Problem with Inductor Self Resonant Frequency - Page 1
Forum section: electronics / rf-microwave

author: bittumbler
author_rank: Contributor
timestamp: May 01, 2024, 08:48:16 am
content: Hi,i am trying to build a 5uH LISN. I started with the LISN developed in other threads on this forum, and modified it to be usable with more current. I dont need precision, just a tool to find where noise comes from and what changes reduces it.My problem is with the 5uH inductor. I expected the inductor to become more like a capacitor over higher frequencies. To compensate for this, i used multiple different inductors in series:From Supply to DUT in series:SRP1770C-2R2M 2.2uH, SRF 21MHzSRP1770C-2R2M 2.2uH, SRF 21MHzSRP1038AA-R47M 0.47uH, SRF 78MHzSRP1038AA-R15Y 0.15uH, SRF 190MHzSRP7028CC-R10Y 0.10uH, SRF 300MHzThis did not help the way i expected it. See plots from a nanoVNA from LISN DUT port to Supply Port.How is SRF compensation done properly?TIA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2024, 11:26:37 am
content: Could you be more specific about "problem"? I see strong attenuation up to 30MHz.What does layout look like? How is the VNA connected (show photos)?Tim

author: bittumbler
author_rank: Contributor
timestamp: May 01, 2024, 12:07:29 pm
content: Hi Tim,my problem is, that around 18MHz the combined inductor starts to resonate / become a capacitor. That is about the SRF of the 2.2uH parts.So adding the smaller inductors in series seems to make no difference.I was hoping to have somewhat of an inductor left until maybe 100Mhz.My "probe" is a pogo-pin soldered to an SMA connector. Calibration was done at the SMA cable.Matthias

author: shabaz
author_rank: Frequent Contributor
timestamp: May 01, 2024, 12:15:34 pm
content: Hi,If it's a one-off, then a hand-wound air-core inductor could be an option, it wouldn't be very large. Coil64 software is quite accurate, and 0.7 mm wire wound around an AA cell would only need about 22 turns to reach 5 uH. The software also provides a SRF calculation, and it would be > 30 MHz at those types of dimensions.

author: bittumbler
author_rank: Contributor
timestamp: May 01, 2024, 12:23:47 pm
content: Quote from: shabaz on May 01, 2024, 12:15:34 pmI was hoping to avoid that. With a selfwound inductor i expect problems on shielding; it would have to be a toroidal, with huge size.So instead i try to use standard parts with "oversized" current ratings to stay away from saturation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2024, 01:27:30 pm
content: I suspect this is a big part of your measurement:Try attaching the SMAs directly to ground plane. If the---oh, I forget if ground even wraps around the full perimeter, or if it's broken under the connectors? If so, patch over that with copper tape, or put the whole thing inside a metal enclosure (screwed down) to be sure. In any case, the SMAs need to be tied to the reference plane, and if you don't have a reference plane (such as because GND is a feeble hook rather than a plane proper) you need to make one.Alternately, cut L3 and put generous ferrite bead stacks on the cables -- at least one of them, to break the ground loop, but preferably both because some common mode is expected in this arrangement. Ground might be poor, but you get closer to a differential measurement, at least given enough ferrite beads, or turns through a generously sized toroid, and this should be good up to the frequency limit of such method -- ferrite bead impedances peak in the several hundred MHz, less for multi-turn but depends on how closely spaced the turns are.I suspect the observed peaks/valleys are too low of frequency to be due to this effect -- more likely the HF asymptote is due to this -- but this will still affect what percentage of residual you're seeing, since it's a small fraction (-20dB = 10%, -40dB = 1%, etc.) and every little bit matters to that measurement. And, frankly, -20dB peak still isn't bad for supply isolation outside the range of interest.Tim

