EEVblog #1176 - 2 Layer vs 4 Layer PCB EMC TESTED!

hi in this video we're going to take a look at a two-layer piece of B and a four layer PCB and what difference having four layers makes to EMC slash EMI or radiated emissions from the board now this is the gigatron TTL computer which you've no doubt seen in previous videos and this is the original from the designers and it's a two layer PCB there is no ground plane internally to this so all the signal traces you can see they go horizontal effectively horizontal on the bottom and more vertical on the top and that's a traditional two layer PCB layout yes they've got some ground flood fill in here and stuff like that but generally it's it's actually quite a nice neat layout get all the chips are in the same direction and it's quite a neat traditional 2 layer layout but the problem with a two layer PCB is that well as we'll get into the finer details later but it radiates like buggery so I took this exact layout and I've done a video on this old link to it down below and at the end if you haven't seen it where I took exactly the same layout and actually produce a four layer PCB it's an absolutely identical layout I've changed none of the traces whatsoever it's completely identical the only difference is you can see in there that I've actually got a huge internal ground plane so ground and 5 volts internally for there all the bypass capacitors everything else is absolutely identical and what difference is this going to make to radiated emissions which is important if you're designing a product commercial product for sale you have to meet various electromagnetic conformity requirements in there varies in different countries we will won't go into that in this video so you can see our computers operational here it's got a six megahertz main clock which is not particularly fast but remember it's not the fundamental clock rate it's in fact you can have a one Hertz clock on this and it can still radiate like burglary because it's the edge rate of the signal it's not just the fundamental frequency so we can actually use one of these H field magnetic probes to measure the near field emissions it's called and I might go into a bit of that later but we can measure the emissions from this board and we can put it under here and we can see the difference between the two layer board and the four layer board this is going to be neat now what we're going to concern ourselves with in this video is as I said this is a near field magnetic probe it's called a H field probe there's also a voltage probe which looks quite similar but it actually operates quite differently and I might have to do a separate video on like the differences between these two but this basically measures the radiated magnetic field from the PCB traces and the loop areas as we'll talk about and I've got my jig of repeatability here which just means that the probe will be in exactly the same position and then I can move the board over various points and I can look straight down through there to actually Center things and we can get a direct comparison and overlay various signals on both boards between the four layer and the two layer how much difference do you think it's going to make so we're dealing with their radiated emissions that actually come out of the board rather than conducted emissions which come out through our cables and including the braid and the shield of this this will be conducted mode radiated radiated emissions the USB over here I've got this powered by from an external battery pack by the way so to do these measurements will just remove any cables from there and we can get a direct comparison so what I'm going to do is use my largest H field probe here at largest diameter because that's the most sensitive you can't get smaller ones a bit there's no point the small ones are really useful when you're checking out differences between like individual traces and you know stuff like that and I've got that going into my preamp over here that's a 20 DB gain preamp 3 Meg to 3 gig and we'll use our rogue old DSA 8 1 5 spectrum analyzer here we're using the EMI the electromagnetic interference filter type which gives us a industry standard 120 kilohertz resolution bandwidth filter here you don't necessarily need that but if you're going to sort of get ballpark pre-compliance measurements that you know try and match what you might get in the field then you know 120 kilohertz and our frequency span here will just go to 100 megahertz at the moment I can show you a wider span that just allows us to look at some nice detail in here and our amplitude we're got units in DB micro volts that's just a bit easier than DB millivolts doesn't really matter and we've just got a input attenuation of a fixed 10 DB here because depending on where we put it put it right over chip we can actually see it saturate I can probably show you that now there we go out of range it's a start saturates oops and we've got a the input preamp on as well and I'm going to put it directly over the six megahertz crystal here and this is the response we get if you have a look at this first peak here bingo that's our six megahertz our fundamental and if we skip through 12 18 and they're all the harmonics and that will extend all the way with LBJ it will actually keep going if I actually change the span let's go to 500 megahertz there boom look at that it extends all the way right up we can go to like one gig or something like that but it starts to and drastically drop off there but you can see that it really extends all the way up because of that edge rate okay so what I've done is that frozen that yellow reference there and let me take it away and let's plug