Pcb design black magic [Fact Checked]


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Last updated : Aug 22, 2022
Written by : Cammie Maymon
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Pcb design black magic


Comment by Lakiesha Boivin

bill heard from hackaday today I'm going to talk about inductance and PC board layout specifically along the lines of unwanted unanticipated when we're done hopefully it's simply unwanted inductance and you may find out why PC boards like this are kind of evil actually there's something wrong here and you'll learn the spot it so why talk about inductance well it's a fact of life that inductive reactance X sub L means that all inductors have some form of resistance to it not just the DC resistance that a piece of wire has but also this makes up flux turns it into an inductor that's it and that's a inductor and adds a frequency component to what we're trying to do now this can be nasty in power supply distribution and also in signal propagation and so the trick is to make it turn it from being unanticipated to simply unwanted and then we learn how to deal with it and we do things like gridding and stuff making alternate paths and reducing the the inductance wherever we can so at the beginning I showed you a small prototype board here's that small prototype or at the bottom on the tops a bigger prototype board and this was $49 brand-new from vector and it still has the same issue that small board had at the bottom so see if you can spot it go back stop to film if you need to but we're going to talk about how to address that okay let's get this conversation out of the way I'm going to go fast but often we have a shared ground path and the resistance that is in that path creates a voltage drop so if this guy is one amp point and there's point 1 ohms there's going to be point 1 volt dropped in the tips guys square wave if you're going to see a point 1 volt drop that's a square wave now if this guy shares that same path and let's say he's only point 1 amp but he's got an entire ohm there's actually going that's going to all add together and we're going to get point 2 volt drop so point 2 Bowl right off our noise margin even before we get to the circuitry and you can even reverse this you can put the 1 amp guy down up here and the point one amp down here and the other guy he shares the grief of his neighbor so a real quick example about why resistance is bad in our grounds and in our vccs and let's move on another name for this video could be three of my favorite books and and and here's one of them I'm going to be showing a lot of books today where I got this information from initially about high-speed design which involves inductance so these data books for free this one's from the 70s this is the Motorola Mihkel system design handbook and I'm going to show you a chart from a robot just copper resistance but mate once mentioned you can tell you're old you say data book instead of data sheet so here's a chart from the Mihkel system design handbook again from the 70s I'm sure they knew this way before then but if we look here's a 20 mil Trace we see that it's point 2 ohms per foot so suddenly the example I showed you point 1 ohms hey that's 6 inches of a 20 mil trace so what is inductance and for that matter what is capacitance and you know I try to come up some visual aids and they they just saw I don't care for these I'm just going to show how see how they yeah the mag the flux interacts with each other but you don't really get that I'm just gonna take this kind of thing and the little shiny stuff helps you imagine the flux and you could even twist it together and see the flux becomes like canceled right it's like screw that this this was kind of useful though and in that it helps demonstrate the left hand rule which is if you put your fingers in the direction of that's of flux travels your thumb points towards the north and actually you can if you have enough fingers which I'm missing one there you can determine the the e the electro part of electromagnetic and the magnetic there at right angles to each other so the visual aids good for almost nothing so what is inductance what is let's let's try and talk about that all right simply put a capacitor resists a change of voltage when it's in parallel to something it acts like a mini battery it stores electrons and and doing so the voltage it's fighting you as the voltage tries to change so it acts as a low-pass filter and in series with something a signal going through it it acts as a high-pass filter meaning the high frequencies can get through and direct current can and all and I was lucky growing up in that I could visualize the capacitor easily because they were in all the radios there was something like that sometimes huge ones right and it uses air as a dielectric and when the plates are fully meshed the capacitance is maximum you could get that you could give Y direct current can't go through this there's no direct path unless it arcs over so I'll show you here real quick a quick example of just an LED a resistor and a capacitor and how it you know stores it resists a change in voltage all right again this is something you guys already know but let's go over it real quick here's a small tail and cap it turns out the biggest capacitor I have around here is 47 micro farad can you believe that in the old days I had 10,000 micro farad caps laying around and and I could have lit this led for a half a minute but if you watch I disconnect the battery and the charge drops slowly instead of instantaneously my first cycle is it charges this cap and then when I disconnect the battery it supplies the it resists that change in voltage I went from nine to zero and it's fighting me right and it's in doing so it's providing the current to keep the LED lit longer what is an inductor inductance it resists a change of current it does that by storing energy in the flux that surrounds the conductor all right it also acts as a low-pass filter if it's in series with something so now growing up again transistor radios were full of these these are little coils and they're adjustable and it moves a little slug in and out I know that because the slug would fall out when I was a kid when I over adjusted it and the radio was no good y'all probably know more from seeing these things like on cords USB cords that whatnot where they have this ferrite material and these are acting simply as low-pass filters to keep all the stuff that will make it fail FCC compliance testing which means it won't screw up your TV signal and nearby radio wireless devices radio devices by putting a signal through a core you can actually wipe wrap around the core or go through the material either one so here is an inductor just not much of one but you'll be surprised how much of an inductor that is I think before we're done so it's obvious that that's a small inductor compared to something with coils now I had a whole collection of girth coils with my grid dip meter which I cannot find that is with my neon light bulbs I was going to use to show you inductive kick I'm sure it is so but it's easy to see that this has more inductance than this right and it's easy to imagine that that has even yet more inductance by putting a core in it a place to store the flux all right so an inductor when when this is in place the voltage is jumping up and


