5uH Aerospace LISN: How dumb would I be to "throw one together"? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: July 24, 2020, 05:55:05 pm
content: September 7, 2020 Update:Please this thread for final design files and discussion:https://www.eevblog.com/forum/projects/lisn-for-rtcado-160-aerospace-emc/Hi all,I've been following this lovely thread for a long time:https://www.eevblog.com/forum/projects/5uh-lisn-for-spectrum-analyzer-emcemi-work/I have two projects that need to comply with RTCA DO-160G (Aerospace) EMC. The end-game is 3rd party testing with "correct" equipment. Like other standards for vehicles, the LISN defined is the 5uH type. I have the option to rent monthly at $600/month for a pair of LISNs. To purchase, it's about $2000-3000. Purchasing is out of the question for the moment. Obviously, the commercial LISNs are already characterized nicely, and since there's already plenty of things that can go wrong with in-house EMC pre-testing, it's nice to keep unknown variables at a minimum. At the same time, I still have a hard time justifying the cost of the commercial LISNs.For the sake of ballpark pre-compliance testing, how dumb would be to try to DIY the LISN with the simplest possible design? (FYI, I need to pass >12A DC (for project #1) and >3A AC (for project #2). I have a 1.6GHz SA with tracking generator to help characterize. I see most people are using VNAs. I'm not sure the difference in this case.)-----A few questions to help me understand whether this is practical or not:1.The Tekbox 5uH design uses a "segmented" or "staggered" inductor design..with taps that go to RC networks. What gives? Is this to help flatten/control the frequency response? What's the effect of breaking down into segments as opposed to one big "lumped' inductor?2.Some designs have small value series resistors in line with the 1-10uF capacitors. Same idea as above? Add some damping to the frequency response/prevent oscillation?3.Generally speaking, are you more likely to have issues with relatively large air-coil inductors or smaller ferrite/powdered core inductors? Seems like air-core is generally preferred, and probably easier to reach at only 5uH, as opposed to 50uH.4.For designs with series inductors, some have parallel resistors across each. What's the rationale? (Jay_Diddy_B shows 330R across his.)5.The measurement is taken across a 5K resistor but with a 50R receiver, so the RC filter is really 0.1uF + (50R || 5K = 49.5R). I suppose the value of that 5K resistor barely even matters then. since it's always paralleled with 50R. (Some LISNs use 1K.) I was planning on copying the HP 11947A transient limiter...Is this generally a "universal design" that can be used with any 50R system? Obviously need to adjust for 10dB attenuation in the receiver.I know these are pretty naive questions. I think it's obvious I'm still a novice in many of these areas. Maybe that's a sign that now's not the time to dabble in something I'm not too certain about, especially when I have a deadline to meet and customers to please.Thanks,Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 07:59:41 pm
content: The answer to most of these, is: dampen resonances of the inductor.Play around with this, for example; read the supporting information:http://hamwaves.com/antennas/inductance.htmlIt uses a helical waveguide model, and a bunch of corrections, to estimate the voltage and current through a single layer, wire solenoid. Apparently this geometry is remarkably difficult to analyze!If the calculation succeeds (there is some root-finding that can fail, particularly at small pitch angles), you get the complex impedance of the element -- resonant modes accounted for!The first (parallel) resonant mode is fine, that's what we want -- a high impedance between DC and EUT sides. What we don't want is the second (series) resonant mode, which acts in parallel with EUT, shunting signal and distorting the frequency response.Knowing that standing waves are the culprit, we can employ resistors in strategic locations to dampen those modes. The series resonance has an antinode in the middle (voltage peak), and nodes (current peak) at the ends, which is why we measure a low impedance at the ends, at that frequency. A resistor from one end to the middle will act in parallel with that mode, dampening it. The resistance acts in parallel with the voltage peak, and gets 1/4-wave-transformed into a series equivalent resistance at the terminal. When R = Zo,* parallel R is transformed to series R, and that's simply the resistance you will measure at the port.*For an ideal transmission line, Zo is simply Zo. Helical waveguide however is dispersive (velocity varies with frequency), meaning the Zo varies for each resonance; the resonances also aren't harmonically related. So it's not quite as simple as knowing the wire length of the coil (though that's close for the 1st mode, I think?), and it may be worth playing with the resonances in a calculator such as above, or testing real hardware.[Note that helical waveguide can have quite high impedances, low kohms -- this makes them useful for bias tees for one, but also useful for delay lines in certain applications.A vintage application was color TV sets: the impedance must be high (low kohms) to suit to the vacuum tubes used in early sets. The delay is applied to the luma signal (which is simply detected directly from the radio signal), to "catch it up" with the chroma signal, which ends up delayed due to additional filtering and processing stages. The required delay was about a microsecond. A typical delay line was a phenolic tube, wound with fine wire, and lined with a strip of foil -- not a complete wrap-around foil lining, that would make a shorted turn, defeating the helical mode; just a narrow strip to give some ground reference for the travelling wave.]We can also employ loading materials; if we have a lossy ferrite or powdered iron material, we can use it to both increase the inductance (reducing the number of turns required, potentially raising the resonant frequency) and dampen the resonance (by magnetically coupling material losses to the resonances).The loss has to be appropriate, of course; a high-mu powdered iron will be too conductive, and actually increase the capacitance more than dampen modes. A low-mu ferrite might have too high of a Q at these frequencies, and just not do much, or maybe make things worse. It may take some trial and error to perfect.I haven't done this specifically for solenoids, but I have plotted a few resonances on a 100% coverage, single layer toroid winding. Here, the boundary condition forces the first mode to be a full wave, and it is series resonant. Again, the resonant impedance is quite high, which makes it difficult to dampen at the terminals -- if we're using this toroid for a current transformer, we can't afford much burden resistance. Maybe the resistor needs to be a few ohms for the application, but the correct value for damping would be 300 ohms, or a few kohm even -- highly impractical to dampen.Knowing that it is a resonant mode, however, it can be shunted by simply distributing the applied current around the core. Instead of using one loop in one place, use two loops, wired in parallel but positioned in quadrature (at 90 degrees to each other on the toroid). Or hexature, etc. (I was using a high-mu toroid, which is rather lossy at the 10s of MHz these modes were showing up, so the 4th mode was already very weak; I didn't try looking any higher.)Anyway, what we're after, is knocking out series resonances (impedance dips at the EUT/RF port) and replacing them with resistances, hopefully resistances that are high enough not to worry about (i.e., R >> Zo).If we have discrete inductors that manifest as lumped single RLC networks (i.e., effectively some simple equivalent parallel capacitance, without having ugly modes at higher frequencies), we can employ the same methods, applied to a lumped-constant circuit, without having to worry about waves and modes necessarily. Here's a bias tee by Picotronics, flat to, as their name suggests, some picoseconds (many GHz):https://www.seventransistorlabs.com/Images/Picotronics/MVC-349X.JPGhttps://www.seventransistorlabs.com/Images/Picotronics/Schematic.JPGIt seems they opted for the parallel damping resistance instead. I think that can save a little on impedance -- you don't have the lossy cap-to-ground loading it down. Which one is best probably depends on the inductors used, and desired flatness.Don't forget, too -- inductors have capacitance between their terminals, and to ground or free space; a 2-terminal model of an inductor or capacitor can be erroneous at these frequencies. This may also be a factor.As for some other questions -- air core is preferred over ferrite because ferrite saturates. Rod cores aren't too bad here, as they can be made with quite high saturation currents, and the advantage is small anyway (mu_eff maybe only 2-5 say), enough to be helpful without getting in the way.Saturation current should be quite high indeed for a mains LISN or CDN -- loads with poor power factor can draw quite high current peaks, and those current peaks are also likely where most of your EMI is transmitted (the FWB acts as a PIN diode), so you don't want your network pooping out on the peaks!Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 24, 2020, 09:30:52 pm
content: TimNJ and the group,I can probably help design you a LISN that will work in this application.Your current specification is higher than the maximum for the LISN that I shared before.I have a few questions for you:1) Does the EMC specification that you are testing require one or two LISNs?2) What is the maximum frequency that is required in the EMC specification for conducted emission?3) Which EMC specification are using attempting to meet?Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 24, 2020, 09:51:51 pm
content: Hi,From here:https://www.com-power.com/uploads/pdf/LI-325C-1.pdfI find this:So it looks like you need two LISNs.and this is impedance curve limits:The limit lines are in red. It looks like 5uH // 50+/- 20% to 152MHz.This is all that is needed to design the LISN.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 24, 2020, 10:05:07 pm
content: Quote from: T3sl4co1l on July 24, 2020, 07:59:41 pmWow, thanks Tim. As always, you find time to give the most informative answers possible and I appreciate that. I guess the takeaway is that as resistance (or lossy material) added to an LISN (or any similar style network) is to add some damping to keep incidental tank circuits from messing with the "flatness" of the frequency response. From playing with the calculator you attached, andthe GCI-Wcalcmentioned in the other thread, I've noticed that making a single 5uH coil with a reasonable diameter (let's say 10mm) might push the SRF to <100MHz. (The conducted EMI measurement goes from 150KHz-152MHz.) But, if you break up the 5uH coil into 4 or 5 coils, then each coil's SRF will be >200MHz. Presumably, if the coils are far enough apart (physically) any proximity effect related issues (that would contribute to parallel capacitance) would be practically gone.Perhaps I should do some SPICE simulation based on the results from Serge Stroobandt's solver. Even if the SFRs of each coil are above the frequency band of interest, I suppose there could be a way the 4 or 5 coils interact to cause distortion at a lower frequency...maybe? (Talking out the wazoo here.)By the way, is the "first (parallel) resonant mode" you refer to the "intended" LC interaction as shown in a typical LISN schematic. And the "second (series) resonant mode", that's the interaction between the Cp and the Ls of the coil?Thank you!