in our four layer board and right under the crystal it's exactly the same look at the difference here yes you still get all the peaks but very significantly reduced broadband noise here we're 5 r DB micro volts per division here so we're talking about you know a good 5 10 15 DB difference in sort of like the bulk noise here and the peaks there simile like 5 or 10 DB down from that so you know it's a huge reduction there and you wouldn't have thought so you thought AHA it's all just coming from the crystal it's exactly the same crystal in exactly the same location no it's the radiated emissions in this case the near-field radiated emissions coming from the traces and everything else on the board but because we've got the ground plane in there it's actually it lowers the loop area and does we'll talk about this towards the end of the video so stick around for that and it just lowers the radiated emissions it makes a drastic difference 15 DB is absolutely enormous that could be the difference between passing and failing your compliance and you know costing you you know five grand or ten grand or something like that you've got to Rhys pin your product you could easily spend tens of thousands of dollars because you've failed your compliance so you know and then you go oh well should have used a four layer board II begin with you'll see that probes actually quite a substantial distance above that board - but the heads are those radiated emissions they're Aquila let's just try another random spot here straight over the addressing mode decoder chip here this is the four layer board and that's our spectrum there you can see it was it's much lower than we've got before you can still see the fundamental and the harmonic spikes still in there but it's going to be and pick up a whole bunch of broadband noise because this is a digital computer that's you know refreshing the screen that's doing all sorts a process in the background everything's going all over the place and it's just you know generating a whole bunch of wideband noise oh by the way if you want to see what happens where we disconnect the power it of course completely vanishes now let's try the two layer board and this is the two layer board so four layer board in the yellow there and you can see that actually the four layer board actually has more prominent peaks in there because all the rest of its being kept more down in the noise by the ground plane whereas the two layer board has once again you know a good 5 10 15 DB difference in the like the average broadband noise level there of course the peaks are once again 1015 DB above the peaks on the four layer board as well that's a huge difference and let's try riding on top of the rom here just for just for kicks so that's our two layer board and there's our four layer board much much law just over the accumulator chip there that's our four layer board and there's our 2 layer board so once again you can see the broadband difference it's it's quite remarkable this is our accumulator chip again over a 250 megahertz span this time this is on the 2 layer board you'll see what happens if we physically take the board away I've got the board like a good like foot away from it now but you can still see some of the radiated emissions there even though it's not in the correct plane which we'll talk about in the minute there you go there's our four layer board once again I've changed the scale here to 10 DB per division now and once again it's a 15 DB difference it sort of gets a little bit closer up here you know around the like a two hundred hundred and fifty to two hundred megahertz range but still significantly under that could make or break your compliance for sure and the thing with these H field magnetic probes and it's not like an issue with them in fact it's a feature is that they are dependent upon the orientation they work in the plane so if you've got your coil like this it's picking up magnetic fields that are in that flat plane there so you'll notice that if we take this there's a spectrum and that that's over 250 megahertz and if we simply rotate that like that it picks up different components look at that so you can actually use that as a feature using a smaller diameter wind you can get down there and you can trace down oh you're offending our components and traces better things like that so I probably have to do a whole separate video on this but yeah it does make a difference the orientation we've seen quite a significant difference here between the four layer and the two layer board makes a heck and a difference like typically like broadband noise in this particular case about you know 15 DB or so and that's a lot but does that translate if you measure say a 15 DB difference here does it actually with your near-field probes does that actually translate to a 15 DB difference on your EMC testing when you put it through the test house and you test it against the compliance standard well the answer is unfortunately not these near-field probes both the H field magnetic field and the electric field probes all this is as I said the Near Field and whereas all of the compliance testing is done in the far field and I'll explain that in a minute because I have a Dave card so what's the point of using these near field probes if they're not sort of like quantitatively equivalent to what they do in the test house well the good thing about it is that at the design stage or maybe if you fail compliance or something or you need to or you're doing some pre compliance testing you can go around your board and sniff all around your board and with the H field and the e field probe to see if there's any issues see if they're you know anything's