Michael Ossmann: Simple RF Circuit Design

thank you Chris Thank You hackaday and thanks everybody for coming it's really exciting for me to get a chance to do this talk it's something that I've been thinking about doing for a long time and I haven't done it most of the mostly because I go to a lot of conferences in the information security community and that's that really the right audience for this talk the audience for this talk is is people who have made a circuit board before how many of you have made a circuit board before yeah all right how many of you want to make something that's RF excellent okay so you know if you answered those two questions then you are the target audience for this talk I wanted to kind of share my experience and how I've made circuit boards that are RF designs and how you can do that too so now my qualifications just to be perfectly clear my qualifier my qualifications are that I've made some fire app circuits I am NOT an expert RF engineer not at all I do not have an engineering degree I'm simply a guy who's made some RF circuits and made them work and I want to share with you how I have done that so this isn't necessarily I'm not going to talk about the best way or every way that you can create RF circuits I'm going to talk to you about the way I create RF circuits so some of my projects are up here probably the most famous is hack RF one which is a software defined radio platform and we're going to talk about these designs in some detail probably not hack RF one and I end up with a whole bunch of extra time I wasn't expecting we probably will not go into that one in great detail just because it's more complex and would require a longer period of time for us to actually delve into its architecture but everything that we're talking about today everything that I'm going to present is is represented in all of these designs including hacker f1 even though it's more complicated than the others it is made up of simple parts and those simple parts are things that I'm going to talk about today so the traditional approach to RF circuit design involves a lot of knowledge you need to have a lot of books on your shelf and you need to refer to them often and you also need a pile of test equipment and some of this stuff can be very expensive even if you do find it on eBay and we get all these ideas like I don't know s-parameters and Smith charts and Q factor and VNA s and RF transistors and many others carry letters that present themselves when we look into RF circuit design and I think it scares a lot of people off I talk to a lot of people who said well yeah I've made a few circuit boards but I've never done RF that's black magic and its really not black magic in the vast space of all the different possible RF circuit designs you could create there is a large significant subset that is very easy very accessible and you don't have to deal with any of these things in order to start working with RF and start making your own RF circuit designs so there's a simpler way and that's what I'm here to talk about today the simpler way is what has led me to be able to produce successful open source hardware designs that are RF circuits so this is this is the core of the talk right here if there's one thing you get out of this talk it is these five rules these are my personal five rules for RF circuit design now this talk is going to consist or less of three parts and that's going to be part one is going through these five rules in detail part two is looking at some examples from my own designs for that can show you how I have put these rules into practice and part three is going to be talking about components and how do you select components for your design that's what this will talk will consist of so let's dig right into the pipe rules first rule is used for layers use a four layer printed circuit board and it should look like this put RF signals on the top layer and then just below it there should be a ground plane and then on the third layer you should have your power supply and on the fourth or bottom layer that's where you put any signals that don't easily fit into the top layer but not RF keep your RF on the top layer now I know there are people in the room who are thinking but but can't I do two layers I like doing two layers like can't I get away with it with this one design that I have can I maybe do this two layer RF thing and I say yeah sure you can but you better start reading and you better have a pile of test equipment so you can figure out why your design isn't working or fix it or make the performance what it should be we're not talking about those that stuff that I consider advanced cockpits in RF circuits so maybe you could get away with doing some two layer RF circuits but the easy way is to go with four layers though the four layers follow this type of stack up where you have a ground plane that is very close to your top layer and you have your RF circuits all your RF signals are on the top layer and things will go so much easier for you so that's rule number one just use four layers four layer PCBs are very easy to get assembled these days thanks to our friends at OSH Park and other prototype assembly houses it's not that expensive it's not that time-consuming to get four layer PCBs done I just ordered some well like I think it was in early November that you know I have with me today already delivered assembled so but you know before I came here so it only took a couple weeks and I had some four layer PCBs an RF design that actually is featured in this presentation because I had it in time to do that even though it was a you know just done a couple weeks ago so do four layers it's it's a it makes your life a whole lot easier working with RF and it's pretty accessible to be able to get four layers or even more layers these days in a PCB so use a consistent stack up stack up matters and buy stack up I mean how how those uh how that PCB is physically constructed like how thick is the substrate between layers one and two how thick is a substrate and what has properties between two and three and three and four and how thick is the copper and that sort of thing and if you use the same PCB house all the time and that's not a problem but if you start a design with one PCB house and then the next