author: TimNJ
author_rank: Super Contributor
timestamp: July 24, 2020, 10:20:53 pm
content: Quote from: Jay_Diddy_B on July 24, 2020, 09:30:52 pmDO-160 generally speaking requires 2, one for (L/+) and one for (N/-) if AC or DC respectively. However, some aerospace systems are allowed to return power through a chassis connection, so in some cases you can use a single LISN on the L/+ line, and bond the return direct to the chassis/ground plane.In my case, these are not typical aerospace metal boxes, and no expectation of returning power through a chassis, so I need two LISNs. The conducted EMI test is from 150KHz-152MHz, which is a departure from the "normal" 30MHz upper limit I'm used to for CISPR.There is also the radiated emissions test which is, of course, much harder. I have never tried it, but known there are reasonably accurate ways of deriving radiated emissions from CM currents on the cables, as here:https://www.tekbox.com/product/AN_-RF_current_to_electric_field_strength_extrapolation.pdf.I may consider trying it.These are switching power supplies operating at normal ~100KHz switching frequencies. In my experience so far, there's never usually significant radiation about 250MHz...so if the LISN could at least operate someway consistently up to about 250MHz, that would be ideal.Thanks for your help and interest.So, far I've come up with using 15AWG wire (1.45mm) @ 16 turns, on a 3/8" former...yielding 1uH with an SFR of about 250MHz. But not sure what happens when you put 5 in series!