radiating wildly and stuff like that you can you might be able to see a big spike or something at one particular frequency you might go or we need to knock that down even though you don't even know it might be compliant at the design stage you might go well you know I'm not gonna take any chances and I'm going to knock that problem on the head now before I send it across to the test house so we'll briefly talk about near-field and far-field here and how it relates to the electromagnetic radiation now a you might have heard the term electromagnetic radiation it's electro and magnetic contains electric and magnetic components and you can look at it this is like the standard visual representation of it the electrical field might like would go up in the z-axis like this and the H field is 90 degrees from that so they actually propagate in different orientations and of course this is the wavelength and here's a cute little animation just to show you how that works as it propagates down now what we actually have to look at though is what's called the wave impedance and this is where the difference between nefi does everything on this side and far-field is everything on this side now the wave impedance in ohms like this in there for this particular scale please excuse the crew didn't have time to build up the scale a lot of pain it from 10 ohms to 10,000 here so this is where you have to define far field and near field well the electric field and the magnetic or H field there is a difference between H and B by the way B is flux density you might sometimes be here at chord B but it's actually H magnetic field as opposed to induced magnetic field as opposed to magnetic flux density anyway ain't going to the details so the H or magnetic field actually has a very low impedance source in the near field whereas the electric or a field has a very high impedance it'll clarify that in a minute but basically it all comes down to the wavelength lambda here and this is normalized to 1 here and it's lambda on 2 pi which is basically we're going to normalize to that value and of course let's take for example 100 megahertz is a wavelength of 3 meters so pi on 2 that's about 1/2 meter so when you get to 1/2 meter away from your product this is where the electric fields and the magnetic field actually start to converge it's not really clean like this there's a bit of you know overlap in here and this is like the transition there's going to be like a transition region in here where the two fields eventually combine and anything over roughly half a meter away at a hundred megahertz the electric and magnetic fields combine to give you a singular impedance which actually happens to be 377 ohms in free air so anything over the wavelength on two pi is deemed to be the far field and anything closer than physically closer than that like we just did with our probes here is the near field now this is why we have two different types of probes one is the H field probe the magnetic probe the other is the e field or electric field probe and the magnetic or H field is going to be generated by higher currents ie sources that have a very a lower in he danced so for example if you've got a lot of current flowing in a in a particular art race either due to an actual like heavy current switching or even very fast switching that's dumping a lot of energy into the bypass capacitors and the capacitance between the power planes and everything else then that's generating typically be generating a magnetic field due to the low impedance and the high current but very high impedance things that don't generate lots of current then they generate electric fields and hence the bigger source impedance so you'll generate electric fields from say just like a static power supply for example your 5 volt power supply whereas all you're switching stuff will dominate down in the H field here because there's lots of current being dumped into the trace or the load capacitance or the particular load itself when you switch in things so that's why you need to use these two different probes and the magnetic field probes they are sensitive to orientation like this and like that as well as I talked about on the plane whereas the electric field is not sensitive you can just put that in any orientation and it's not going to make a difference so if I use my ear field probe like this and let's say I probe this power trace over here like this you can see it's really not going to make any difference in the orientation that I put that in it's just completely insensitive to that because there's no magnetic field coupling it's electric field coupling and it's just purely the distance but if you take a magnetic loop probe like this and I just change the orientation like that Wow that makes a big difference it really brings out the peaks if I put it vertically like that if I put it horizontal it gets more of the current flowing through the trace and if we use our smallest H field probe let's just have a look at let's say this like blank area over here this is our four layer board like this or maybe right over on the edge of the corner of the board over here like this and let's compare that with our two layer board here bingo look at that because we've actually got a power trace actually running right around this corner as well which we actually physically reemerge and you can actually see that the power trace actually running all the way around there like that so that's just going to radiate like buggery but even if we go over just the ground plane there you can see it's much much higher than we get with the four layer board and this is why at the emc test house they'll test in the far field here because