EMC Essentials Explained: Theory & Low-Cost Lab Measurements

In this video, we are going to have a look at EMC because everyone who design boards uh knows that these boards, these systems, they need to go through EMC tests. And we are going to have a look a little bit about what is behind the test. Then little bit about theory and also some practical examples how we can do some measurements in our lab with some affordable equipment. Is this what we are going to talk about Yost? — Yes, that's true, Robert. Uh and so this topic of today is for uh electronic developers who sometimes worry about EMC and um I know how that feels how to to fight uh invisible interference and therefore I would like to present one um very powerful strategy to get more um control over EMI during the development process of electronics and then we'll share four measurement techniques that you can apply in your own lab without fancy um expensive equipment and it will give you a lot of insight in how your product is uh is operating in terms of EMC um without going to uh to the expensive labs.

— Mhm. So you know how to do this because you this is your job. — That's correct. Yes. I'm an EMC consultant and I would say that almost 50% of my uh my activities are in troubleshooting EMI issues. So that means that uh clients call me if they fail an official EMC test. So for example, radiation cross limits and that means that the product doesn't pass the test and the product is not allowed to uh to be sold. Um or um the product interferes with itself. So think about really sensitive equipment where uh sensor trips when an activator kicks in, stuff like that. Um so those are really the troubleshooting activities and uh the other part are more on the preventive side where I try to prevent EMI from uh from occurring. So the main goal is there to reduce EMI um yeah in the development phase by reviewing PCB layouts and doing a risk assessment. — Okay. So now everyone knows that you are the expert for this topic. So let's start from beginning. Uh I think uh when we go at the beginning of your presentation is a little bit about this EMC testing itself. Correct. — Correct. Yes.

— Okay. — So this this session is not about uh all standards and all tests because EMC is a niche but it's still a very wide topic. Um but if you um zoom out a bit and you look at test, it's also some always something to do with a with pa failing or passing a test. So in this case, this example is an emission test. You have your device, there's something switching that creates a spectrum and you have your system that creates off cables, the PCB housing that uh that has some property self resonances and um if you are unlucky um the emission caused by that resonance um yeah will uh will make the test fail. — Mhm. I would like to just uh go maybe a little bit deeper because someone who never went through this EMC testing uh they may think like okay it's not my problem I'm not going to have this uh so from your experience very quickly what are usually the problems cables power supplies or you know just to give them idea like you have power supply you may have problems with DMC Um yes yes or now we're talking about um uh risk identification. So what are the most common causes of um of high emissions and of course it always starts with the source and um in most of the cases that's electronics. So electronics is switching otherwise you don't have any signals and um the the highest risks indeed are uh power electronics because um they're switching really rapidly to make the switch mode power electronics as efficient as possible. You want to switch uh really fast. Um, and the downside of that is that if you then look into the signal spectrum of the square wave that is produced by the MOSFET and the harmonics of that switching frequency, they go up to well tens even hundreds of of the harmonic number. So if you have a switching frequency of let's say one MHz, won't be don't be surprised if your system fails at 100 or 200 MHz because there's a harmonic of that one MHz square wave. — I went a little bit sideways because I wanted to point out I remember there are always problems for example like HDMI cables like — but HDMI that's another uh Another example of a fast switching signal. So I would say that the faster the signal switch switches, the higher the spectral density will be because you just have simply harmonics that's that spread out even more to higher frequencies and in combination with um resonances on your boards on your cables on how you mount your PCB in a housing. So that's the big square in the on the sheet. And — mhm — that combination of a source and the resonating properties of your whole system that defines the amount of emission that comes out of your product. — Yeah. Okay. So what do we have on the other side? Other slide number three. — Um yes. So that's um when we talk about EMC, EMC stands for electromagnetic compatibility and sometimes people forget what compatibility means because it says something about um yeah compatib compatible with what and I think it's good to understand that it's not only about um standards and regulations and so if you want to sell your product you uh depending on the product type and where you going to sell it um there are laws um and that point to the standards and ways that you test and that points to test limits. But there is also something um and I call that self um self-compatibility and that was what the example I just gave you where a system can interfere with itself um meaning that the performance of the system is just uh yeah not what it should be to be uh commercial viable. So your product still can pass the certification but you may have some random behavior on your board. — Exactly. Yes. — Mhm. And I would like to point out you actually have in this picture like conducted emissions and radiated emissions. Uh because some people they don't know there are actually multiple kind of tests which are like kind of under this umbrella. Correct. — Mhm. Yes. So um em emission basically means that energy is leaving your system. Unwanted system and there are multiple ways that can happen. So one is radiated via radio waves and the other one is that it uses uh the cables to uh to to leave your system. And it's not entirely true because um a lot of the radiated emission is caused by um current on the cable. Um but when we talk about radiated and conducted emission, we actually mean the way we are measuring the emission. So um if we talk about radiated emission, it means that we use a radio antenna to um to measure the field levels. And we were when we were talking about conducted emission, we're using um a listen line impedance network um and then we plug that into the cable and we attach a receiver to that. So then we actually measure the currents in those cables. So this is a question to you uh Robert. I was wondering if you ever worried about EMC. — Of course I return sad from EMC test. you return sets. So what will happen? — Yeah, there were some times when something was failing and you know we just had to go back and then try to find out how to fix it. — Yeah. And um what was the consequence of failing the test? Was it a big thing or — uh the two times what I remember the best are one was uh actually it was not DMC it was ESD test and the one of the chips was always uh burned uh after ESD testing uh so we had to it we didn't have to redesign the board we just had to change components Fight bit for zero ohm resistor and it fixed the problem. The other one was EMC and it was when I uh split the plane ground plane under audio or audio circuit from digital. — But uh luckily my senior engineer at the time he told me like this not good either. So I just placed number of empty footprints around this split plane and I was able to fix it by fitting zero ohm resistors to connect the uh audio ground plane with the digital ground plane and it fixed the problem. So no redesign luckily but yeah time wasted. — Time wasted. Yeah. Then well I would say it is a redesign when you have to add new components and you can maybe make it work but if you want to really make a product it's I think it's a redesign right if you — component I didn't have to change the layout. Okay. And then you're lucky. And I think you're also in a really great position where you had a senior engineer with a lot of experience about EMC that could help you and review your uh your design and point out what to do.