time you get that design built you use a different PCB house you better make sure that they use the same stack up that your first PCB house do you want to make sure that that's consistent dedicate the entire inner layers to unbroken power supply planes the entire layers layer to is just ground layer three is just power that's it don't put anything else on those layers I know you think oh it's all that wasted area where I could be running things and no if you really need extra traces that can't fit in your top or bottom layers go to a six layer design go to an eight layer design if you absolutely have to rather than break up your ground plane or break up your power supply plane really this is this will make you her life a whole lot easier everything will work right if you have unbroken power supply planes and keep the RF signals on the top don't run them through vias don't do anything complicated with them pick your RF signals a signal simple as possible and we'll talk about that more later the second rule use integrated components use the most in degree rated component you can for your design that means you'll be looking at RF ICS integrated circus for radio frequencies there is a huge array of RF ICS available today they're wonderful and but it also goes for other types of components things like balance and filters passive components not active circuits that are available as a as sort of a monolithic or an integrated package these things are available today and I highly recommend that you use the most integrated component that you possibly can for a particular application so RFI sees in particular come you know all sorts of different functions are available in RF ICS you can get mixers you can get amplifiers you can get frequency synthesizers modulators demodulators all sorts of different things transmitters receivers but the one most integrated component that I imagine most of you would find useful for your designs is going to be a wireless transceiver IC it's an entire transceiver on a chip and I've listed a few part numbers here that are just examples of ones that I think are interesting that are on the market but there are many many more that could be useful for your design so if your design is something that has say a microcontroller and you want to give it some kind of wireless interface to something else this is the type of chip that you should be focusing on as being central to the RF part of your design a wireless transceiver I see they do it all it's amazing how integrated these things are really is awesome and you should try to find one that fits your application the best try to find one that is the most integrated so that you have the least other work to do in order to make it function now I mentioned in addition to RF ICS there's also like passive components for example filters that you can buy in an integrated component now if you don't buy them an integrated component you have to build RF filters out of like capacitors and inductors and and there are tools to help you do this filter design tools that let you create this this big network of capacitors and inductors and simulated and try to figure out what is frequency response is going to be and it's kind of a messy business because simulating these things requires actually like intimate knowledge of the properties of the components that you've selected not just the capacitance but much more than that it's frequency response and also knowledge of the layout nuances of the layout can affect the performance of the filter in different ways let me show you my favorite filter design tool oh I'm missing a slide how did that happen I'm going back and forth and see realizes blah blah blah blah blah blah it's totally not there okay ah the moment is gone uh my my favorite filter design tool is supposed to be a screenshot of the searching for RF filters on digi-key that is that is my favorite filter design tool use your distributors website use parts dot IO use resources that are online and available to you to find RF filters because you can get a filter that and and we'll see one later on in one of my example ports but you can get a filter that is just a single component and is not all these and it's just one little surface mount component that you solder on your board and it has a known performance known frequency response known impedance matching it's super easy to use and they only cost a few cents and for almost any application you're going to find a good filter look for RF filters also look for saw filters s.a.w surface acoustic wave filters for many applications like something in is M ban like the 2.4 gigahertz band or the 900 megahertz band for for many applications you can find a very low-cost soft filter and saw filters are amazingly effective they just have the the most the sharpest cut-offs and you know the biggest difference between the how much they attenuate the the stop band versus how much they pass the signals that you want them to pass like they're really good performing and they're easy to use and low-cost so look for RF filters look for saw filters I bet you'll find one that will figure application very well and it's a more integrated component that's easier to use than making a big network of passive components and trying to figure out how to put it together so the next rule is use 50 ohms everywhere so impedance matching is a thing and we have these tools like Smith charts this is actually a Smith wheel that you can physically manipulate on the backside it has a circular slide rule which is cool but one of my former students set me this as a gift that I think it's awesome but I've never used it because because I avoid using Smith charts I avoid doing impedance matching which is one of the major uses of Smith charts I am void doing impedance matching myself so impedance matching is a thing that you have to worry about sometimes in RF circuit design but there's a super easy way to match impedance all the time and that's just to use 50 ohms everywhere have 50 ohm outputs at it from every component and then run that signal over a 50 ohm transmission line across your circuit board to a 50 ohm input on the next in component you don't need to worry about impedance matching other than just doing 50 ohms everywhere and why 50 ohms well I'm not going