it binds the electric and magnetic fields together and basically the typical testing distances would be like one meter three meters five meters 10 meters for example away it depends on the type of product they're testing into which are standard they're actually testing too but say if you put it 10 meters away then you can have a larger rotating turntable so that your product rotates around like this on the turntable and they can measure all the axes like this when they while they have they're super expensive you know buy a conical super calibrated measurement antenna ten meters away measuring over say 30 megahertz to ten gigahertz far-field for example might be a typical measurement range and then there'll be a standard like envelopes that you have to get under and also peaks and things like that and it gets you know the standard gets are quite complicated but yeah just the near-field testing that we do here it doesn't really translate to the far field but you can certainly I get an indication of whether or not you've got any nasties on your board so why does this happen why does a four layer board make a huge difference compared to a two layer board when it's an identical layout all the traces are exactly the same length all the chips are in the same location it's got just the same number of bypass capacitors everything's hunky-dory they should be identical right well it all comes down to loop area which you've heard me talk about in many videos before and a huge general rule of thumb when you lay an hour Gords is not only to keep your traces as short as possible but to keep that what's called the loop area as small and tight as possible so the tighter your layout and the tighter your loop area the less problems you're going to have with EMI in generation and EMI and susceptibility to electromagnetic interference as well so you have a source and you have a destination on your PCB a trace going from one side deep from the source to the load now the loop area is actually the total area that includes the ground for that entire loop but it also includes the power system with the bypass capacitors and I've done a whole video on that actually showing the return path for currents and why you need bypass capacitors I'll link that one in it's really fascinating so that it has to do with the entire loop area and the bypassing and on two layer board like this one here the original one you just don't have the luxury of having or often don't have the luxury of having a very tight loop area for all of your signals you might buy either accident or good design have them for certain traces but when you've got you know what two dozen chips spread over on this on a large board like this you just can't possibly have every one of the traces having a short loop area so something's going to radiate somewhere in fact probably the majority of them I just have large loop areas and hence are generating a larger magnetic field in this case a bit electric field as well electromagnetic field will say generating a larger electromagnetic field and hence why we see the huge increase in radiative emissions so this is the original two layer board and what will go in here is will go and just inspect a single signal let's say the address 0 pin to the RAM chip that's it because it's only got 2 connections on there now we can actually go in in there and have a look at this this is the RAM chip and this is the driving chip here for that address decoder now look it's got a nice short trace there look at that that's really neat isn't it and in this particular case the under the driver chippers here and the ground of the RAM is over here now this is actually a reasonably short path for a look it has the snake it's got to go all the way around here that's a reasonably short path for a two-layer complex to lay a board like this but if you turn the top on there might even be a shorter path not yet like it might jump back over a veer here yes it does look at that that's handy they just happen to put in a veer here and a veer here so the path is actually shorter it goes through here like this and goes back over to here and that is kind of the shortest path otherwise it's got to go all the way over here on the bottom and all the way around on that green layer there but still that's relatively short so you might think that's not too much of a problem but aha what about the bypassing when your signal transitions like this the capacitive load and the capacity of traces and everything else actually a capacitor when you apply voltage to it it first when you transition up like that it appears as a short-circuit and that generates a little gulp of current and that current should come from the bypass capacitor so let's have a look at the bypass capacitor for these two devices look at this here's the VCC pin of the RAM chip the bypass capacitor goes down to this ground here and look the the two chips are actually sharing a very short power path so that's almost ideal so if you just look at that from a point of view of like the loop area from a point of view of just the power pins everything's reasonably hunky-dory for this particular trace remember we've got hundreds of these traces on the board each with their own loop area because they're all switching and they all intermix that's why you get all that huge broadband noise measured across the spectrum now but the problem is the ground for this here's where we might come a gutter he goes up here oops where else does it go look at this it's got to go all the way over here all the way over here or all the way over here if it's just on this layer then it's just snakes its way back through there and it comes back through the trace that's