on October 12, 2023, 12:33:20 amOkay this makes sense!My AWG (Uni-T UTG932E) actually has a 50-ohm termination mode; does that mean I can skip on R1, or should I include R1? Furhter, for C1/C2, is a 5uF ceramic capacitor large enough or should I use something larger?I assume that R1/R2, and C1/C2 don't need to be "perfectly" matched; it's fine if there is like a 5% variance between the values? Is that correct? Will it effect the SRF measurement?

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 02:10:24 pm
content: Matching between R1/C1 and R2/C2 is not critical: the important thing is that the reactance of each of the two capacitors should be much less than the resistors, over the frequency range of interest.R1 and R2 are in series with the signal path: both are needed.The trick is to get very low resistance in series with the inductor (parallel-series conversion of the impedances at each end).5 uF? What frequency range are you using?At, say, 10 MHz, that capacitor is probably above its series SRF and looks like an inductor.At 10 MHz, I used about 5 to 10 nF.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 12, 2023, 02:34:45 pm
content: Quote from: TimFox on October 12, 2023, 02:10:24 pmSo I guess R1 is not needed with my AWG...?Quote from: TimFox on October 12, 2023, 02:10:24 pmMost of my inductors were pulled from either switching regulator circuits or power supplies, so I wasn't planning on testing above 1MHz initially. However maybe it is a good idea to measure the SRF of the ceramic capacitor before I use it? Should I go with a capacitor in the nF range instead, as it'll reduce loading of the AWG?

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 03:52:09 pm
content: I have always used R1 with a 50 ohm source, to avoid loading the generator with a seriously reactive load.If you are testing around 1 MHz, then you need to increase the capacitor value by a factor of 10 from that used at 10 MHz.Ceramic capacitors above 100 nF tend to be bad dielectrics: X7R or worse, that may give bad results in a measurement jig.At or above 100 nF, your best bet might be a polypropylene film capacitor, although C0G/NP0 are available at 100 nF.

author: mawyatt
author_rank: Super Contributor
timestamp: October 12, 2023, 04:03:08 pm
content: Here's something that folks might find useful wrt capacitor/inductor resonance.https://www.eevblog.com/forum/testgear/lcr-meter-plot-software/msg4774100/#msg4774100Best,

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 04:11:12 pm
content: That's a nice series of posts.Note that in his measurement of a 5 uF polypropylene capacitor, the self-resonant frequency (series-resonant mode) of the 5 uF polypropylene capacitor is about 300 kHz.In my simple circuit, you want to make sure C1 and C2 are operating well below their SRF.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: October 12, 2023, 05:57:52 pm
content: Good Morning LooseJunkHater, go here:http://www.learningaboutelectronics.com/Articles/LC-resonant-circuit.php

author: fourfathom
author_rank: Super Contributor
timestamp: October 12, 2023, 08:35:31 pm
content: Quote from: TimFox on October 11, 2023, 08:35:55 pmTim, can you explain why you use capacitors instead of resistors in the shunt path? My typical test jig for measuring inductors looks like this (attached image). I've got a series capacitor in the circuit to so I can calculate the inductance, that cap value will change according to the target inductance and frequency, or will be shorted out if I am only looking for self-resonance. If I want a sharper response I can always reduce the shunt resistor values (at the cost of more attenuation).