author: shabaz
author_rank: Frequent Contributor
timestamp: May 01, 2024, 01:38:17 pm
content: This probably won't help unless there was a redesign of the PCB, but out of curiosity I quickly tried a ferrite core.With a FT-50A-67 core, 14 turns provides 5 uH (approx 0.7 mm enamelled wire will fit the core).The inductance rises to 7.5 uH at 30 MHz however. Maybe a bigger core would be better (or split it across two cores). I didn't have a larger core of that type to try. I think the saturation current will be higher than your inductors, but that may require someone to calculate it (I'm calculating a value of 17A which seems incorrect, I may have worked that out very wrong).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2024, 01:43:35 pm
content: Quote from: shabaz on May 01, 2024, 01:38:17 pmFerrite saturates under bias, but a gapped ferrite or powdered iron is effective. Downside: that means you need more turns, more wire, which lowers the SRF (it's basically the electrical length of the winding, give or take loading effect from the core). So, you get back to the original problem.Geometric scaled values work if the impedance is sufficient, and well damped: note that the inductance of the next  inductor resonates with the capacitance of the previous, and if that impedance Zo = sqrt(L/C) is mismatched to the natural loss R of the components, you get an impedance trough of Q = R/Zo (for parallel, or Zo/R for series resonant). A small R+C along the chain can be used to provide that loss.Tim

author: bittumbler
author_rank: Contributor
timestamp: May 01, 2024, 03:05:06 pm
content: I added a ground plane (fully copper PCB + an isolation-PCB) and soldered the SMA connectors to it. So L3 is somewhat gone. But i want to keep the terminal blocks in the path for now.I also disconnected the big Caps on the upper half of the picture on their ground side. They are now floating on one end.The measurement port is now terminated.But i still get the resonance at about 18MHz.

author: shabaz
author_rank: Frequent Contributor
timestamp: May 01, 2024, 03:26:54 pm
content: I'm definitely not knowledgeable on this (so please take with a pinch of salt anything I type on this topic!).Apologies if it is not helping.When I was trying to experiment with a LISN, I used the layout below. The coils are separated greatly, for little interaction. Everything was taped to the inside of a (sanded to be conductive) metal cookie tin, so it was shielded.Unfortunately I don't have this setup any more to run any tests.