And so then I think uh this process uh will look familiar uh to you where I try to sketch um from a high level how electronic development process uh works from an EMC perspective and um so it all starts with a with a PCB design. It will be produced. it will be integrated into the system and then there's always a step like internal verification and is it functionally okay and all kind of other specifications you have to that product and when it all passes you'll also still have the uh the external compliance test um like the big EMC uh test we were talking about and if you pass those then um uh you can put your product to market and uh earn some Okay. But if you don't um you go back into the loop and you have to go back to the uh to the beginning because apparently you have to make a design change and this loop is uh is really uh large but it's also really slow because um in hardware yes you have to reproduce your board. you know as of course you know it takes some times to do the design changes but also the production you have to get all the components there all the logistics involved you have to integrate it again and all the tests um if you compare this to software for example um yes they also need to uh to change the design of software although you can debate if AI can do that but if you want to make good software it still takes time um building it takes seconds maybe a minute and integrating it and internal testing it takes also minutes and then you can I would say you can complete a loop like that in software easily within a day and hardware it's not the case so — no — in hardware if you are very optimistic PCB one or two weeks and then for assembly and it takes one month just to get your part again. — Exactly. And then you're only talking about the PCB itself because if it turns out that you need a larger filter or a different way to integrate your boards with the mechanics, it also heavily affects the let's say the housing design or the mechanic aspects of your product. So the impact of hardware changes at the end of the phase they are huge. Um so um this is a very uh large loop and uh if you ask a project lead like what's EMC about? Yeah, it's all about uh avoiding loops, right? Uh but in the end I think it's uh if you take look at it from a pro process perspective about adding more loops. So, uh, one the first thing we could do is add, uh, a check in there. Just having your PCB reviewed by a senior or more experienced person who can just view. — That's what happened to me. — Yeah, exactly. You can look at your design and you get feedback that's always directly applicable to uh, yeah to your level of uh of engineering. Um, and if it It's it's by far the fastest loop and it's all would say it's all way also the most effective way of uh of learning. It's by finding someone who can guide you and help you there and don't wait till the end. But yeah, even then if you integrate it we can still have a fill at the end. So let's see if there are other things we can do. Um and luckily there are um we can find ways to test as soon as we can. So when the PCB production is completed and you have a PCB, you can already do some tests. Um when you integrate it in your system, you can already do some tests. And when you um buy offthe-shelf modules like power supplies or displays or radio modules or well uh printer modules, all kind of off-the-shelf modules, as soon as you decide or you think about integrating those in your system, you can buy them and when they're on your desk, you can test them. And um if if you if you find out that uh that the MC performance is not that good, you still have time to uh to replace them and the impact will be significantly smaller. And I know for sure that there are people watching now that have hardware on their table that they haven't tested and they are planning to test that in the end. And I would say this is the moment to uh to perform some tests and to see um how your system is performing and if you can uh make some modification or at least be aware um that there might be some issues and that also triggers you to already think about what you can do if you will fail in the end and help you prepare to uh to mitigate those issues. So um again uh it's all about uh the test and if we take about if you take a look in the emission test it's all about the amount of field that is um that your system produces. um in the actual uh formal EMC labs that's tested with an antenna and um some people think you really need that uh that fence equipment to do meaningful tests and um we're now going to sh I'm now going to share four ways that you can do uh on your own uh on your in your own lab um without buying very expensive equipment I'm curious. — Yes. So, um, when we focus on radiated emission, um, the antenna that picks up the fields, we call that a far field.