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2023, 10:25:38 pm
content: I originally used this test box to measure inductance (possibly affected by its parasitic capacitance) and coilQby adding a known good capacitor in series with the coil under test, between the two large capacitors.Roughly, the relationship between the parallel resistance and reactance, and the equivalent series resistance and reactance (at a fixed frequency for the reactances) is calculated from:Rp/Xp=Xs/Rs=QTherefore, ifQ>> 1Rs=XsxXp/Rp=Rp/Q2<<Rp(sinceX<<Rp); andXp=XsGo to the textbooks for the exact formula, sinceXchanges for lowerQ.I used the shunt C method to get very low resistance for the originalQmeasurement, with a slight correction to the known capacitor from the two large capacitors in series with it.Your circuit works, but has a larger resistance in series with the coil.Note that C1 and C2 must be chosen for the frequency of interest.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2023, 10:58:35 pm
content: Mind, there is no "one" way to measure an inductor.Anycircuit you can make, in which the impedance of the element in question is sensible at the input, output or both, can be used, given appropriate calibration (or the circuit and given values).Consider these simple methods:https://www.seventransistorlabs.com/Calc/RLC.htmlL/C by dividerassumes an ideal reactive component in the divider, and based on the ratio measurement (can be done with any AC voltmeter that reads the driven frequency accurately enough), tells what L or C that would be, given the assumption.I normally use this when I want a crude measurement of a component, letting R = generator impedance (50 ohms) and adjusting F until drop is in the 10-90% range.Vector impedance, is basically the same thing, but with a series resistor added (we ignore source resistance because it's measured as reference; this requires a simultaneous measurement, and hence the extra resistor), and using a low distortion sine wave, and the scope's phase measurement function, we get a more accurate value -- also we know unambiguously not just what reactance it is, but its resistance as well; at least to within measurement error: the phase measurement tends to be pretty rough (+/- degree or two), so for element phase near 90° (note, that doesn't mean measurement phase near 90°, it's less due to the modest divider ratio!), resistance is proportional to phase difference from 90° and it can easily be measured near-infinite, or negative. (Note that, for electrical purposes, real numbers are a ring, i.e. there is a continuum from positive-infinite to negative-infinite numbers. So this is expected behavior.)Frequency and Q factor, is practical when measuring higher impedances, and at select (tuned) frequencies. Because the inductive reactance is canceled out with capacitance, the resistance can be measured accurately, and at a chosen frequency F <= SRF. Note that, for very high impedances (large divider resistance, small capacitance), and high frequencies, probe impedance itself is a concern, but for modest values at 10s of MHz or below, a typical 10x probe is generally fine.Note that we can measure Q both ways, i.e. by resistor divider and by fractional bandwidth. I didn't bother writing a calculator for that, but simply noting the frequencies where amplitude is 70.7% of the peak, and dividing center/peak frequency by that difference, gives the same value. Same give or take -- if the network is more complex than assumed (repeated/overlapping poles at the frequency, or the resonance of a continuous structure like a transmission line), the methods might differ, which is useful info about the network/element itself, though how one might use this difference, depends on the network.Capacitance by difference, is adding a tweak and measuring the shift in frequency. I probably use this most often when measuring switching circuits, the ringing frequency of a stray oscillation I want to dampen. First introducing a plain capacitor Cx, I determine the in-circuit capacitance; then I use >2C, plus a resistor R = Zo, to dampen the oscillation (which gives a Q factor under 2 or so).Matching capacitor, is as it says in the description; I use it to measure high-Q inductors at level. Taking careful measurements, using high-Q capacitors (C0G are practically ideal, they are very good capacitors indeed), I get values as accurate as the measurements themselves. So, repeatable within some percent, which is enough to see the effect of, say, bits of metal near an airgap in a ferrite-cored inductor.The above-posted methods are variations on this (or these) approach(es), with parallel or series arrangements instead. The flanking-capacitors motif effectively swaps series and parallel resonances, so instead of an input dip you see an output peak, etc. The capacitors skew the response -- reactance is lossless, you don't get a straight-up / naive resistance divider, it's impedance, and the transfer curve contorts as a result. Ultimately, all we're doing is rotation and translation on the Smith chart; so as long as we haven't completely lost visibility of the test component (which is guaranteed when lossless components are used around it), we're just picking what frequency we want to measure the element at, and what range of impedance it will present to our instruments at that frequency.Tim

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 03:10:30 am
content: Lots of ways to do an impedance measurement.For inductor self-resonance, the important thing about the fixture is to avoid uncompensated parallel capacitance added across the inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2023, 04:15:31 am
content: Or a known amount of capacitance, of course.Any kind of coupling will do:In the resistor divider case, you're coupling with a resistor, measuring the peak into the element's resistance (anywhere d|Z|/dF = 0, i.e. at local minima/maxima, the impedance is real), and as long as the resistor divider is a resistor divider (you aren't introducing reactance), then there you go.For inductors with external field, an inductive link can be used, in which case, as coupling is varied, the notch depth and width vary, with maximum depth and minimum width (jointly that is) occurring at critical coupling. This is a common way to test loading / choking coils for antenna and matching use, and it used to be there was a specific tool to test this (grid/gate dip meter). I mean, you can still use one too, but these days you might as well get the cheap(ish) VNA and do so much more.For the C-coupled case, it's the same way, coupling varies with its value, with the difference that the C loads the tank as well. So, use the C+Cx calculation to correct for that. For filters constructed this way (i.e. parallel resonant tanks with small C spanning between their tops), it's why you get asymmetric skirts, a highpass-ish characteristic, for example.Tim