Thanks for your comment Lakiesha Boivin, have a nice day.
- Cammie Maymon, Staff Member


Comment by potonulok

well I liked it I thank everybody for joining us today on the webinar about talking with the pros and the topic today is gonna be RF designs and today that we're gonna be Jorge and myself and we're gonna be featuring today Joel more set which has recently joined the team of fusion the out of this team and working off with us and Eagle Joe could you tell us this a little bit about you really quick if you don't mind sure currently you would see New York's designer working on eke ads with a company and as Edwin mentioned recently joined prior to that 15 years at Intel doing a whole range of things for design comps research to the labs new business development and decided to make a career transition into the design world about four and a half years ago I've always had a passion for design and wanted to really combine my background with technology new business innovation with what I was seeing in the Portland design community this tech Directorate a couple of agencies including doing user experience research and then everything sort of got combined with an opportunity to come here and and work on some software I've been using for a long time and use it Eagle for ages and really have a have an opportunity to combine my passion for design use experience with with all those random things I picked up as a as a system engineer and hardware design development software and everything in between so and this is my first webinar it's thank you very much I appreciate the information you know so the topic today is about radiofrequency and you know it's one of those exciting applications to build these days in electronic designs virtually all up and running technology today smartphone sensors robotic security all of them basically demand some level of radio frequency and some of the factors that need to be considered some are like impedance trace lengths and interface so I'm gonna go ahead and stop sharing our screen because I actually want to jump over to Joel that he's gonna be sharing his screen and giving us some more information about it so if you have any questions throughout the presentation just go ahead and put them on the chat and Joel and I will be manning the chats and help you up Thank You Joel I appreciate it it's all you know great thanks so much I will start with some introductions slash level set just so expectations are kind of set we only have about you know 20-ish minutes on the books we can always run a little longer this as as some of I know you've you know this is a very deep well Aref is a very fascinating and in robust corner of engineering and design so my hope today is to kind of get the ball rolling for folks that may not have done much of this work or have sort of a cursory experience if you're RF Pro and you and you've really got some chops which I know I saw in the chat a few of you do please jump in both keep me honest and and share touch tricks details what have you because that'll definitely make this conversation richer and while I have some experience I do not claim to be an RF professional I play one on TV but but I do have enough basics to kind of pointed you in the right direction and show you some core concepts so that you're grounded in kind of the fundamentals and as you go forward down this path that's how it's saying to Edwin earlier either know how to do the basics and get the things working that you need or know when to call on the professional because some corners of doing this kind of design really are so domain-specific and require a level of expertise that's it's really quite fascinating so without further ado I kind of wanted to start and I'm intentionally maybe I'll just turn my cam on so you have something to look at there we go that's me so perhaps we'll start with with just some kind of core concepts best practices things to keep in mind around radio frequency design as as Edwin was alluding to and then we can kind of use that as as guides to the rest of our discussion and it's just no in particular I tried to sort them a little bit last night as I went through if at all possible first if at all possible use the reference design some manufacturers that are producing RF chips technology will have better documentation than others but in generally in general the stuff that's provided in reference Docs has been has been prototype manufactured and tuned by say Nordic for instance and and so it's at least in most cases known good and if you go to their support folks you can you can get some help getting started there and and then intentionally and carefully diverge from that we'll talk about this in a second about how to actually set up the circuit and what's in there but there's an order of operations for getting things placed on the board and routed to avoid noise crosstalk and general pollution from the system noise we want to avoid at all costs as we do in many high-speed designs especially in RF that's going to very quickly kill our efficiency which means range dropouts you know corrupted data all kinds of other things we want to in general ground reference everything and do it well so large the largest floods we can have we want to tie them together with ground vias to see this in the reference design in a bit to make sure we have a good solid ground plane available underneath all of our signals and then not under our antenna again we'll show this in a second because we want the antenna to basically be as close to free in the air as possible with the constraint that if it's a PCB trace antenna that we're going to look at then you're you know you're confined to the plane of the board but you can certainly you can certainly leave yourself plenty of space for for being able to get you know an omnidirectional antenna and be able to get a nice drum signal and finally there's when it comes to manufacture and I'll mention this again it's like make sure your fab house know is exactly what your constraints are when you go to get the board fact because this is one of those things that's pretty sensitive to your board tolerances stack up your board stack up even adding like some fab houses will add like an ID number on your board in an open space somewhere put a note if you have to even in the silkscreen on the board like do not put additional copper here in the spaces that need to be cut out for the antenna because that can have an adverse effect on your design so let's let's do this I'll share my screen here in a second knife because all this takes time to put together I'm going to start with a reference design that's a that's a known good won't turn off my cam because you all have seen enough of me already so let's let's bounce to ego this is the courtesy of smartphone this is their an RF 24:01 chip breakout boards it's a great example because it's 2.45 gig so it's you know high frequency it's got all the things you expect to see in an RF design inspect to having your ASIC with which about here so this is the NRF 24 this is going to break out for you know are doing or what have you all the usual suspects you also notice that there is and that work here for doing impedance matching so in general you can d


Thanks potonulok your participation is very much appreciated
- Cammie Maymon


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