author: TimNJ
author_rank: Super Contributor
timestamp: July 24, 2020, 10:32:09 pm
content: If I wanted to do some lumped element modeling in SPICE with coil parameters derived fromhttps://hamwaves.com/inductance/en/index.html#input......how might I approach it? For example, if I set the design frequency to 1MHz, Cp comes out around 1nF. If I set it to 100MHz, it comes out at 0.3pF.Perhaps trying to do some resonant tuning/damping in spice is a futile endeavor due to the fact that a lumped model is inherently only relevant in a limited range of frequencies?And, I'm having a hard time believing the capacitance could really be 1nF @ 1MHz. At 150KHz, it says it's about 65nF! That's a heck of a lot of capacitance...What am I missing?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 11:29:10 pm
content: The Cp equivalent model is a poorer approximation, and as you note, varies with frequency.The only fundamental thing you can really do, is take the R and L as given -- which are the series equivalent resistance and reactance, reactance being divided by frequency to give inductance -- and plot them over a range of frequencies, then construct an equivalent network that fits those. Bit of a PITA.You might build such a model to hit the resonances without putting too much effort into the overall shape; in that case, hunt around for the resonant frequencies (points where reactance goes through zero, from increasingly large L to very small C; and vice versa) and construct the equivalent accordingly.Quote from: TimNJ on July 24, 2020, 10:05:07 pmNo relation to a circuit -- just the two-terminal component itself. Any small 2-terminal component can be modeled as a parallel combination of R+L+C networks, or a series combination of R||L||C networks; or ladder and other topologies can be used. (The ladder topology has the advantage that it's relatively easy to calculate: the alternating series and shunt branches are the terms in the continued fraction expansion of the impedance.)For example, here's a fit to a common mode choke:https://www.seventransistorlabs.com/Images/CurveFit1.pnghttps://www.seventransistorlabs.com/Images/CurveFit2.pngyou can see the resonant mode(s) at the top end aren't well documented anyway, and aren't tightly fitted.In this particular case, most of the resistances will be core loss; in particular, the stack of R+L's, in a geometric progression, fits a diffusion characteristic 20-400kHz, skin effect of the wound metal strip core. It looks like it has some actual capacitance intrinsic to the core as well, presumably the capacitance between layers (C1).L3 is probably leakage between winding and core, and L4 could be a partial leakage inductance between parts of the winding itself. The fit breaks down here, partly because the data probably aren't great (and cut off suddenly), and because the impedance up here is due to standing waves in the winding, a transmission line or wave effect not easily modeled by lumped elements.R5 isn't part of the model, but used to test it as an impedance divider.The impedance plot of a helical resonator will show multiple peaks and valleys, like the high end in this plot, but more of them.Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 25, 2020, 12:04:09 am
content: Hi,I think you could try and build one using off the shelf inductors.Consider these models:The models are set up to measure the impedance of the circuit by injecting 1A.On the left is the standard 5uH//50In the middle I have a Wurth 7443640470 inductor. This has SRF of 20MHz and a saturation current greater than 40A.The model in LTspice includes the following lumped parameters:The right hand model is this inductor in parallel with 50These are the results:This is almost good enough the impedance is 37at 150MHz.If I replace the inductor with two pieces of Wurth 7443630220 in series I get:Which is fine.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 25, 2020, 12:22:21 am
content: Hi,I am going to share the results of some quick experiments that illustrate the challenges of measuring the impedance at the higher frequencies.I am measuring the 5uH LISN that I designed. I am using an HP3577A and a clone of the HP 35676A R/T Test Set. I have calibrated the network with SOL at the reference plane indicated with a red line.At 30MHz, the marker position the impedance is 50above 100MHz the impedance rises and at 200MHz the impedance is around 60.If I measure two adapters, terminated with a 50load I get this result:You can see that the two adapters connected back to back is worse than measuring the LISN.Alternate Calibration standardsIf I use these standards instead of the normal SOL calibration:So now the network analyzer is calibrated at the banana jacks, I get:This result is text book.I just wanted to show how small amounts of inductance can effect the high frequency impedance measurement.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 25, 2020, 05:21:54 am
content: Thank you all. I realize I need to go back to my LISN fundamentals so I’m spending some time with Henry Ott and this rather insightful pdf from Michigan State:https://www.egr.msu.edu/emrg/sites/default/files/content/module11_conducted.pdfAt least where I work, the LISN is a somewhat “mythicized” black box type thing, so this is forcing me to think about it a little more.I guess my question now is Jay_Diddy, if it’s seemingly so straightforward to put together the network with off the shelf parts, and with reasonable frequency characteristics, why are there so many LISNs with fancy damping techniques? Maybe that’s what’s required to get true flatness in the 50ohm band.Your plots were very helpful in my understanding. Initially my thinking was that if the inductor’s SRF was anywhere in the measurement band, it was useless..but now it’s obvious that a resonance will mostly not affect the 50 ohm impedance, and mainly affects the HF roll off of the 50 ohm impedance. Still, there certainly can be many other things to which make an inductor look, err..not so much like an inductor, as Tim showed in his complicated model of the common-mode choke. Clearly, there can be several resonant interactions and an impedance which doesn’t quite rise quite in like with 2Pi*f*L.Seemingly those Wurth inductors were pretty well behaved, ferrite core and all.Whats SOL by the way?

author: 2N3055
author_rank: Super Contributor
timestamp: July 25, 2020, 08:09:12 am
content: Quote from: TimNJ on July 25, 2020, 05:21:54 amAir core inductors won't saturate.. Cored inductors will also change characteristics as they get loaded. So with cored inductors you have to be mindful of the current going through..SOL : Short, Open Load calibration of VNA....