And if you ever heard about far and near fields, near field and far fields, it's actually refers to the distance you are from the source. Um, and not in a measure of meters, but wavelength. And the way I always um to explain that is by making the analogy with water. So if you have a boat on the water or if you're in the swimming pool and you dip your toe in the water and you wiggling it wiggle it around, you get all kind of splashes around your toe. Um but if you zoom out a bit, you also see some waves occur and they eventually will reach the other side of the pool. and those waves that's the far field and the the splashing around that you could say that is the near field and the same happens with uh electromagnetic fields. If you have a fast switching node in your switch mode power supply or a loop that generates magnetic field, it creates a lot of near fields and but it also creates some far fields. And if the structure the conductive structure of your whole system acts as an antenna, um it will actually also produce the far fields that will um will travel outward and will later be picked up by the uh by the EMC antenna. Mhm. So we would like to clarify. So in these big uh measurements they measure the far fields and uh what is easy to measure in lab are these near fields but there is uh correlation. So we can uh we don't really based on the near field measurements we can actually get the idea how good the board or system what we are designing is. you're you're getting ahead, but you're absolutely right. So, let's first understand a little bit on how the far fields are produced. So, — okay. — Um let's turn it around. Um what would we do if we want to um make the most emission as we can because if we understand how we can do that then we also prevent it.

also understand how we can prevent it. — Mh. So um let's build an antenna and one of the most effective antennas is a dipole antenna and a dipole antenna consists of two conductors. So two wires and a voltage source and if that voltage source is zero volts then all electrons on the on both uh on both cables both conductors are evenly distributed. However, if we apply a voltage, so positive voltage, then electrons are repelled on one side and they are attracted on the other side. So we so we can basically steer where the electron density uh is how the electron density on the conductors behaves. If we do it the other way around, um the side that first repels now attracts the electrons and the other way around. So by um putting an alternating current over there, we're just basically uh moving up and down back and forth the electrons and that leads to a current distribution and a voltage distribution on the wires and um I think it's good to to understand that um moving electrons that is current. we are by by applying a voltage at the uh in the center of the antenna we are able to create a current in those wires although they're not connecting to something else. So if you would make a schematic drawing out of this uh this antenna you would say that the loop is open um current cannot flow but that's not the case because we are pulling uh we are moving the electrons just by changing the electric field and that results in a current in the in both wires of the uh of the dipole antenna although they the end is not connected. Um the electric field in combination with the currents through those conductors um generates both an electric and a magnetic field and they um they create a structure where they propagate. So a changing E field creates a changing H field and create a changing E field and that goes on and on and that causes an outward pointing uh vector meaning that the wave flows out of the center of the dipon and this is exactly how radio transmitters work. Um but the only difference that those are uh intentional and uh the the thing is that sometimes we by accident make structures like this and then our product starts to radiate and produces far fields. — Okay. — So um the correlation between the far and the near fields. So first the far field property. So um the far field strength is significantly lower than the near field. So that's property one. Um when we talk about far field far fields those are the propagating radio waves the concept where um the the wave propagation occurs as we just explained that's uh valid um in an open area. So let's say vacuum or at least there are no nearby conductors and this is not the case for near fields because near fields are always very close to the conductors. um E and H field in a far field they have a fixed ratio and they're always in phase and the properties of the medium where they are traveling those uh define the uh the impedance of that wave. So um yeah vacuum or air that's about 3077 ohm. And now the coupling to uh to the the near fields again. So the thing is that if you have um a near field, it doesn't necessarily mean that you also have an H field. Uh sorry, if you have a near field, it doesn't necessarily mean that you also have a far field. So um any electronics you will pick, if you are going to probe that with your new probes, you will always measure something. — Mhm. But it does doesn't have to be an issue as long as it doesn't reach doesn't reach a structure that acts in an antenna which convertes into a far field. The other way around um is true that if you have a far field, you will always also have a near field. That's a near field that's exciting the the antenna. And that's what makes the uh the near field uh probes so uh so useful when you are debugging an existing uh emission problem where you already know like okay we are failing at 12 MHz you know where to look for you can use a nearfield probe to uh to find that source. — Mhm. — Then there's one um other thing I would like to uh to point out. So I always used to think that the near field is produced really close to the source and then a certain moment it converges into the far field. But that's actually not the case. The if there's a far field this is all also it's also present near the uh near the source. However, we're not really measuring it because the near fields are significantly uh stronger than the far fields. So the farfield is actually also also present really near the source. — Mhm. — Okay. So let's go to the next slide because um you never know um you only know about if you are suffering from EMI if you measure it. If you don't measure it, you have no clue about uh if you if your product is producing any EMI.