author: bittumbler
author_rank: Contributor
timestamp: May 01, 2024, 04:09:12 pm
content: Quote from: shabaz on May 01, 2024, 03:26:54 pmOk, i am asking here, because regarding the analog/RF part, i am also lacking experience. (But i have a grid license).A bit of additional background:I want this in the end to be usable with mains supply/DUT. That means 2 or 4 parallel LISN. If i just use open wires as inductors, i would expect them to couple between each other (like a transformer), pick up all sorts of foreign noise (like an antenna) and transmit some of the DUT noise.So i expect to need some sort of shielded inductor and/or a toroidal setup to keep the magnetic field loop short. Using standard inductors seemed the easier thing to do, but i may have to try again with different ones.Because i intend to use this on mains, the special thing i plan to do, is have separate "Protective Earth" and "Functional Earth". So i need a groud arrangement, that is separate for electrical safety rules, but connected for RF rules.My plan is to have a LISN that is more useful to me, but i compromise on standards compliance, precision, etc.My problem with the inductor parasitic capacitance /SRF is much more basic. I wanted to work around the parasitic capacitance of one part by adding others in series. That seems not to work the way i expected it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2024, 06:24:32 pm
content: I can't see the screw holes in the photo; are they just loose, open? Or do you have screws grounding them to your plate? 'Cuz the plane will be complete with those joined, and otherwise you have the huge hooked return path from the bypass cap, and it's probably even worse than before.The peak does seem to be worse, which is interesting. Although I'm not sure what/why you're concerned about the valley at 18MHz? Surely you mean the peak at ~40MHz (~-13dB)? Parallel resonance of a LISN choke is good, it's definitionally as high an impedance as it will ever achieve, and therefore the most isolation between RF and DC paths. The series resonances, and resonances between parts, are where you lose performance. But even with the -13dB peak, that's still within a couple dB thru-path (EUT to RF).Quote from: shabaz on May 01, 2024, 03:26:54 pmYeah, that's fine. The EUT leads should probably be routed away from the RF connectors to keep CM coupling down, and/or the ground plane could be joined to the enclosure nearby (i.e. put a bit of foil between the connectors and solder it down).The coils would probably be better elevated a radius or two above the plane: as shown, you're shunting some of the field, reducing inductance from the free-space value. But that's a small tweak. It's not like it needs to be crazy accurate: -10% here or there will hardly be noticed.The USB shell should probably be grounded to the enclosure, or the cable jacket stripped where it enters the enclosure and a ground clip used to join it there. Same for the RF connections I suppose -- rather, use bulkhead connectors to pass thru.Huh, that's also a USB-A providing power? That's rather cursed.Well, you'd do it nowadays with a USB-C, where it doesn't matter, heh.Quote from: bittumbler on May 01, 2024, 04:09:12 pmLook at where the inductors are connected: they are in shunt to the RF path. The main magnetizing signal on them (ignoring DC or mains), is the RF signal itself. Any signal coupled in or out, is simply in relation to that.If you're testing EUT on an open bench for example, induced waves from the EUT acting as an antenna, receiving commercial broadcast channels for example, will easily dominate over anything a compact inductor can pick up.In a shielded test chamber, LISN pickup is equally shielded, no problems there.Even if the LISN ground isn't very well bonded, say, and there's significant fields say from EUT to LISN to reference plane (test chamber interior), and some of that field couples into the inductors -- that's still only the tiniest percentage shift in readings. If your EMC readings are fractional-dB precise, I have news for you, you're doing it wrong!Likewise coupling between channels, what's it matter? They're already intimate with each other in the power cable! You might get some gain/phase error that makes a CM/DM network less useful (less accurate/representative), but not knowing what effect the cable has is already working against you, and, simply part of the mysteries you have to solve in the course of an EMC project. Whether that's enough of an error, an added mystery, to materially affect the project timeline, might be a good question, but I would be inclined to suspect it's small.Anecdote: I built this thing some years ago (per-channel schematic:https://www.seventransistorlabs.com/Images/LISN_20MHz_30A.png),With all the RF ports terminated, one EUT port driven, and looking at each RF port in turn, there is a peak evident on the neighboring channels, on the order of -30dB or -40dB. I assume this is due to stray capacitance resonating with the inductance, and thus acting as a matching network at one narrow, unlucky frequency; the coupling ratio is still small so the peak is low, but it is detectable in a lab setting. It's basically irrelevant in a practical setting, and I didn't notice any such peaks in use (being aware of them after testing).Or wait, was it with RF ports open-circuit? I forget now.I tried adding ferrite shield plates between inductors, with no effect.QuoteEarth is easy -- run all the EUT cables into the LISN as the applicable testing standard dictates, and, if that includes safety earth, just wire the DC side to chamber/earth, done and done.Note that equipment earth is often hard-grounded in EMC tests, so, I do mean "applicable standards". Some may stipulate otherwise, so, make careful note of this either way, and set up accordingly.I regularly keep a jumper wire on the above network, actually, so that one port can be shorted to ground, and I just let that also be reference plane / RF ground for convenience. Note the 7th screw terminal joins to plane, so everything can be wired up, and copper clad out to the edges means EMI tape joining it to EUT / chamber reference plane is convenient.QuoteAs mentioned, EPR of one resonates with EPL of the next, and so on; it is a basic problem, as you say, and is as basic a modeling problem. SPICE is excellent for such application -- find or fit models to the components used, and put them together, you should be able to recreate your measurements. But, again, be mindful of what ground really is -- if that DC port or bypass cap isn't actually referenced to ground, you need to model the inductive loop that takes it back there.Compare with shabaz's example, which bypasses the DC ports early and often, direct to GND plane. You can't use ceramic chips at AC mains voltage, of course (well... sort of, but to say: maybe better not to), but a couple film caps, direct to the reference plane, and maybe some loss and additional filtering if you need more isolation to the DC port or freedom from impedance mismatch (which is what motivates my above circuit), will easily do as well.It's just important to emphasize the role and nature of that reference plane, and how little trace or wire length it really takes to spoil the plane-ness of it.Another example of mine:2 x 6uH (actually more like 8uH at zero bias, but dropping to 5uH at ~100A), uhh although it's 8AWG wire so "100A" is a bit generous in continuous operation, but whatever, it's just a matter of temp rise and voltage drop. Notice the ground plane is contiguous underneath the chokes and signal path, with the RF path made towards the terminals (rear) and coax bringing the signals up to BNC connectors. The DC port is C || (R+C) bypassed, to be well-behaved over a modest frequency range without going crazy on attenuation. It's made with 600V capacitors so I could actually run mains on it if I wanted.Again, all copper clad construction, so I can put EMI tape down to the reference plane no worries, and, EUT bolts onto the terminals with whatever cables are necessary.Tim

author: shabaz
author_rank: Frequent Contributor
timestamp: May 01, 2024, 06:57:22 pm
content: Very useful info! Those two implementations look really nice.. I do want to do it again at some point, and that detail and photos helps a lot.