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 25, 2020, 01:10:56 pm
content: Quote from: TimNJ on July 25, 2020, 05:21:54 amHi TimNJ,You are right there are a lot of myths associated with building LISNs. Which has resulted in a lot of 'traditions' and not a lot of engineering. I believe that that there are many reasons for this:1) MIL 461 one of the earlier standards was introduced in 1967. In 1967 we didn't have the same tools that we have today.2) Some of the early EMC specifications described the construction of the LISN with air-cored solenoids. This started the tradition of using air-cored inductors.Fast Forward today, we haveaffordable VNAsSimulation toolsModern componentsInternetConceptThe concept of a LISN is simple. It provides a standard impedance to the DUT. In this case 5uH in parallel with 50.The requirement is that the LISN looks like 5uH//50Myth destruction1) You can use inductors above their self-resonant frequency.At high frequencies the LISN impedance is 50so the requirement is that the impedance of the inductor is 'large' (say 200) comparedwith 502) There always benefit in using a string of different inductors with different SRFs.This is NOT true.If you connect inductor in series with different SRFs then they can interact.3) Inductance changing with DC currentThis is not an issue providing the impedance remains 'large' compared to 50Air cored InductorsAir cored solenoids typically need many turns to achieve the desired inductance. This long winding may transmission line effects within the target frequency range.The air cored inductors need to be in a large case, typically 3 diameters of the coil, to avoid coupling the case.Cored InductorsCored inductors are really air cored. They have an air gap or a distributed air gap. The magnetic material is equivalent to a conductor in an electrical circuit.The winding in a cored inductor is much shorter. This helps eliminate transmission line effects in the target frequency range.The flux is confined to the core, reducing the effects of coupling.Inductor MeasurementsI am going to measure the Wurth 4.7uH Inductor, 7443640470:I have solder 2x 100resistors in parallel and a BNC connector:This allows the impedance to be measured with my VNA.This is a log frequency sweep from 10kHz to 200MHz.The vertical axis is linear 5/divThis is the LTspice simulation of the same circuit. The LTspice simulation uses lumped parasitic components.There is excellent agreement between the model and the measured results.This implies that we can accurately predict the performance of a LISN built this way.I don't have the proposed inductor 7443630220 in my inventory, or I would have measured it.SOL is Short, Open , Load. This one a techniques used to calibrate the VNA.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 25, 2020, 06:45:33 pm
content: Thank you very much. I think I may give it a try with either 2x2.2uH or 3x1.5uH of the Wurth high current inductors. Seems their models are pretty good, which may be important Do these models account for effects at high current? Thank you for running some tests on your VNA!Any reason I can't verify the same parameters with a spectrum analyzer with tracking generator?A few more questions related more to the actual test standard/requirements:1. The standard calls for 2x10uF connected between line and ground plane, and between neutral and ground plane. The setup is as below. For our normal CISPR setup, our LISN is actually earthed to the building's earth wiring. But in this case, if I was to earth the chassis/ground plane, wouldn't that be a ton of earth leakage current, possibly messing up other devices, not to mention potentially dangerous?I'm trying to figure out if these capacitors are really only to be attached to a "local ground plane", insulated from actual earth. Does that make any sense?2. On that same note, any reason why I can't put the two LISN halves in the same box? Traditionally, two boxes are used, but I think that's mostly because there are certain cases where you only need one. (Although, from reading the other thread, some people have vocalized opinions about why only using 1 LISN is a bad thing.)Thanks again.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 26, 2020, 11:17:58 pm
content: Quote from: TimNJ on July 25, 2020, 06:45:33 pmHi TimNJ,1) I am not aware of an easy way to do the impedance measurement that I did with the VNA with a SA and TG. You can probably get an idea if you have a directional coupler or a return loss bridge.2) Most of the LISNs that I have built have a capacitor in the position of the 10uF capacitor. It may not be quite as big as 10uF but it is certainly an effect RF short.3) There is no reason why you can't put two LISNs in one box. Here is a picture of a low current dual 50uH 50LISN (CISPR 32) that I have built:It works very well.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 27, 2020, 04:44:30 am
content: Thank you.My ignorance to most things RF is showing. Indeed, VNA and an SA+TG are not intended to measure the same things. Not sure how I got that in my head. Maybe I’ll get a $50 nanoVNA but I haven’t done much research to check if it’s a suitable device.Regarding the 10uF caps, I guess I was just looking for a sanity check if putting 10uF between line/neutral and earth (at 400Hz) really made sense. Obviously the standard calls for it, but sheesh...that’s 40 ohms at 400Hz...and there’s two of them. I’m going to research more in the morning, but just thinking about putting 20uF to earth in a public work area just sounds bad, personally. Interestingly, the DO-160 conducted EMI measurement actually doesn’t use a direct measurement at the LISN. Rather, it uses a current probe. So, in this case, the spectrum analyzer can be isolated from the measurement setup. I may need to move the transient limiter to a small dongle outside the LISN in this case, presuming it still applies for current probe measurements.In CISPR measurements, the SA earth and LISN earth are one. In this case, I wonder if the LISN chassis can just be referenced to a local plane, instead of a plane referenced back to the building’s electricity. Just spitballing, will update with my findings.I’ll work on a KiCad project this week with some beefier inductors. Thanks!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 27, 2020, 02:50:49 pm
content: Hi,Don't worry about the transient limiters being included in the LISN. They are 10dB 50attenuators in the measurement range. This means that if you forget to terminate the LISN in 50the maximum return loss is 20dB. It is 20dB because the signal goes through the attenuator, getting attenuated by 10dB and then is reflected going back through the attenuator. You simply add a 50termination on the BNC connector.The transient limiters are a good things.The LISNs I have designed so far were designed using OrCAD. I am moving to KiCad. I have started putting the parts into my KiCad library:I just threw them on a board so I could see what they look like.I make the 3D models in Fusion 360.Coilcraft make an alternative inductor to the Wurth 744364 series that is cheaper and has a higher SRF and is footprint compatible.For the DC testing the 10uF capacitors don't matter.What voltage is the AC testing at 400Hz?Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 27, 2020, 04:41:32 pm
content: Thanks. I mostly understand the function of the 10dB pad + HPF. The CISPR LISN we use (I believe) just has a very low capacitance diode across each BNC. Perhaps I'll just do that to provide some level of protection. For the current probe (https://www.tekbox.com/product/TBCP1_200_Manual.pdf), I think the bandwidth is probably low enough at low frequency to prevent front-end overdrive, although I suppose transients are still possible if something is left unterminated (??) Once again talking out the wazoo.Thanks for the Coilcraft suggestion. I also found Bourns PQ2614 series inductors. (PQ2614BLA-1R5K). 3 of these in series. Typical resonant frequency is 60MHz so that might push the upper-end bandwidth out a little, might help for radiated emissions pre-compliance testing on the input power cable.https://www.digikey.com/product-detail/en/bourns-inc/PQ2614BLA-1R5K/PQ2614BLA-1R5K-ND/7695895The voltage at 400Hz is 115Vac nominal. So, that's on the order of 6A through the earth conductors of the building with 2x10uF. Just seems crazy.10uF box-style film cap presumably has a fair amount of inductance anyway, so I wonder how effective is in the MHz range anyway. The recommended type per the standard is a feed-thru type with metal shell, with the metal shell bonded to earth/ground plane. Anyway, one of the commercial LISNs says this:Quote...so I guess it is what it is. We'll make sure the earth connection is very solid, if that's what keeps it safe. Otherwise, I imagine reasonable voltage could even build up on the enclosure.

author: Pitrsek
author_rank: Regular Contributor
timestamp: July 27, 2020, 08:08:40 pm
content: Depending on your voltages, the textbox lisn might work for you (different box/connectors, bigger wires). I would not be afraid too much if you skirt the impedance limit lines for pre-compliance testing. fan might be needed to cool the inductors.You can measure impedance by shunt s21 method. Basically you calibrate/normalize for through connection and connect dut between hot and gnd. Google for "2-Port Shunt-Thru". Check your setup against a known impedance to asses accuracy.

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 12:11:28 am
content: Thanks! My company doesn't have a VNA, but as I mentioned, I might consider NanoVNA, assuming it's good enough. (I assume the $5000 Keysight VNAs are expensive for a reason? Thanks for the pointers, as my RF knowledge is at the "don't even know the right things to Google" stage.Regarding the design, I've decided on 5x Coilcraft SER1590 1uH parts.https://www.coilcraft.com/getmedia/81a5b014-f8b3-44cc-abe2-de459d23320f/ser1590.pdfI simulated the design with the provided LTSPICE frequency domain model. The theory is very good...I'm not sure if the upper end frequency range would really behave that nicely in practice, but who knows.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 28, 2020, 01:46:59 am
content: Hi,The SER1590 inductors look like they will work. They may be slightly low on saturation compared to some of the other parts. The price is right.The 10uF feed-thru capacitor is bit harder. This is what the 'official' parts look like:I found three manufacturers of these but they all looked similar.I found this listing:http://www.icwltd.co.uk/products/ft.phpThey show this style:I don't really know much about the RF performance of these capacitors.I would be tempted to try 10 pieces of a PHE850ER7100MR03R06L2 from Kemet. These are Y2 rated.This would be one of the two LISNs:I included the larger SER2918 Inductors.I did not place the parts in the transient limiter.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 02:24:40 am
content: Just did a sanity check on the Coilcraft Model...The SFR is not showing at all. Might explain why it's so flat...although 115MHz SFR sounds good regardless. You're right about the saturation point. Perfect inductance is really only maintained to 10-12A which isn't a huge upgrade over a standard Tekbox model. On the other hand the SER2011/SER2013 series are bigger and more expensive. I'll think about it.Here's the model. Not sure what everything is exactly. Wurth model in the same setup shows SFR just fine.QuoteRegarding the 10uF capacitance...you are right that Y-class capacitors are technically appropriate here. I was looking for Y-class caps in large value today, but somehow did not come across this series, so I decided X2mightbe okay for *laboratory use only*. The cost of Y2 film caps is very high, not to mention the size is enormous...close to 10x the size of X2.I am definitely hesitant to say any of this on a public forum.Warning to anyone reading:X-class capacitors are built to be safer than normal film capacitors, but still are not intended to go from line/neutral to earth. If a capacitors in that position fails, it has the potential to put the chassis at a high potential. X capacitors are not advised in the position I mentioned.At the same time, for a properly earthed system, an internal short to earth should blow the internal or building fuse/breaker. I'm a little bit on the fence.