to get into the history of it which I'm not even sure I know but uh well I mean I've heard things heard rumors but I'm not sure that I necessarily believe all the rumors but we have a standard of 50 ohms in the industry almost any component some components you might find have sort of random impedances but anyway that has paid attention to like trying to give you an easy-to-use interface to their component any manufacturer pretty much is going to target the 50 ohm single ended interface occasionally you see 75 ohms like in the cable TV industry but like in the cabling end of things mostly but other than cable TV almost everything RF is 50 ohms as a standard so just go with 50 ohms everybody is using 50 ohms you should use 50 ohms and selecting components selecting components that give you a 50 ohm output it's very easy selecting components that give you a 50 ohm input is very easy having a 50 ohm transmission line on your PCB is something that you have to take responsibility for but it's pretty easy to and what you need to do is to think about the microstrip impedance of the or the characteristic impedance of a signal that's going on your top layer some distance above your ground plane and if you have if you have a a component that gives you something other than 50 ohms if you look in his datasheet and it says like I don't know 34 + 11 J or something like that like wow what is that don't worry about it just well first of all see if there's another component you can use instead that does give you a 50 ohm impedance but if there isn't then just convert to 50 ohms right away just convert that signal as close to that component as possible into 50 ohm and then run 50 ohm trace across your circuit port and how do you do that you know what I think I am missing more than just one slide so I'm going to change things up a little bit here start over no we're not going to start over we're gonna put through make sure all the slides are here you'll get to see the the one we missed which is coming up here it is my favorite filter design tool woohoo all right um now here we go transmission lines okay so if you have 50 ohm output and 50 ohm input you still have to have a 50 ohm transmission line that goes from one to the other how do you do that well you use an impedance calculator a characteristic impedance calculator for your PCB how many of you have a impedance calculator on your computer how many of you have kicad on your computer oh then you have an impedance cut code how many of you have a web browser on your computer okay then you have obedience calculator because you can find tons of different impedance calculators on the web here's just one of many that I happen to like it's simple easy to use at man taro comm they have a whole bunch of different calculators that are useful for circuit design and and here are the things you have to plug in you plug in the width of your traits in the mills you plug in the thickness of your tracing Mills like how how high is it so on the default is 1.4 Mills as you know it's a thin layer of copper and then you have to have the dielectric thickness that's the distance between the between layer 1 and layer 2 and then you have the dielectric constant which is a which is a property of that material that substrate that the PCB is made out of okay so where do you find these things well you find them at your PCB manufacturers website so let's say you're using OSH park you look up there for layer stack up and specifications which are on the left here and then you plug those values into the calculator so your toy trace width well that's something that you get to define when you're making your circuit board right but your trace thickness 1 point 4 mils oh yeah that default actually is the same anything that's called one ounce copper is typically going to be 1.4 mil and Clark tells us that now what's our dielectric thickness 6.7 mils that came from right here six point seven mil prepreg layer between these first two copper layers okay that's where they got that number and plugged 6.7 mils from the OSH Park site into the calculator now what is this dielectric constant well it says down here dielectric constant of three point six six at one gigahertz and the dielectric constant may change depending on what frequency you're working with but the if you look further down on this webpage you'll find a link to the datasheet for this particular substrate material and you will see that it hardly changes at all across a very wide range of frequencies so you just plug in three point six six and you're fine for other substrates you may need to be a little bit more careful about what number you use at what frequency so we plug in those numbers and hit calculate and it tells us the characteristic impedance in ohms and notice I plugged in 12 Mills and it gives me almost exactly 50 ohms so all I have to do all this work was just so that I know my RF trace is on the top layer of my circuit board should be twelve Mills wide that's it and guess what I only had to do that once if I make many RF circuits and I use the same stack up and I make everything 50 ohms everywhere I only have to do this once can I find out what my trace width is yes question just Oh 50 up plus or minus 10% is probably fine for most designs if you're going like higher power or you have really rigid performance requirements you might have to go a more precise than that like maybe 5 plus or minus 5% but plus or minus 10% is probably totally fine for most most designs especially low power designs so it doesn't have to be precise but it has to be in the ballpark if we if we made our traces you know 6 mil wide things are going to go badly for us if we make them 50 mil why things are going to go badly for us before you make them 12 mil wide everything should be fine of course a mil is a thousandth of an inch so you can do this in units of millimeters if you want to but most PCB houses do you use imperial units to use mils so that's usually what I use when I'm dealing with PCB specifications so that's it this is like the most complicated part of the top right here it's just using this PCB calculator and you only have to do it one time because all you're figuring out is how wide you need to make your 50 ohm traces on your circuit board so what if you need