So the four uh measurement methods we already touched a little bit upon uh near probes. And I have a set sets right here. And um they come in in different sizes and I have a professional set over here. You can also see it on the picture. Um I used to also ma make them myself. It's really easy to make a loop or a piece of copper and you have a have an E antenna. Um however there there are some downsides and that is that um if you buy professional probes they are covered in isolating material. — Mhm. And I once shorted uh a 400 volt system while probing which caused permanent damage to the device under test. And that's um yeah that event uh changed the way I think about nefield probes and isolation. So nowadays I use the proper isolated ones. Mhm. — You can of course also make them yourself and but really focus on the isolation especially if you're probing circuits that uh that have higher voltages but even low voltages if you if you short them you can just easily damage your uh your equipment. So be careful — and I think a little bit later we will explain why we have different shapes and also how we use them. Correct. So if someone is wondering like now like oh why there are these different kind of shapes we will talk about this. — Yes exactly. So the first point I wanted to make is to make sure that you have the right isolation because you see a picture of me probing a 400 volt system. Um was in a big cabinet with live electronics behind it finding uh the source of emission and well uh it's good to have some isolation when you do that. So if you consider uh a probe set also consider how you're going to use it and um yeah uh if you if you design your own take that into account. That's uh and another thing is that if you just make a loop on uh on a coax cable and you have a probe and it's not shielded. So that's an argument that also people make like yeah make your own probe but it's not shielded you better buy an official one because those are shielded but when you think about it um the shield effect and there's a good reason why you need it but it's not what you think because people explain it as um you have a shield because otherwise you will pick up unwanted signals but usually use those probes to find the sources. So, um if you have a shield or not doesn't really matter. You want to know where the uh where the source is unless you really use it to do some absolute measurements, but that's not where we're talking about now for troubleshooting. The shielding effect of the shield is not that useful. Um, but it is really useful to protect your measurement equipment because if you do have a breakdown and you do touch the live electronics, um, I prefer to have that breakdown in the uh, protective earth shield and not uh, straight into the front end of my spectrum analyzer. — And especially for the evil probes, which is basically just a floating piece of copper. Um, that's really something to uh, yeah, to think about. And there the shield basically acts as a uh as a lightning distractor uh where the energy goes into the frame of your measurement equipment. I understand from — Yeah.

— Um yeah, then the big question of course is what are we going to choose the Hfield or the E-Ifield probe? What's your preference? When would you pick an Hfield probe and E-L probe? depends where you are measuring around inductors, around capacitors. I don't know. — Mhm. I'm not expert for EMC. I don't know. I only learn from making these videos. Okay. Let's uh let's find out with an experiment. So, we're just going to try. Okay. So, — yeah. Okay. — Can you can switch to the to another camera. — Yes. So, here we have our device on our test. This is a piece of electronics with some switch mode power supplies on there. And I have a small uh spectrum analyzer attached to uh my laptop with a USB cable. And you'll see the uh the spectrum on the screen. Um you want to start with a with an H or with an E field probe, Robert, it's a switch mode power supply. Which one uh would you start with? — With the magnetic one because there will be inductors. I don't know. Okay. So we start with the magnetic one. We can now search the area around the power supplies and you'll see all harmonics pick up but it's not really clear yet which one it is. I do have an ID but picks up quite some uh some some harmonics — when you rotate the probe. I'm curious. — It changes a bit. It all depends on uh how the leakage field of the inductors is positioned. — Mhm. And you are using this big one because uh you can cover a bigger area. Yeah. — Um I always start with a big one indeed. Um and that's because you can easily find the the spot. It just uh the larger the area the larger your signal will be because more magnetic flux will penetrate this loop. But the downside is that it picks up a lot of uh flux. So the spatial resolution will be really low. If I change it to a low a smaller loop this loop is so small you don't even see it. It's just a dip in the — in the probe. And this enables us to to really pinpoint the inductor. So have three inductors here. This one — is uh showing this spectrum, you really have to put it on top. — Mhm. — This one is showing another spectrum. So if you have your radiated emission test results and you know what frequency you're looking for um using this test you can identify what's causing the uh main emission — and if you place the probe on something else or some other components everything disappeared. Yeah. — Yeah. So this is also an inductor. This one doesn't show anything. — Mhm. But if we use an e- field probe, put it on the magnetic component, we will also see harmonics. — Mhm. — And that's of course because one of one sides of the uh switch mode power supply inductor is switching really fast. So we have a an alternating uh electric field over here. And what I just described about wave propagation where changing E fields also uh generate an H field and changing H fields also generate an E field. That's also happening here. So um the idea that you can have only an changing a field without a changing E field is just a myth that's not practical at all. So in reality you always have H and U and E combined. — So it's not important which probe you use the first one. — I think it's highly overrated. Yes. So if you go to the next sheet um people always uh show this graph