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 02:38:51 am
content: This doesn't necessarily make it right but the Tekbox TBLC08 LISN (teardown EEVBlog #993) uses Vishay MKP1847https://www.vishay.com/docs/28172/mkp1847ac.pdf...not even X2. Just a normal film cap.The LISN at work (Atten brand) also only uses X2 capacitors. I wonder what a Rhode and Schwarz uses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2020, 03:02:37 am
content: Honestly, I'd feel safer with MOVs on the mains side, and regular whatever-voltage-rated caps in the network, not X or Y. 630V or up is probably a reasonable choice.Also, at some point we went from happy little DC LISNs, to mains LISNs. Probably better to make one specific to each, so you don't have these problems with scale. (The mains one probably won't do so well above 30MHz, either.)The Coilcraft model can be plotted here, and also converted into a format usable in any SPICE engine:https://www.seventransistorlabs.com/Calc/Coilcraft1.htmlAhh fuck, they updated their website, didn't they. All my links are broken...These are the parameters required:https://www.coilcraft.com/en-us/models/spice/I don't know what the other parameters are, in the above post.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 03:12:24 am
content: Thanks. Any reasonnotto use X-class?Sorry if it wasn't clear but I think I stated that it was an AC (115V/400Hz) + DC (14V/28V) LISN from the start, but maybe that's buried in the mounds of text now in this thread.. In my case, I'll be using an isolated programmable AC source to produce the 400Hz (Chroma 61503) and an isolated supply for the DC. So...maybe notmainsin that sense since there's some isolation.I guess you're just talking about issues >30MHz due to sizes of parts for the mains rated version, if there was one? I agree it would probably make sense. Unfortunately, don't have a ton of time so I'd really rather only have one...if it was "good enough".Thanks for pointing to your website, shall give it a look.Thanks again.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2020, 03:26:10 am
content: X caps are safe for mains transients because they self-heal. Self-heal dumps spikes into the system; not the greatest idea.Running from a synthesized source? Even better! No worry for transients at all. Hrm, well, beyond hot-plugging I suppose. Even nominally rated capacitors will do (but a little extra, and/or the use of floating-electrode types, isn't a bad idea).This may be one of those situations where staggered capacitor sizes and values proves beneficial -- you can get the stray inductance between them low enough that it doesn't make things worse at the inbetween resonant frequencies. Also may be a good reason to choose PE dielectric (MKT and other types) over PP, the Q is a bit lower. Anyway, bypassing on the DC side shouldn't be too important thanks to the dominant impedance of the decoupling inductor, which is really the star here.Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 28, 2020, 03:29:26 am
content: Quote from: TimNJ on July 28, 2020, 02:24:40 amHi,The Coilcraft model is only valid over a limited frequency range:*======================================================================* SPICE Model generated by Coilcraft* Coilcraft Part Number : SER1590-102* Inductance = 1uH*======================================================================* Model Parameters:*Valid Frequency Range = 0.1MHz-10MHz* Ambient Temperature = 25 degC* Inductor Frequency Model* Use model for Frequency Domain simulations*======================================================================They say here:https://www.coilcraft.com/en-us/models/howto/using-coilcrafts-models-in-ltspice/When running simulations, note the upper and lower frequency limits for which the model is valid. This information is shown on the Coilcraft SPICE model document for each inductor series.Which make it pretty useless for looking at the SRF frequency.The Wurth model seems to be correct. It matched the measurements that I made with a VNA.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 28, 2020, 03:45:39 am
content: Quote from: TimNJ on July 28, 2020, 02:38:51 amThe datasheet for the MKP1847ac capacitors used by Tekbox says:I like the idea of using a better cap that is intended for connecting either from L to N (X rated)orL to GND (Y rated)Although you might not need the mains rated caps in your application at 400Hz with the Chroma AC Source, why not build one that is safe as possible to use on the line?There is some logic in building a small one for the DC application and a bigger one for the AC application.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 04:32:04 am
content: Quote from: T3sl4co1l on July 28, 2020, 03:26:10 amFloating electrode as in MLCC with flexible termination? If so, interesting thought, though since these will be hand-soldered, I'd still feel a little queasy about the whole idea. Inductance of those 40x20mm MKP caps is probably not so great at the high frequencies is I guess what you are suggesting. I suppose I could bypass with Y-caps or similar to get some better MHz range performance. I suppose those should be located closer to the AC (or DC) input? Or maybe it's more important to keep the inductance between parallel caps down, exact position of HF bypass caps less critical?Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 28, 2020, 04:44:32 am
content: Quote from: TimNJ on July 28, 2020, 04:32:04 amThe floating electrode, is a type of capacitor construction where there are two (or more capacitors) in series inside the capacitor.The Wima FKP1 shown here:Has two capacitors in series.This reduces the ac voltage across each of the internal capacitors by a factor of two.Typically one section is used for every 200V of ac voltage rating.The 10uF capacitor in the EMC application do two things:1) the define the impedance at low frequencies say less than 100kHz. Good RF performance is not needed for this part.2) Act as a filter for noise coming from the input side. This is probably the main reason for having them. Good RF performance is needed for this.(In your application with the Chroma Source) you might not need this function).Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 04:51:23 am
content: Quote from: Jay_Diddy_B on July 28, 2020, 03:29:26 amThanks. I saw this too. Not sure what Wurth is doing to get a reasonably accurate model and why Coilcraft can't (or doesn't care) to model the high frequencies. Perhaps they decided it's outside the "useful" frequency range of the inductor (i.e. few people would try to design a switching power supply above a few megahertz anyway.) Maybe low frequency model accuracy suffers if try to fit the model to high frequencies as well. Not sure.