something different I talked about this just briefly before I realize that that previous slide was missing uh if you have a component and you need to use this like you this component is perfect for your application in every way except that it doesn't give you a 50 ohm output you need to convert that output to 50 ohms as close as possible to that component how do you do that well the traditional way that the books tell you about is to like use the Smith chart and all kinds of stuff but know the way you do that is by following the manufacturer recommendations almost any manufacturer of RF components that gives you an input or an output that's not 50 ohms is going to provide for you the information that you need to figure out how to transform that impedance how to convert it from whatever it is into 50 ohms so let me give you an example here's a the application circuit like a first schematic that's referent that is present in the CC 1110 datasheet now the CC 1110 is a popular wireless transceiver I see it's the one that's in my favorite pink toy the I am me and it's pretty easy to use like most wireless transceiver ICS and this diagram shows you everything you need to make its radio work the only thing missing is some power supply decoupler capacitors it also doesn't show you the digital side of things like how do you actually how does your microcontroller talk to it or something like that but how do you program it but everything you need to make the radial function is right here all there is is this one funny decoupling cap over here a crystal oscillator crystal and to load cap this bias resistor which is probably a precision resistor that they specify and then over here we have this mess of capacitors and inductors that convert between this port this this two pin port pins 23 and point pin 24 over to a 50 ohm antenna so this is the circuit that you can use to convert this to 50 ohm RF input an output and since it's a 50 ohm interface you can actually just replace that antenna with anything else if you want to run a 50 ohm impedance trace over to some other component like a filter or an RF switch or whatever you just plug that in right there because all this impedance matching is taken care for you for you now we actually see if you know a little bit if you've ever looked at analog circuit design before you might notice that there are a couple things going on here this is actually a vallon section and this is a filter section here altogether it effectively transforms the impedance for us but it also gives us some filtering and it gives us the Balan function a Ballen means balanced unbalanced a balanced signal is a differential signal see how we have two signals ones negative and ones positive that is a differential interface and then an unbalanced signal is also known as a single-ended signal which is what we have over here an antenna is typically a single-ended signal and transmission lines that we use across our PCBs most of the time our single ended signals and that's what I recommend you use 50 ohms single ended signals so this balon converts us to a single ended signal and this filter probably also transforms the impedance and we end up with a 50 ohm interface now this is the most complicated part and the manufacturer tells you exactly how to do it now as I mentioned earlier these types of circuits these bundles of capacitors and inductors are very sensitive to component selection and they're very sensitive to particularly layout so make sure you follow the manufacturers recommendations very carefully they will often recommend particular components they will often give you the layout tool even give you Gerber files they'll give you an example board like an evaluation kit that you can buy sometimes but they will give you the design details that you need and you just copy that section verbatim and I guarantee you that design will involve a four layer circuit board so use that follow the manufacturers recommendation and that way you can use something like this non 50-ohm non single-ended interface you just transform it into a 50 ohm interface and then you go from there so the last rule is to route RF first when you're designing your circuit board route RF first you should keep RF traces short and direct so if you can put your components closer together do that you should when I say short I mean short with respect to the wavelength how do you compute the wavelength well approximately the wavelength is going to be C the speed of light divided by the frequency so if C is 300 million meters per second and you want something running at 900 megahertz that's nine is 300 million divided by 900 million okay your wavelength is a third of a meter as long as your traces on your PCB are short with respect to a third of a meter you have nothing to worry about in terms of the effects of wavelengths on your design of course that becomes a bigger problem the larger your design is it becomes a bigger problem the higher the frequency is but just do that in your head just remember 300 million divided by the frequency and if that seems big compared to your circuit board don't worry about it keep other signals away from the RF signal make sure you know don't run traces right next to your RF signal if you can avoid it just keep things a little bit keep a little buffer zone there and characteristic impedance matters we talked about that earlier make sure you use and people's calculator and that you follow it and every single RF trace you have every other 50 ohm RF trace should be consistent with following what you learned now you should route RF traces first but of course you should route clock signals first and you should wrap power first but but pay power is taken care of most of the time if you're using the four layers stacked up properly power if you've never done four layered circuit board design and you do your first one you'll be like ah this is so nice like I just drop a via anywhere and there's my power it's oh it's great so yeah you'll get spoiled very quickly I I sometimes use four layers labs for things that don't strictly need to be for later science now just because it's so easy front during the design process shell let's go through some examples those are the rules that's all you need to know to be able to build a design your own