a huge Lou period now of course if we turn on the top layer we might be lucky and we might get some vir stitching in here let's have a look actually this one's not too shabby the ground from here goes on the bottom layer the green layer over to here then jumps up onto the top layer the red goes through this via and there's another one up here as well goes down here and then can drop down to the bottom layer like this and then go to the pin like that but you know it's it's a bit higgledy-piggledy it's not ideal there's only one they might only be one lousy vir in there which is going to be higher impedance at higher frequencies it's got more inductance right so that is the problem the inductance of your veers and all the and your inductance of your ground paths and your loops and all that sort of stuff comes into play so you know we're still kind of within this area but it's going to be much higher inductance and it's just running all over the shop and if you were the person to be layout person you've forgotten if you didn't put these and like via stitching in here like this then Wow it'd be like traveling all over the board and the loop area would be larger and larger and generating a greater electromagnetic field for a given particular switching current if we go over to the four layer board we have a look at exactly the same signal like this you'll notice that well there's no more flood-fill ground planes on the top and bottom because the whole board is one ground plane so now the trace is exactly the same but this ground pin can go all the way over to here this ground pin not only it's the same length path but it's lower inductance because it's a big solid ground plane going right over and then that's solid ground plane is also connected over that entire area right up to this bypass capacitor here and likewise for the power because we're going to big power plane on there so the loop area is a little bit smaller in here but it's much lower inductance so it's going to be much more effective and also you've got the shielding provided by the ground and the all planes which makes a difference but magnetic electric fields we won't go into the details but yeah it makes a big difference and that loop area if you don't keep that small Faraday's law is going to screw you over and the greater the area just like the reason why you have two different sized magnetic h field probes that they've both got one loop in there there's just one loop that's it but this one is a larger diameter so it's more sensitive it works in the reverse if you've got a larger loop on your PCB it generates more and it receives more if it's bigger and that's just a singular example of one particular trace remember you've got hundreds of different traces each with their own loop area and let's have a quick look at the power system on once again we're back on the two layer board so this is actually the VCC or power pin you'll notice how it's just running loop it's got a run right around here like this so imagine if you had a source up here and a destination down here and it's got to go all the way back through that power and the local decoupling you know it wasn't right and thus ground planes was split and that's the problem split ground planes are always a killer because they're instantly going to generate more loop area and then if you have tracers running across splits that's really bad for AMC and there's all sorts of stuff like that so you know look it's just it's just a problem and it's nothing wrong with this layout this is a good to good layout to layer board they put in via stitching in there to try and sort of you know shorten the grounds everywhere when you're flood fill a two layer border like this yeah you should just pepper everything with this sometimes you can screw it up and it might cause a loop where you didn't want it to go or something like that but in in general yeah the more you stitch those planes together the better you know the luckier you're gonna get and you're going to avoid Murphy and you're going to hopefully you know not be too bad but there's absolutely no competition compared to a solid ground and solid power plane across a board now if you had a three-layer board you would do a three-layer board because they manufacture them into in even numbers but let's say you did and you only had the ground then you'd still have the power like this and you might still have at lower frequencies for example because the bypass capacitors work at different frequencies and they have different responses done videos on that so the lower frequency current has to flow in much bigger loops then the higher frequency stuff does whereas if you have that nice solid ground and power plane then the loop areas are going to be much much smaller check this out I found one that's worse this one has a source in two destinations it's the y0 pin i don't know what it does it's a TTL computer so it connects this chip here this one and this one here and the ground is actually pretty good if you look at this see between this chip here goes over to this one here right so it's up and there so right so that's a relatively short ground path but let's look at the bypass capacitors which as I said is where all that high frequency stuff comes from the high frequency bypass and that's why you have the bypass capacitors look this one goes up and goes up to here and then it's connected to this bit of ground up here and that bit of ground is like split on this half it's split by this power rail running right through the guts of this so unfortunately that's like it's going to have a hard time getting backs that a loop area is going to be much larger for this actually trying to get