author: TimNJ
author_rank: Super Contributor
timestamp: July 28, 2020, 05:21:31 am
content: Quote from: Jay_Diddy_B on July 28, 2020, 04:44:32 amAh! Didn't know about those capacitors. Learn something new every day. Thanks.Regarding the capacitance, I don't really know how the AC source synthesizes the AC voltage and what the noise signature of it is. Definitely some sort of switching topology, maybe some sort of DDS thing.I simulated with 1uF and 10uF in that position. As you said, the low frequency impedance is indeed affected, although when comparing to the DO-160 LISN limits, seems it would still be in the allowable range. Then tried with 4.4uF (2x2.2uF). Impedance is really not that much different between the two.Yellow = 1uFBlue = 4.4uFRed = 10uFPerhaps the 10uF is somewhat of a historical artifact. Maybe the 400Hz sources of the past were very dirty/noisy. (Perhaps EMC test labes were even usingmechanical400Hz generator?)Unless a higher impedance below 10KHz is for some reason a problem, seems like a lower capacitance would be okay.edit: As we know, something's gone wrong with the attachments function. It was showing an image I posted in another thread. Should be fixed now.

author: TimNJ
author_rank: Super Contributor
timestamp: July 29, 2020, 01:09:16 am
content: Hi all,Here's the planned schematic and layout. Earthed mounting holes mate directly to the die-cast aluminum chassis via M3.5 screws. Planning to use panel mount BNCs with a small 50ohm coax jumper from PCB to BNC. (Too many misaligned holes on previous projects.)I've changed the inductors to SER2011-122 (1.2uH). 100% inductance to over 30A. I've added the option to populate two ceramic Y-caps on the "mains" side. Undecided if it's a good idea yet. The bottom side will have a matching earth polygon pour.Any feedback welcome. Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 29, 2020, 06:16:16 am
content: Hi,What capacitors are you using?I like to use Y caps for the 0.1uF. If they fail they will destroy the analyzer.You need to use a transient limiter (or at least a 10dB) attenuator with this LISN if you connect the output to a spectrum analyzer.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 29, 2020, 03:08:32 pm
content: The 3 * 3.3uF caps are X2. Planning to use this one:https://www.digikey.com/product-detail/en/kemet/F861DY335M310ZLH0J/399-17029-ND/8346549Since posting, I've added a MOV across each, to keep the voltage stress on the caps as low as possible. For my case, should always be powered from a well controlled source but for anyone else who might attempt this, can't guarantee that.The 0.1uF were also X2. As others and you have mentioned, large physical size and high frequency usually doesn't bode well, so was trying to keep the size and series inductance low. Not sure if it's relevant in that position. However, based on your recommendation, I will plan to use this:https://www.digikey.com/product-detail/en/wima/MKY22W31005D00KSSD/1928-1791-ND/9370699Regarding the transient limiter I've installed two back-to-back 5V ESD diodes. I know the Tekbox 5uH uses a Bourns GDT rated at 60V. Necessary? Recommended? Anything else? If I add an HP11947 style limiter with 10dB, I'm just a little worried about insertion loss roll-off/deviation with frequency. Presumably I'd have to compensate on the SA somehow.By the way, I bought a NanoVNA and am excited to learn how to use it. Hopefully I can give an accurate analysis of the impedance characteristics of this LISN once built.

author: Pitrsek
author_rank: Regular Contributor
timestamp: July 29, 2020, 06:52:08 pm
content: The measurement I've mentioned/describedcan be done with tracking gen. and SA. It is equivalent to what VNA does as s21, minus the phase. If you are measuring just passives, phase can be calculated out of the measurements. But amplitude is usually all that is needed.Ad 10uf capacitors:some LISNs have the capacitors as add on module - both due to safety reasons, and it is more versatile. There are other regulations that are using 5uH LISN, but do not use the cap. If you are only after the required impedance profile, big foil cap across terminal is all what is needed. If you need to filter out hf crap from outside, as mentioned previously feed through cap is way to go. If you want to implement it on the pcb, and want really the best performance, layout is important.Ad Layout:The signal has to be forced through the capacitor pins, ie. fuse - pins of first cap - pins of second cap - pins of nth cap. Just trace, which is narrowed down just to solder pad of the capacitor at the cap pins. If you go with polygons, I'd slot it around the pins, so signal has to go through the pins. Same on the return. I'm usually a big proponent of solid unslotted (ground) planes, but in this case slotting is in order. It will definitely work as it is now, but the effect of the routing will be measurable at the higher end of LISN bandwidth.No ground plane under the inductors/traces. I'd put the inductors further away from each other, and provide connections for possible damping resistors/RCs.Minicircuits has nice power limiter VLM-33W-2W-S+You might be also interested in noise splitters and parasitic coupling of components - google for "Characterization and Cancellation of High-Frequency Parasitics for EMI Filters and Noise Separators in Power Electronics Applications" - by Shou Wang

author: TimNJ
author_rank: Super Contributor
timestamp: July 29, 2020, 08:52:02 pm
content: Quote from: Pitrsek on July 29, 2020, 06:52:08 pmOi! I guess I misinterpreted your post then. Well, I guess I have two options then, though I still have to learn about the measurement principles.Regarding the 10uF capacitors:It's a good point you make. There's a possibility we'd want to test CISPR25 (auto/vehicle), but I really can't be certain. CISPR25 is 1uF.CISPR25 also uses a more "standard" voltage measurement via BNC/N-type connector on LISN to measurement resistor. DO-160, on the other hand (what I'm currently after), only uses current probe measurement. I suppose it's possible to convert the limits between voltage and current measurements?? But per the standards, that's how they want you to test.Regarding the layout:I've seen this kind of slotted polygon layout many times before in EMI filter designs, but to be honest, I don't understand why they help. I assumed the lowest inductance/impedance connection would be appropriate, so what's the mechanism here? High frequency currents maybe tend to scatter and bypass the capacitors? (Just seems counter-intuitive from what I know, but I'm sure my current mental models are flawed.)Is this what you were thinking?Indeed, no ground plane under the inductors as that will likely affect the SFR among other things.Regarding the Minicircuits power limiter:This is a new type of component for me. As I understand it, it will pass 100% of the power between 0 and 12dBm. Above 12dBm and below 33dBm, it will limit the power to approx 12dBm. Sound correct? VSWR looks a little high at 200KHz. (Does it matter?) Also, seems like SA damage/problems often occur due to low frequency overdrive, i.e. at the fundamental switching frequency of a power supply...which could be <100KHz. Would this do much to attenuate those signals?Regardless, this could be an interesting quick solution for me. I just do not have much time at the moment to work on transient limiters, etc, especially since the standard I'm trying to test to doesn't even use that method. If I go for a "DO-160 only" LISN, then I might just remove the BNC outputs altogether, and just put a 49.9R dummy resistor on the PCB and call it a day.The other option is to maybe include 1uF on the PCB, then allow for connection of ~9uF outside the enclosure. Or no capacitors inside and swap out 1uF or 10uF externally, so that it can be used for CISPR25 or DO-160. But, then I'd need to add provision for voltage measurement internal to the LISN.Thanks!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 30, 2020, 03:56:18 pm
content: Hi,Don't work about 10uF versus 1uF for the CISPR 25 LISN. It will be fine, especially for pre-compliance.The 0.1uF 22.5mm capacitor will have an inductance of about 20nH.You could tighten up the layout at the grounded end of this capacitor. I use 50coplanar wave guide. There is only low voltage higher.This model shows how the inductance impacts the LISN impedance:Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: July 30, 2020, 04:54:31 pm
content: Quote from: Jay_Diddy_B on July 30, 2020, 03:56:18 pmThanks again. Feel like I'm getting annoying at this point with the non-stop questions. But, thanks for your help.What do you mean the grounded end of that capacitor? Which side is that?The way I've chosen to approach this is to add SMA connectors on the board to allow for a future upgrade to actually use the measured voltage signal. For now, the bottom resistor is 5K (4.99K) per the standard, and then I will screw in a 50R SMA terminator to get the 50R impedance when using current probe measurement. In the future, I may bring the internal SMA connection out to a panel mounted BNC/N/SMA connector. Maybe add one of those Minicircuit limiters in between, or add HP11947A clone on a daughter-board inside. For now, I just don't have the time to think about it.The trace going to the SMA connector is coplanar waveguide 50R (1.5mm width/0.3mm gap/1.6mm FR4). I don't know if that's helpful or necessary. Maybe I should add a few stitching vias to tie the planes together.Where's the coplanar waveguide you're talking about go?Thanks.