which is a graph that describes um the uh the the location where the near field converges into a far field and as you will see that it's highly dependent on uh on the wavelength. So of the frequency of your signal. So for example the if you look at 30 meghertz you'll see that the wavelength is around 10 mters and then the uh boundary between the near and the far field is around 1. 6 m. And so we say this is this just a definition. We say distances below 1. 6 m. we say that we are in the near field area and above we're in the far field area. What you see in the graph is the impedance uh graph. So what you'll see is that for different frequencies. So let's say uh 1 ghahz it's a green graph. — Mhm. Um you'll see that it converges to the 300 uh 377 337 oh forget it about 350 ohm something like that the CD wave impedance in free air um and this is the distance so at certain moment it converges and the for really high frequencies that happens in close proximity in terms of meters And for the lower frequencies that happens uh well far uh far way further away. — And what does it mean? What does it mean once it has the impedance of the air? — Yeah. Well, this is this is a theoretical um uh model where you would say well if we have an E field source um we say that it has an high impedance um so the impedance will be high and if the impedance is low of your source it will mainly generate H age Hfield but it's — this is a graph that many people just um show you when you ask them should I use an H or an E field probe Um but honestly um it's it's a theoretical story. Um in reality you don't have a purely Hfield probe Hfield source or an E-field probe. So um it's nice for simulations but even in simulations if you take parasitics into account you cannot make an Hfield probe an Hfield source that doesn't generate an E- field. So — so don't look at it too much. it's uh it doesn't really provide any uh any useful information I would say and I think the most pragmatic way to look at it is that um in the sense that if you're looking for the emission source if you don't know what the source is you don't know if it's an E or an Hfield source so that if it's high or low impedance you do you do not know because you don't know what the source is right so uh you cannot use that as an input to select between H and and E. — Mhm. — And as I just showed you, it's it doesn't really matter. There are yes, there might be some differences in signal amplitude, but um that's not really the point of uh of probing. You just want to localize the source here. So, — so what is important from this slide is the table. That's what is more important because the table tell us the uh length of the wave. So uh the signals with very long wave we can't really measure them in the chamber because the chamber has uh size limits. — Yes, that's the explanation why there's a difference between conducted and radiated emissions. And the other thing um to take away from this sheet is that this graph which is often used um as inputs to select the right probe is actually quite useless. Okay. Um where are we? — 24. — Yes. So um really short introduction uh

explanation on how the Hfield uh probe works. So Hfield probe basically a loop and when magnetic flux passes through it generates a voltage and you'll re you will see that in your spectrum analyzer on your or on your oscilloscope. Um and that the H field is typically uh produced by either current flowing through a cable. So you can also put it on cables. If you have excessive common mode current on your cable and you put those probes nearby, you can also pick that up. So you can also use it for that um or to capture the u displacement currents. — Mhm. That's why it's easy to pick up something from inductors because around the inductors there are these fields and we just put our probe inside of the fields and we can measure them. — Exactly. So every inductor so if we're talking about switch mode power supply inductors every inductor has some field leakage some magnetic field that leaks out. And some of them have a small amount of magnetic field leakage. You can pick that up. And other ones you have a tremendous amount of uh of field coming out. So you have that's the difference between the shielded and the non-shielded inductors. And a new nefield probe test like that. It will it's like day and night. You can easily just be 30 dB of difference. Uh wouldn't be surprised. It's just use shielded inductors. That's the tip of the day. Yes. Yeah. Okay. Yeah. So, we already uh did some experiments and we showed you how that worked in practice by probing a device. We already — But you can show also what is on the slide 25. Yeah. — Yeah. It's not really new. It's just the experiment we did. — Yeah. Some probing it the probing electronics. Um Yeah. Yeah, what we already discussed during the demo, the larger the loop, the more fields it picks up, but the lower the resolution. — So, we always start with a larger loop and then narrow it down.