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 13, 2023, 03:29:59 pm
content: Quote from: TimFox on October 12, 2023, 03:52:09 pmI made a [Falstad circuit](https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCDsB0AcYCYCsBTAtAThA6TIYAYBmIjDMSMDIggNlpABYGkQkC30wwAoIyDrUYg0RBOASwRY8CA4EeAJwlTR4igzVy2CgMYgawrYZAYGHMNALWbtu2HSQRBK5MgJGsT0iTVGjAmo5KwJeAHMDAmEzSOFEKXkeABtwSE0ZZHTxeStWNBdbBFp3DCRGDEh+IiRaL2EFZUzpcRMtDnYeCI1TBm747T1UrKGe7Us7CdsHNCcXBAQxHwCCBGIMWGsEkN4KQSMZMGH1cAnZF14AezYQITH58mDrf1qfMwRCdw96kNDtcVYiDwrhZtAEyOBoB4yNCYWRyt8VuUMJJ4ARYEVaMQ4thtICgA)and it appears that at 1v out of the AWG, the load could potentially be 20ma (but I think it would actually be more, because as you increase frequency, the resistance of an inductor increases?). I can't find the max current output of the AWG Uni-T UTG932E. If the AWG can only output 20ma, will that effect the measurement accuracy of the inductor?

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 04:07:12 pm
content: You want to make sure that the generator can drive 50 ohms to the desired amplitude without distortion.I just obtained a Rigol DHO914S with a (somewhat wimpy) AWG capable of 5 V pk-pk into an open circuit, and verified that it delivers 2.5 V pk-pk into a 50 ohm load.Most generators with 50 ohm source impedance are designed to drive a 50 ohm load: some audio generators are rated for 600 ohms.My circuit as drawn presents a minimum of 50 ohms to the generator: that's why I included R1.Thereactanceof an inductor increases with increasing frequency: therefore, the current through it (not the same as the current into my left BNC connector) decreases with increasing frequency.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 13, 2023, 04:32:22 pm
content: Quote from: TimFox on October 13, 2023, 04:07:12 pmSorry, can you explain this in further (simpler?) detail? I'm still not very knowledgeable with electrical engineering.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2023, 04:32:36 pm
content: Paper from Coilcraft:https://www.coilcraft.com/getmedia/8ef1bd18-d092-40e8-a3c8-929bec6adfc9/doc363_measuringsrf.pdf

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 04:50:18 pm
content: Quote from: LooseJunkHater on October 13, 2023, 04:32:22 pmWith R1 = 50 ohms between the BNC and everything else, the minimum impedance seen by the generator will be 50 ohms.With large C1, whose reactance is much less than 50 ohms, the actual impedance will not be much higher.A generator is characterized by its output resistance (50 ohms for most high-frequency generators): in most generators, that is a physical 50 ohm resistor between the guts and the output connector (often with an attenuator in between). It is rare for such a generator to be incapable of driving an external 50 ohm load, so the output current capability is enough to drive it (although the maximum voltage across a 50 ohm load is 1/2 the maximum voltage into a high impedance load).Do you have access to any of the Spice variations, such as LTSpice?Rather than work through all the algebra, the ".AC" mode will do all the algebra for you on a linear circuit (no diodes, etc.).The equivalent series-parallel conversion is very useful in RF design.For a two-terminal network with one resistor and one reactor (capacitor or inductor),at a single frequency, there is no difference in the impedance of the two networks.Of many on-line articles, seehttps://aaronscher.com/Circuit_a_Day/Impedance_matching/series_parallel/series_parallel.html

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 13, 2023, 05:30:22 pm
content: Quote from: TimFox on October 13, 2023, 04:50:18 pmSo my generator can output 20v, does that mean it can actually *likely* output 20v @ 400ma (as 20v/50ohm = 0.4A)?Quote from: TimFox on October 13, 2023, 04:50:18 pmI primarily use Falstad. I've tried LTSpice before but it's quite complex, so it'll take a while for me to learn. Is there a reason I should be using a simulation tool, and what for if I'm seemingly "just" testing for the SRF of an inductor?