author: bittumbler
author_rank: Contributor
timestamp: May 02, 2024, 08:54:02 am
content: Hi Tim,many thanks for all your detailed info!I did some more measurements:"combinedinductor..." is the 5uH combination of my LISN, but with all the capacitors removed. So only the inductor combination hovering above the ground pcb."smallind..." is the 0.1uH alone.Then I tested some self wound combinations with the stuff i have here. They were all horrible.I will have to order some parts before i can continue experimenting.ThanksMatthias

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2024, 02:20:22 pm
content: Yes, this is reduced to the most basic. Now, test each one individually and extract the RLC circuit (your tool may have a function for this, I don't know?). Put it in SPICE and have funTim

author: joeqsmith
author_rank: Super Contributor
timestamp: May 02, 2024, 02:25:36 pm
content: 300MHz, not controlled impedance, no de embedding. Maybe something here is of use:https://www.eevblog.com/forum/beginners/measuring-the-self-resonant-frequency-of-an-inductor/msg5112405/#msg5112405

author: wilhe_jo
author_rank: Regular Contributor
timestamp: May 04, 2024, 12:40:39 am
content: I really don't understand why one would build a 5uH LISN... (I know why/when you would need it, but after years I haven't found a single application other than very high power inverters) ...the common one is 50uH...Anyhow, what you can do is to place some resistor accros the inductors to "broaden" the resonance. This helps you with the series resonances.Btw: choosing better inductors could also help. SER2918H-472KL gives you 30 MHz and leaves abt 1/10th of the inductance. You could fill this with SER1412-301ME... That'll give you 150MHz.Another possibility would be VER2923-332KL and SER2915L-152K that might be even better...To build a Lisn, it's always important to consider the min. acceptable impedance.73

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2024, 01:30:06 am
content: FCC, CISPR 16, etc. mostly use 50uH, for mains, true. But CISPR 16 also defines a 5uH + 50R network, and various automotive standards do as well. Even just a few uH is sufficient if you want to measure higher frequencies.Tim

author: bittumbler
author_rank: Contributor
timestamp: May 04, 2024, 06:55:50 am
content: Quote from: wilhe_jo on May 04, 2024, 12:40:39 amThe reason for 5uH is my personal situation. I dont need strict standards compliance. The 50uH version has an 8uF/5R capacitor on the supply side, that results in high "leakage currents". The parts are mechanically much bigger. So the 50uH version is much more impractical to use for me.On the other side, the main difference seems to be the lower usable frequency limit (8kHz for 50uH, 70kHz for 5uH). I cover the low frequencies with a current probe.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: May 04, 2024, 09:16:47 am
content: Quote from: T3sl4co1l on May 04, 2024, 01:30:06 amWell, that's one reason, but I doubt it...Quote from: bittumbler on May 04, 2024, 06:55:50 amWell, no. You change the impedance presented to the EUT pretty drastically.If you want/ need to go below 150k, you need the 250uH inductor as well.This will also attenuate noise on the grid. You correctly found that an insulation transformer is needed to get rid of the leakage. But it will also remove low frequency noise.In my lab, I use 90% of the time an audio amp to make some clean mains. However, sometimes I use the real one and I'm always surprised how dirty it is. The 40dB+ attenuation from the Lisn is really key in these situations...If you just want to have "some" results, you could just multiply the current with the LISN impedance and you get something similar.For everything but mains, just take some capacitor and 100R in series to some 50R input.If you have more than 1 conductor (more than just some cable shield), just take N times the R/C values for each conductor.That'll simulate a CDN. But here you also loose the defined impedance....Yes, you can do pre-compliance without well defined impedances, but you'll never be sure that you're fixing an actual problem.73