— Mhm. — Um and then something interesting about E-Lield probes. Um and that's uh yeah, how e- field coupling works. So, um if you pick two conductors um you could say that uh you can easily represent it as a capacitor. So that would also mean that a conductor is half a capacitor and that's the way you should visualize how an E- field probe works. So imagine you have a circuit — and according to Khov um the sum of all currents into a node is zero. Um however if you have some parasitic capacitors um the current is not zero anymore unless of course you include the paric capacitor current. Um so if you have a switching node and it's moving up and down, it's changing in voltage and you um have a parasitic capacitors to the neighborhood or you use the the the electric nearfield probe some current will flow into the neighborhood into the environment or into into the probe if you if you position it right there. — Mhm. Um well as with capacitors the larger the area the more current will uh will flow in because the capacitor is uh is larger and also uh if you reduce the distance the capacitors will also increase and that will make uh that that will result in the actual current that will flow into your capacitor. So the it's just basic electronics the current through your capacitor is the capacitor value times the change in voltage again. So changing voltage if you have a parasitic capacitors current will be injected into that other system. So if you have a circuit and one node is switching if you have some kind of active electronics you always have some switching nodes. If you have a parasitic capacitance to something else, it will all there will always be a current injected to into that parasitic in parasitic capacitors and the smaller uh the distance the larger the capacitance and the larger the surface the larger the capacitors will be. So this is also one of the reasons that if you have um a switching node in your power supply which is switching really really fast because there's a MOSFET switching really fast and you want to make this surface as small as possible because otherwise you just have more um energy flowing out through the parasitic capacitances your system. um a summary of um the yeah the coupling properties of the H and the Hfield H and the E- field probes. So the magnet the signal that you pick up with the magnetic probe um is determined by the change of the flux that penetrates the loop for the E field probe. the electric field. The speed of the change of the electric field and for both the area of the probe matters. The larger the area, the stronger the signal and also the distance is uh is really important. So if you put it closer, you will pick up more signal. And for the Hfield probe, there's another property and that's that there's some directiveness in uh in the in in the loop. So — the the field lines have really have to really penetrate the loop because if you uh place the if you position the loop in parallel to the field lines, you won't pick up anything. So you can play around with that. Okay. Um a practical uh me and mental exercise. So imagine you have a voltage source. So you have your PCB and you have a switch node for example. Um that of course will generate an E field. So the E drew some E field lines here. And now the question is um the solution you oh sorry already saw the solution. The question is how will the E field lines uh go? So this is the source and this is your e- field probe which is basically a coax coaxial cable where you created a small surface on the on the center core and you hold it in the uh proximity of your switch node. How will the current flow? You have any idea? So what happens is that it the signal that travels through your coax — through the shield and then from the shield. — Exactly. Yeah. So there will also be some parasitic coupling between the coaxial shield and probably also the housing of the spectrum analyzer back to the device. — Yeah, I understand — because the coax is also kind of capacitor. Yeah, exactly. So we so we using um capacitive coupling. So the displacement current is flowing here and the same principle applies in the in what some people say the return current otherwise loop is not closed. — Mhm. — Okay. Um yeah, we already discussed the smaller probes. The same applies for the E probes. — Oh, I didn't know. I never seen this kind of E field pro with the ball. — Yeah. This is a quite a large one. So, it increase the service area to pick up even more signal, but it's not really that uh convenient, I would say. This is the size of a gold ball. So, it's Yeah, I do like the isolating properties of this probe, but — it's not really convenient to pinpoint something. — So, it has to be like ball or that's how they do it. it, I think, to make it more um how to to say that um less dependent on the direction because you — it doesn't really matter where the field is coming from, you will always pick it up if you uh if you construct it in a spherical shape. If you make a small uh square like like this one, then of course you have the best coupling if you place it like a capacitor really close in parallel to another trace. — Mhm. — And if you have a ball or spherical shape, then you can also pick up fields from the side more effectively. That's the — that's the idea. — Um sometimes um it's not enough. So the signal that you will pick up here um will be determined by the coupling that you achieve and sometimes that coupling is just really small and then you cannot pick up the signal especially if the frequencies are a bit lower then it's quite hard to uh to catch that with an electric uh field probe like this. And in that case, um, a solution would be just to put a capacitor in between. — I need to know. — So then it's not wireless anymore. Just yeah, increase the capacitors by adding a capacitor. That's the contact probe. And you can use that to uh to create a better coupling with your circuit to the to your spectrum analyzer. Mhm. — However, be careful because if you uh if you're probing a really strong signal, it also means that a lot of current is going into your spectrum analyzer and you can easily damage it. So, only apply this to really sensitive or really small signals. If that's not sufficient, you can also buy systems like this which is also an active probe uh sorry also a contact probe where you have a capacitor in as a component in near the tip and this one also includes an amplifier. — Mhm. — Okay. Um

then um there's another uh idea to uh to increase coupling and that's by um adding some um uh some ferret core material inside the flux probe sorry inside the loop and um one of my mentors Lex he brought this under my attention and it's a really nifty idea to uh to increase the coupling. And I have one created one myself because you cannot buy them. They're extremely useful, but Can show it here on the camera. It's it's really really small. It's a small core with a few windings — and you can use that to uh to pinpoint in even more detail. You can even uh put it on a trace and see uh local localize which trace is uh is causing the it's causing the image. — If you want to uh have a fun do it yourself uh experiment recommend building one yourself. — But where you get this kind of uh you know fite or what is it? You can you can uh well the first decision you have to make is what kind of size are you looking for? Um and I think this one I R word has some really small ones but you could also um pick a magnetic uh coupler. So the magnetics of a of an Ethernet uh connection. — They have small ones. — They have really small ones inside. If you break them apart and you just uh just break them, then if you break 10, a few of them will break right in the middle and you pick one and then uh do some just a few turns is enough and then uh you can make your own probe. — Mhm.