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 05:52:08 pm
content: Quote from: LooseJunkHater on October 13, 2023, 05:30:22 pmIf your generator can deliver 20 V into an high-impedance load, then it probably delivers 10 V into a 50 ohm load.If both of those voltages are peak-to-peak, then 10 V pk-pk = 5 V pk, so the current into a 50 ohm load would be 100 mA peak.I believe AWGs are typically specified for peak-to-peak output voltage into high-impedance loads: check your manual.The AWG built into my recently received Rigol DHO914S is rated for a (wimpy) 5 V pk-pk, so the maximum current into 50 ohms is 2.5/50 = 50 mA peak.My boatanchor Wavetek 98 1 MHz synthesized generator is capable of driving a 50 ohm load to 5 V rms (14 V pk-pk), or 7/50 = 140 mA peak.My recommendation for Spice was to help you understand the algebra: you don't need it for analyzing SRF data.Note that some of the other replies deal with compensating for capacitance added across the inductor under test.For example, if you connect a normal oscilloscope (digital or analog) directly across the inductor, you probably add at least 15 pF of capacitance.An expensive active probe might add only 1 pF.Whether that is too large for an accurate measurement is a quantitative question.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2023, 04:09:38 am
content: Datasheet for a Coilcraft 0805CS-821XJL:https://www.coilcraft.com/getmedia/dd5f20e4-1ff7-43df-8317-b693eb2dce3e/0805cs.pdfS-Parameters may be found here:https://www.coilcraft.com/en-us/models/spice/?seriesName=0805CSTouchstone file for the above part is attached (.txt).Shown using the LiteVNA to measure the parts SRF. From the Coilcraft paper I attached,QuoteDatasheet shows SRF of 310MHz for this part. +15% is 356.5. We measured the phase shift at 333MHz. For a homemade fixture and low cost VNA, it's close enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2023, 03:50:52 pm
content: Here I am using the LiteVNA to measure an 821, 391, 151 & 820 and comparing it with Coilcraft's data. I've made no attempt to characterize the standards and am using an ideal model. I have an old 4-receiver Agilent VNA that supports more complex calibrations which could do a much better job making these measurements but still, for $120 the LiteVNA does a reasonable job making these SRF measurements.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2023, 04:36:05 pm
content: Looking at smaller parts, things start to brake down. Here is a 15 & 10nH inductor. The LiteVNA was specified to work to 6GHz. Beyond this it can use harmonics and I can get some sort of somewhat meaningful data out of this one up to about 8GHz.The fixture is nothing more that a section of ridged coax that I cut and polished sitting inside a block to keep the ends aligned. It's now spring loaded to make it easier to insert parts but you can see it is not easy to align them. In this case, the parts are just sitting in air. To use the fixture with the Agilent, I use the unknown thru (and somewhat better standards).For large inductors, simple clip leads could possibly be used. All depends what you are after.

author: MrAl
author_rank: Super Contributor
timestamp: October 14, 2023, 04:36:36 pm
content: Hello there,I think another way to measure the resonance is to simply connect a series resistor Rs to the inductor, then apply a sweeping frequency while measuring the voltage across the inductor alone. You should see a peaked response if Rs is high enough in value.To understand how the series resistance Rs works, there is something called the discriminant, and if that is either positive or negative it either makes the response sinusoidal or damped. The idea is to choose a series resistor Rs such that you get a sinusoidal response, which isn't too hard to get I don't think because the discriminant becomes more favorable for a peaked response when Rs is somewhat large. It depends on the values of L and respective C though, as well as the ESR of the inductor, so it would be hard to calculate. In an experiment however, you could just vary Rs making it smaller then larger, see what you get. When you get a peaked response indicating some kind of resonance, that means you have Rs set to a good value. There is a tradeoff however. That is, if Rs is too large although you still get a peaked response, the amplitude may be harder to measure as it could be too low at that time. In that case try to decrease Rs to get a larger amplitude while keeping it large enough to maintain that peaked response we look for.Just to note, the resonance you see that is indicated by the peak may not be physical resonance, but it could still be useful to know. To figure out if you are seeing physical resonance you'd have to measure the phase as well as the amplitude. You may or may not be interested in that.If someone wants to try this that would be cool. You need a variable sine wave generator, scope, and I guess you can use a potentiometer for Rs but it depends on the frequency range you may have to use a fixed resistor and perhaps try a few different fixed values.