author: TimNJ
author_rank: Super Contributor
timestamp: July 30, 2020, 05:55:20 pm
content: From a safety standpoint this is questionable, but a fat HV ceramic seems pretty nice in this application.https://www.digikey.com/product-detail/en/kemet/C2225X104KFRACTU/399-10506-1-ND/4172868About 800pH series inductance according to Kemet's K-SIM. Unfortunately, even with boardflex/flexible terminations, I'm not sure if this makes sense, especially granted it's position near mounting holes.Or, use 2 x 0.047uF Y2 film caps in parallel. In theory, it should halve the inductance, but in practice do not know if this winds up making a resonance point or something else.

author: Pitrsek
author_rank: Regular Contributor
timestamp: July 30, 2020, 07:14:31 pm
content: Paralleling same value capacitors works well. Paralleling same capacitors actually it works much better than the 1uf 100n 10n triad of resonance...Make yourself a impedance test pcbs so you can see for yourself (Or buy themhttps://www.sv1afn.com/en/products/rf-experimenter-s-pcb-panel-of-8-pcs-diy-kit.html).Each piece of trace adds an inductance. Depending on the inductance of the capacitor and added inductance this might have big or negligible effect. If you have power plane/ground plane pair with very tight spacing, and caps connected directly to the planes, added inductance is marginal. In case of 2L board, spacing is not tight and added inductance might not be negligible at all. Ie. If you add 2cm of trace you will multiply inductance of a small mlcc capacitor. It's a same here, just capacitor inductance is rather big to start with, so the effect is not as huge.I did a quick test - small pcb, top was signal, bottom ground capacitor(330n/630v mkt, rather big package) connected between the two. SMAs on both ends. 2 samples - one with solid planes on both sides, second with plane slots so it forced flow through pins of capacitor. Difference in attenuation at HF was cca 1.5db. This is not much but: big package - with smaller package the effect would be bigger, and if you have multiple capacitors/multistage filter and on each cap you can gain 1.5db of attenuation....In your case it is a detail, but depending on components size and layout, it might have bigger effect.Imagine you are electron traveling from the fuse to the first small cap. In you original design, you go through the trace(inductance) and through the capacitor. If you want to go the output, you do not travel through the same inductance. You have basically added extra ESL to you capacitor.Actually there will be a HUGE resonance between the 470pF and 10uF. At some point it will be marginally better than just 10uF, and at some much, much worse than just 10uF. For more of the topic I can recommend something from Steve Sandler about flat output impedance. Or Istvan Novak - distributed matched bypassing.But this all is more of curiosity/academic discussion, unless you have extremely noisy power supply your lisn will be fine will be fine either way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2020, 09:49:56 pm
content: I don't think quibbling about nH is all that useful here. The body of the capacitor will be some pF to surroundings, thus having a LCL equivalent with a cutoff at some point, and whatever impedances. Perhaps more intuitive is to consider it as three TL segments: the leads and body.About all you can do about the body is remove ground under it.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: July 31, 2020, 11:02:08 pm
content: Thanks to everyone for their input. I've ordered a few PCBs based on some recommendations here. I will try to characterize the design once I receive the NanoVNA (and once I watch enough W2AEW tutorials to figure out how to use the thing).Pitrsek, regarding your explanation:QuoteWhich capacitor? I guess I just can't seem to wrap my head around how a cutting the polygon pour (in my head, increasing the average distance traveled) makes the inductance lower.---Side question: The main difference between a 5uH and 50uH LISN is the low frequency impedance, below roughly 5MHz...If the noise source is modeled as an ideal current source, then the voltage at the RF port of the LISNin theoryshould always be worse with a 50uH LISN than with a 5uH LISN. I thought about it for a while and figured there was a hypothetical situation where the wiring from the DUT, its input EMI filter, etc. may resonant with the LISN's impedance in such a way that the EMI level is worse with 5uH than it is with 50uH.However, after trying a number of LTSPICE simulations, the only time I could get resonant peaking (with higher RF port voltage on 5uH vs 50uH) was with a relatively large common-mode capacitance (line to earth), in the range of ~5nF and above. I feel this would generally be a rare scenario, especially for mains power supplies or anything with earth leakage requirements.Perhaps a situation still exists, but my question is: Is it generally true that a system passing EMC standard on 50uH should most likely pass on 5uH?Thanks.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 01, 2020, 12:46:52 am
content: Hi,Let me address the differences between the 5uH and 50uH LISNs.Historically the 5uH LISN were used in small vehicles like cars and planes where the wiring is short. 50uH LISNs were used for line voltage and ship board applications where the wiring is longer.At high frequencies the impedance of both LISNs is the same, it is 50. So for high frequencies the results will be identical.LTspice ModelI am going to turn to LTspice to illustrate the differences between the LISNs. I am stepping the value of the inductor between 5uH and 50uH:ResultsThe bottom graph shows how the impedance changes with frequency.The middle graph shows the voltage output if the emission has a 50source impedance.The top graph shows what happens if the emission source is low impedance. I have chosen 1. It could be lower than 1. In the case the choice of LISN has very little effect on the voltage at the output of the LISN.Regards,Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: August 01, 2020, 02:52:31 am
content: Thanks, as always, for your insightful responses. I understand 5uH may closer approximate a more compact distribution system , and 50uH may closer approximate the wiring in a building. That part makes sense from a historic/rationale perspective. But if the response to EMI is so similar..why were two impedances ever created?What if we actually measure the EMI current going into the LISN, instead of the voltage across the 50R resistor? (This is as measured in DO-160 and for signal wires in CISPR25). In this case, the 5uH inductor presents a lower impedance and thus the current below ~5MHz will be higher for a given noise voltage signal. No? By contrast, if measuring at the RF out port, since more EMI current is now shunted through 5uH, then less signal reaches the RF out port.So, by that logic, seems you will show lower EMI with a 5uH LISN if using RF measurement port, but you will show higher EMI if using current transformer method. Once in the 50R region, then both methods should show equivalent readings. Am I making sense?[ Specified attachment is not available ]