Um there's another really powerful um thing I would like to share and that is if that there's a there that there is a relation between the amount of field the final radiated emission antenna will pick up and the current that flows in the antenna. So if you have an ideal dipole antenna, um, some current will flow and that will result in a in a signal level at the during the EMC radiated emission test. And it turns out that you only need a few micro ampair. So in the extreme case, you only need three and a half micro ampair in your antenna to cross the class B limit line — to fail. during a radiated emission test. Indeed. Yes. So, this um shows us two things. One, you only need a very small amount of RF current on your system to fill a radiated emission test. And two, um this means that we can measure current in antenna structures to get an idea about how our radiated emission test will uh will perform. It's not a bulletproof uh way because there are always some exceptions but it's it provides a tremendous amount of value. — Mhm. Oh, that's interesting. — Yes. So what we basically do here so we started with a near field probe. We added the coupling by putting a let's say half a ferite core uh in the loop. And now we close that core by uh yeah making a full uh a full circle of a of ferite material. So um — so this is how we will measure the current through the cables and — exactly this is we call this a common

mode probe of an RF current probe and what it basically does is it's it creates a really good coupling between um the current that is flowing through this wire and um the windings that are attached to our measurement equipment. So the near field probe um and the flux probe um they have a poorly defined coupling because if you change the distance a bit your signal strength will change and with a common mode probe over here the coupling will remain the same because all flux that's gener majority of the flux generated by the current that flows through this cable um will be picked So this is a more precise way of measuring current in a cable — and uh that makes it really effective to make a statement about the amount of radiated emission we can expect. — Yeah. But you can't really use this on PCB boards. That's — not on the components itself. But um the majority of boards uh have cables. — Yeah, cables. Uh you can use it on cables. Yeah, — exactly. So in most of the radiated emission issues I encounter um the emission is caused by cables. — Mhm. — And again this has to do with wavelength. So uh the the best antenna is a fourth of 14 a quarter length indeed of the wavelength. So if you look into uh a typical cables like a USB cable what it's like 75 cm something like that um that the the peaks of emission due to cables they're usually around 100 megahertz something like that between we would say between 40 and and 100 mehz I see the the most radiated emission failures and those are usually the larger structures like cables Um then the other failures that appear up to well let's say in the 400 MHz between four and 700 MHz those are usually switching harmonics of uh of clocks and data signals stuff like that. They most of the time also come out via the cables because even long cables are actually quite good antennas for uh for those higher frequencies. But there are indeed also um possibilities where small details on your PCB can act as a high frequency antenna. And you're absolutely right. You won't capture those with uh with these probes. Mhm. Um I would like to show you uh a very quick demonstration about the how powerful this is. — So I'm now um switching um the near field probe and I'm connecting the common mode probe and I clamp over the cable. Now we measure the common mode current. And if you know the transfer impedance of this probe, you can calculate the amount of current that is flowing. — And so this will tell you if you're in the neighborhood of the of the limit line of the emission test. And a practical example where you can use this is of course first to test if your own device is um generating large amount of common mode currents through this cable. — That will uh that will cause emission. But um you can also use it to test for example if uh a USB uh supply um is something you would like to include in your final test. — And so if you design a battery powered system and you uh nowadays you switch to USBC as uh as a way to charge it many companies don't offer the charger because everyone has a USBC charger. So um but you do have to um to bring it with you to the lab because otherwise your device cannot be tested in charging mode. So you do have to supply some kind of USBC USB source in a um to operate your uh your equipment. So this is what a power bank um that generates the power for our device under test. And this is the um uh the common mode current that you're measuring. If you now replace it with another power bank, turn it on, then you'll see that there's an enormous difference. — So this is like is this saying that the power bank is not good one? — I would say so. Yes. If you use this power bank um in your final test, you the whole setup will uh will fail on radiated emissions for sure. But it's not because device of the source, the power source. — Exact Exactly. But exactly. So um by using a probe like this and really easy tests, you can assess uh both if the modules you are integrating in your system um how they perform on uh on the mission level. You can test uh the side equipment like USB power banks or USB adapters that you use using your charging uh test and you can use it to test how your own design performs. So I would say this is — one of the most powerful and cost effective tools um available to uh to assess radiated emission — in your own lab. And also these probes you can make yourself. H actually this is one of the things uh because we have uh discord forum and uh very often people say like I'm designing this uh board power it through USBC uh and it's failing EMC that very first question is what you are using to power up your board yes it's a it's something to uh that's part of preparation for EMC test and uh this is something to be really aware of uh during emission tests. — Mhm. So it's not only your board or your system what may fail but you have to be very careful also what you are using to power up your system. — Yes. And if you if your device under test also needs other auxiliary equipment like a laptop or a radio connection or — monitor — a monitor or something or Yeah. Um you have to think ahead uh how to test that. Yes. — Mhm. Okay. So what do you have on the next slide? I think there are some parameters of the probe. — Yeah. There's So, so the transfer impedance, you use that to convert your uh the readings on your uh so — current. Yeah. — To convert the voltage you're measuring into the current. Um and you will see that um if you're going to buy those, you can buy them in all kinds of different bandwidths. And if you look closely, you'll see that this is quite a flat — transfer impedance, which is of course nice. — But in the end, you'll see that it starts to drop off. M even here it drops off a little bit more. Um and this is basically what defines the bandwidth and that's do you know what's why oh sorry uh what what's happening here any ID — don't ask me so hard questions — okay so um if you make a probe like this there's a balance you have to to make a decision between do I want to have a high transfer impedance meaning that really small currents can be transferred into a strong signal or do I want a larger bandwidth and the reason is that um if you want more signal you apply more secondary windings so here you don't have one turn but you add more turns then you just it's basically a transformer you just get more uh more signal at the uh at the measurements equipment site. — But the downside is that if you do that the inductance of course of that winding uh will limit the bandwidth but also um if you add more windings there's always some capacitors in between and that will basically bypass the really high frequencies go through the parasitic capacitors and they will never reach your measurement equipment. Oh, that's very important to know. — Yeah. So that's the reason why there are different bandwidths. Um so you can get those in the range from one to 300 MHz. One to sorry up to 300 MHz to 500 and I think also in the range of one GHz. Um I think the majority of the issues you can capture with the one of 300. And well it it goes down and you doesn't mean that it stops measuring. You can still measure a bit further but you have to account for that. — We are almost done. — Yeah. So all in all I think this is uh those current clamps are — your own your own. — I made some myself indeed. I've have one over here. The one is also on the sheet. um but you cannot open it so it's not really convenient. It's nice for test or for do some experiments with ESD guns and stuff like that. Um I also have a few with uh based on fite clamps, but there you need special tools to open it. Then I would say that a clamp like this is uh — is most practical. — Mhm. So I uh I hope this was uh was useful for everyone's watching that I gave you um at least something to think about the electronics development process and some ideas where you can create smaller loops in the design process to get information as soon as possible. This enables you to fail fast and to react on it and don't wait till the end when all other development is already completed. But uh I like to make these videos because I think it helps people to not be scared to maybe make these kind of measurements by themselves. Yeah, absolutely. um just start measuring and if you if you don't measure you never know how it performs. — Okay. So if someone has any questions uh