author: TimFox
author_rank: Super Contributor
timestamp: October 14, 2023, 04:50:31 pm
content: Again, if you drive the inductor under test from a suitable resistor, while the inductor is connected to a voltmeter or oscilloscope high-impedance input, you need to deal with the input capacitance of the meter/scope.Some suggestions above involve measuring with two load capacitors, using math to calculate the inductance and self-capacitance of the DUT to get its inherent SRF.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 14, 2023, 05:38:58 pm
content: This is a brief review of most used Q measuring methods, also shows why a low Z generator is needed:https://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2012/QEX_1_12_Audet.pdfAnother way to identify the self resonance frequency is to use a grid-dip-meterhttps://en.wikipedia.org/wiki/Grid_dip_oscillator, or to improvise a dip-meter from an existing generator, or from an AWG, like seen in any of the attached examples.The coil in the schematics is the coil that produces the field, not the one to be measured. The inductor to be measured has to be put in the nearby of the coil from the schematic, then the frequency must by swiped with the generator. Instead of the analog uA-meter, can be used a normal DMM.The schematic captures are from the book "How to use Grid-Dip-Oscillators" by Rufus P. Turner, 1960.https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Servicing/How-to-use-Grid-Dip-Oscillators-Rufus-P-Turner-1960.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2023, 06:53:55 pm
content: Here is a 0805CS-050XJL 5.6nH 5% part. The datasheet shows a typical SRF of 5.9GHz but when you look at their S-parameter data, they measured 9.5GHz. Much higher than the 15-20% number they provided. You can imagine, the smaller the part, it gets more difficult to measure. Coilcraft's paper states they used an Agilent 8720D VNA to measure SRF with a in-house fixture. For this test, I am using a homemade frequency extender. Consider I am using the cheap standards that came with the LiteVNA and the ideal model, I'm surprised it shows anything close.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 05:51:45 pm
content: I'm trying to measure the SRF of my capacitor but it DOES NOT look like other SRF graphs that others have posted.Image 0. shows my basic schematic.Image 1. shows the ceramic capacitor (100nf) that I'm measuring (no datasheets available; just a generic ceramic capacitor).Image 2. shows my AWG, log sweeping between 1KHz --> 30MHz, sinewave, 10Vp/p, 1second sweep timeImage 3. shows the FFT (with MAX-HOLD, 50ohm load) as well as what the spectrum looks like (neither of these look like the other SRF graphs that I've seen)Image 4. shows what my overall measuring setup looks like (as I'm not sure if the coils of wire and it all being physically on the ground would mess up the measurement).Can someone explain what I'm doingright & wrongwith measuring the SRF of the capacitor (so that I can then determine if this is an appropriate capacitor to then measure the inductor)?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 15, 2023, 06:04:22 pm
content: Its a cap, which is basically open to DC. As frequency increases, so does the loading. You are basically shorting out the signal generator. Pretty much what is expected.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 06:58:00 pm
content: Quote from: joeqsmith on October 15, 2023, 06:04:22 pmSo do I just add a 1k resistor in series of the AWG or...?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 08:31:18 pm
content: Quote from: LooseJunkHater on October 15, 2023, 06:58:00 pmI tried adding a 47ohm resistor as well as decreasing the AWG output voltage to 1V/p and it shows nearly an identical waveform.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 15, 2023, 08:37:24 pm
content: Quote from: LooseJunkHater on October 15, 2023, 06:58:00 pmThen you have just made an RC filter. Roll off or how fast the signal attenuates will increase with frequency, as after all the RC filter is a low pass.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 08:44:15 pm
content: 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.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 08:52:26 pm
content: Quote from: joeqsmith on October 15, 2023, 08:37:24 pmOH RIGHT. Woops that makes total sense. Okay then, how am I suppose to measure the SRF of the capacitor than? Maybe a diagram would help me?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 08:53:10 pm
content: Quote from: TimFox on October 15, 2023, 08:44:15 pmI do not understand anything that was said here.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 08:54:10 pm
content: Quote from: LooseJunkHater on October 15, 2023, 08:53:10 pmI don't mean to insult you, but this can be found in elementary textbooks.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 09:04:54 pm
content: Quote from: TimFox on October 15, 2023, 08:54:10 pmThis 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.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 09:20:17 pm
content: 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?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 15, 2023, 09:34:39 pm
content: Quote from: TimFox on October 15, 2023, 09:20:17 pmSo 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?)?

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 09:57:48 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: October 15, 2023, 11:25:51 pm
content: Here is an elementary application note from a good capacitor vendor:https://article.murata.com/en-us/article/impedance-esr-frequency-characteristics-in-capacitorsNote 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 16, 2023, 02:24:29 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2023, 02:51:04 am
content: 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.Quote from: LooseJunkHater on October 15, 2023, 09:04:54 pmI'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 bewronga 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

author: p.larner
author_rank: Frequent Contributor
timestamp: October 18, 2023, 11:58:08 pm
content: could a gdo not do this?.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 19, 2023, 11:09:03 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2023, 03:54:35 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2023, 04:19:07 pm
content: 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.pdfRemember 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2023, 05:44:35 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 20, 2023, 07:00:10 pm
content: 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.pdfSRFmin 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 24, 2023, 11:58:38 pm
content: 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.pdfEven 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 26, 2023, 02:02:52 am
content: 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.