back between these particular chips and then the bypass capacitor for this chip here is over on this particular ground which is not via stitched over to this one which has to go all the way it doesn't even go all the way across there maybe there's some vir stitching in there but it's it's like connected to a totally different split ground plane and it might make its way back here gordy piggledy somewhere but yeah that's no wonder this thing has a lot more a.m. radiation than the four layer board because the four layer board would just that top and bottom under the chip with very low inductance very low impedance ground and power loop paths for the bypass capacitors and the signal because the return path wants to take the lowest impedance and I'll have to I think I did do a video on this takes a lower impedance so if we have the ground and power under here then the return current will actually follow and float the high frequency return current will flow and follow this particular trace or it'll try to but if you don't let it because you've split your ground plane right through yeah you've got a huge loop area it's just spewing out the radiation so there you go it's all about loop area when you're laying out boards and when you're the difference between two layout four layer everything else but not only just loop area it's particular frequencies of different types of bypass capacitors different currents different capacitive loads and things like that different whether or not it's generating electric field or a magnetic field or a kind electromagnetic combined field and things like that depending on the source impedance and the load impedance and all sorts of things it gets a really complex so does this mean that two layer boards are just inherently horrible and you should avoid them at all costs well no you can actually do quite decent layouts and approaching the performance of a four layer board on a two layer board if you're careful and you're lucky with the layout unfortunately with a design of this sort of complexity this many number of chips spread over this you know convoluted arrangement yeah you're just going to get issues a four layer boards just going to be you know it's just going to bury the performance of for this particular design here but there are some good practices you can do on two layer boards like try and keep your ground and power on top of each other wherever humanly possible don't try and split the grounds do extra vir stitching and flood fields and stuff like that to try and keep it as tight as possible in fact a well laid out two layer board might actually have less radiated emissions I even have better performance then a poorly laid out four layer board so it you know but for heads just go into four layers having the ground and power planes just makes it much easier and you're less likely to screw up just remember split ground planes really bad so if you've got to chop up your ground and power planes on your four layer board that can cause problems too but like there's lots of when you go down the rabbit hole on designing PCB layouts for EMI EMC performance there's just almost an infinite number of things to consider I might do some more videos on that if you want me to let me know in the comments down below but there have been and there are like entire books devoted to just doing this sort of stuff so yeah it can get quite complex and there might actually be a follow-up video to this if I can organise that I've actually comparing these two layer and four layer boards in an actual EMC test house or maybe on an outdoor test area called an Oates and an outdoor area test site so oh but that obviously requires a lot of planning and facilities to do that I don't have that here so you can do that in the far field you can do like a far field test in your lab you can get you can buy an expensive emc antenna you can sort of roll your own but they're a bit how you're doing and but anyway you can give you a decent indication you can put in a meter two meters away you go companies even make their own RF anechoic chambers and and stuff like that but you know you can do that sort of stuff but just using your H field and your F your probes going around your board you can like and just check in for any sort of nasties hidden in there that can really save your bacon when you go for testing later so while there may not be a direct correlation between the near-field test we did here and the far field ones you'd get in a MC compliance testing it's you know it's not a bad sort of correlation so the two layer one would absolutely definitely perform worse than the four layer board in a true EMC compliance test over the full spectrum if there's no doubt about it so anyway hope you liked that video hope you learned something if you did please give it a big thumbs up and there will be more videos hopefully coming on this as soon I want to show you how you can construct your own near-field probes and also I'd like to do something getting a heat map of H field radiation on a large board like this so maybe we'll have a do-it-yourself project for that as well so we'll see what happens catch you next time [Music]

Channel: EEVblog

Views: 170,349

Rating: 4.9452276 out of 5

Keywords: eevblog, video, h-field, e-field, b-field, near field, far field, near field emc, emc testing, emc compliance, emc pre-compliance, pcb layout, ground plane, power plane, inductance, loop area, pcb loop area, pcb, pcb design, how to, pcb tutorial, antenna

Id: crs_QLuUTyQ

Length: 36min 21sec (2181 seconds)

Published: Fri Feb 01 2019

Reddit Comments

[u/aerohoff]

Short and sweet as usual. 😂 Great to see proof of the design assumptions we all follow.