author: TimNJ
author_rank: Super Contributor
timestamp: August 01, 2020, 03:08:35 am
content: A few sanity checks about safety:The LISN is an unusual piece of test equipment due to it's normal high earth leakage current. 60Hz LISNs may leak close to 1A (depending on the standard). For 400Hz DO-160, the earth leakage current is close to 3-6A (!!!)However, if my analysis is correct, with an isolation transformer (or isolated AC source), a high current no longer flows in the earth wiring of the building since the conductive path is broken by the galvanic isolation of the transformer. So, in these cases, no more 3-6A flowing in earth wiring which could possibly cause a ground bounce issue that might affect nearby equipment and/or elevate the voltage on nearby earthed surfaces to something unpleasant or dangerous. The earth leakage current through the 10uF caps should be limited by the capacitive reactance/insulation resistance of the isolation barrier (transformer)...which will be on the order of 100s of KOhms at least.Is that correct?For an ideal isolation transformer, grabbing either the live or neutral secondary wire is not a risk because there's no way for current to flow through your body. But, with two 10uF caps now tied to ground, a loop can be formed on the secondary side of the transformer. It's is nearly "earth-referenced" with such a huge capacitance between earth and one of the secondary. So, grabbing one of the hot wires allows a current to flow out of the transformer, through your body, through the earth, up through one of the capacitors, and back into the transformer. Danger! Still, this requires actually touching a hot wire, which is not such a likely scenario.If the LISN is left unearthed with an isolation transformer, touching the housing of the LISN should still be safe since you're still galvanically isolated from mains. From this, it seems that an isolated source provides a good deal of safety.On the other hand, with anon-isolated source, there seems to be much less room for error. If the LISN is left unearthed, the housing floats to 1/2 the mains voltage with respect to earth. Touching the housing will be nasty, especially since the source impedance will be quite low through the 10uF. And, now, since there is a conducting path back to the mains supply, a large steady state current flows in the building earth wiring/structure.So, it sounds like isolated supply is really the way to go for two reasons. Does this seem accurate?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 01, 2020, 10:50:06 pm
content: HiIf the noise source is current, not voltage, you get this result:If you inject and current and measure current you get a flat response from the CT and the expected result from the LISN impedance.Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: August 01, 2020, 11:19:03 pm
content: Thanks. Sorry, my circuit image is not showing. I modeled as voltage source with 1R source resistance, same as you did before. But then measuring the current into the LISN. In reality, I don’t know if a “typical noise source” (if one existed) is more closely a current source or voltage source.

author: chris_11
author_rank: Contributor
timestamp: August 02, 2020, 07:46:43 pm
content: Try to give you some practical advice. The first inductor data sheet SRF is the parallel resonance of the inductance with its own parallel capacitance. This is in a LISN a high impedance point (against 50 Ohm) and of no concern. The problem are the real series resonance frequencies (low impedance points) where partial parts of the inductor inductance resonate with partial serial capacitance. Since those points are not given in any data sheet you need to characterise them with your own measurements (VNA or Tracking generator with SA). You are better off with several low inductors in series (same values) because their resonances are higher than larger inductances in the same package. I got good results up to 1Ghz with 6x 800nH SER2009-801 Coilcraft inductors in series. Do not mount them on a PCB, since the connections will add capacitance and move the partial series resonances lower. Solder those 6 inductors in the air in a long strip. You might support them with some low capacitance material on the core if you like too, but due to the flat wire with over 50A capability it is usually not an issue.brChristian

author: TimNJ
author_rank: Super Contributor
timestamp: August 18, 2020, 03:20:51 pm
content: Hi everyone,Photo of the assembled PCB is below (sitting unmounted in the Bud AN-1322-A enclosure.) I'll report back with NanoVNA measurements soon. If everything's okay, I'll share the KiCAD project files after.Thanks for all of your help!

author: TimNJ
author_rank: Super Contributor
timestamp: August 21, 2020, 10:35:27 pm
content: Today, I measured the magnitude of the impedance (s11) and plotted in Excel, using NanoVNA V2.2. I ran two sweeps, two calibrations...100KHz-200MHz, and 200MHz-400MHz. (Originally just swept 100KHz-200MHz, forgetting that the DO-160 upper frequency is 400MHz, so I just ran a separate sweep and appended the data.)I tried to get the reference plane as close to the circuit as possible by soldering an SMA directly between DUT_line and earth. Perhaps more accurately would be to measure the impedance at the actual power port on the enclosure. However, didn't drill the holes out yet, and also the "true" impedance of the network of the circuit gets skewed by even a little inductance...as I've been told. (Thanks Jay_Diddy_B.)Anyway, the impedance seems very well behaved when measured @ -10dBm. Definitely within the limits. I am not sure how one would characterize it near its true operational power.Please see the attached plot and measurement setup. Since this was my first ever VNA measurement, I'm hoping someone can give me some feedback. If it looks okay, I'll share the files.I've also seen people measure insertion loss of LISNs...but I do not understand its relevancy, nor do I see any sort of official specification.Thanks!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2020, 12:41:28 am
content: Hi,I think you can take advantage of the fact that you have two identical LISNs.Connect them like this:The two LISNs are connected back to back. Bias current can be passed through the inductors using a bench power supply.You can measure the impedance at the mid point using a blocking capacitor.The impedance that you measure is the two LISNs in parallel. This is easy to correct for, multiply the result by two.I have not tried this, but the theory is good.I might try it on some low current LISNs which are easier to saturate the cores.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2020, 01:54:08 am
content: Hi,I tried this in the lab with a 50uH LISN. The inductor used have a inductance versus current like this:If I test the LISNs connected back-to-back, in parallel, I get this at zero bias current:If increase the bias current to 9A, I get:This should be the same as two 10uH // 50LISNs connected in parallel.This is because the inductance of each inductor has changed from 10uH to 2uH and there are five in series.At lower currents, less than 5A, the bias current has very little effect.Regards,Jay_Diddy_B