then they can contact you how on the last slide I think. — Yeah. So if someone has additional questions or just want to contact me um the best way is to do it via LinkedIn. I also share some other EMC related insights all with the aim to uh to pass EM EMI in uh pass EMC in one go. And if you create your own PCBs, if you design your own PCBs, you can send me your PCB and I will review it and within three days you'll get a report and you with findings that will show some EMI risks I found and ways to uh to reduce that. — Mhm. That's interesting because uh some people they actually ask me uh for this kind of uh like feedback uh but I don't do that. So it's very uh nice to hear you know if someone needs then they can send it to you and get some feedback. — Yeah, exactly. So I need a the layout file schematic is also of course nice to see what I'm looking at and uh in the report you will see some really practical um advices that you can directly give to the engineer who's making the layout to uh to improve the design and reducing the risk of EMI. — Mhm. on this website link emitbuster. com um Robert I will make a custom page where you will also find some more information about the probes and the uh experiments and the hardware I used in this uh during this session. — Okay, I will then link this uh from the video. So below the video there will be this link and everyone who would like to uh have a closer look at what we are talking about they can find there all the resources. — Awesome. Great. — Ah fantastic. Thank you so much uh Yost and uh yeah if anyone has any questions uh then leave comments and then maybe we can make another video and talk about this. — Sounds like a great plan. Thank you. — Thank you very much. That's all for this video. I hope it was helpful. If you would like to learn more about electronics and board design, check out our online courses. You will find everything important there from basic board design up to advanced board design and highspeed PCP layout. We have courses in Alium, Cadence, Keycat and also courses covering many different topics for example FPGA, EMC measurements and so on. Visit our website at fedel. com. Thank you very much for watching and don't forget to leave your comments. See you in the next video. Bye.