RF circuits honestly you'll have success and let me show you how I've had success using these very rules and my own designs so the first thing I want to look at is is the great FET project which is new some of you had seen me carrying this around this is this is one of the this is the main board as a liqu codename is a Lea it's it's a if you're familiar with Travis good speeds good FET project this is sort of a next generation good fit and it gives you a high performing high speed USB peripheral it gives you a device like your own custom high speed USB device that you can control from a simple Python interface on your host computer that's that's one of the main goals of the great vet project but one of the things we've been working on is different pluggable front ends which we call neighbors and plugged in and that's what some of these other boards our neighbors that you plug into the main board and give you some function and one of them I want to look at is crocus here which is a 2.4 gigahertz wireless transceiver chip 2.4 gigahertz wireless transceiver chip is probably something that could be used in a lot of your projects so let's take a look at it I'm going to zoom in on this board actually I'm going to zoom in in the future also because I'm going to change to a newer rebel the board the one that I just got that has this SMA edge connector instead of the other connector we saw there this is the entire RF section of this board we have the NRF 24 L 0 1 plus which is a very popular it's one that was mentioned in the slide I had earlier with different RF transceiver chips it's a very popular 2.4 gigahertz transceiver chip from Nordic and and this entire circuit here almost all of it is taken directly from the manufacturers recommendations and there isn't much to it there's this crystal oscillator a couple load caps there are a few decoupling capacitors there's this one bias resistor that's a very common thing you see in these types of designs and then look over here between the antenna connector and the between the the antenna connector in the IC is this thing right here this is a ballon remember the first ballon we looked at was this complicated ish sematic that had a bunch of capacitors and inductors well you can do that and the Nordic well the the webpage or the the datasheet for this part tells you exactly how to do that but instead I actually went a simpler way I googled n RF 24 L 0 1 plus Balan and I found two manufacturers that just sell a single component that's designed specifically to match the impedance the differential impedance of this chip and convert it to 50 ohms it costs something like 20 cents instead of a mess of capacitors and inductors it's kind of a no-brainer to use this thing so you can you might be able to see it's a little hard to see but there is a differential pair here that goes from these two pins over to this ballon which is a transformer a ballon is it is a transformer that converts this differential signal into this later Aref trace which is 50 ohm characteristic impedance now it's all jammed so close together that the character's impedance might not matter that much but and I actually could move the balance even closer to the IC if I wanted to but I sometimes especially on a brand new design like this I when I'm prototyping I'll separate things out just a little bit so I have more room to get my soldering iron in there so I can drag solder that qfn but look that's all there is to it just a bow one component ballon and a one component wireless transceiver chip and then all I have to do is like give it give it a power supply and connect it to my microcontroller and I can do radio stuff over this 50 ohm interface pretty cool let's take a look at another design this is uber tooth one which is a special-purpose bluetooth sniffer primarily it can be used for other things but the reason I started the uber tooth one project was because I wanted to be able to monitor Bluetooth communications I wanted to be able to discover undiscoverable Bluetooth devices and this is actually why I learned electronics was so I could build over to so let's take a look at this we have USB we have a big microcontroller and then over here we have the cz 2400 which is another wireless transceiver chip from the chip con family by Texas Instruments let's take it let's zoom in on this RF section here from the transceiver chip to the antenna port and see what's going on okay so we have the CC 2400 as a few passives around it exactly as recommended by the manufacturer and then it has this CC 25 91 chip here which is also recommended by the manufacturer is another chip that that is a companion to the CC 2400 Texas Instruments makes it - they're actually designed to fit together and this gives you amplification both transmit and receive amplification integrated into one chip which is pretty handy and there is a differential interface that goes between the two I don't have to worry about the impedance of that like being 50 not being 50 ohm single-ended because these two chips are designed to connect directly together so I just put them really close together run those two pins to each other and they match there is an unused pad a pair of pads here but where I could mount an inductor to repair the impedance match if it weren't good but it's good because those chips are designed for each other and then over here between this is the edge this is the SMA connector over here where actually it's an RPG SMA because it's a 2.4 gigahertz between there and the CC 25.91 there are inductors and capacitors and how do I know how to build those that network of inductors and capacitors I found it in the CC 25 91 documentation in the datasheet and the example circuit that Texas Instruments provides and I copied it exactly that's all I had to do the easiest part of the over tooth project in terms of the hardware design was the RF section because it's all copied directly from manufactured now including it is important that it be copied very precisely for example there are these components down here these passives that have a particular transmission line length and width that goes and because the transmission line characteristics matter and so and so the manufacturer actually specified like the PCB stack-up and exactly how far this would this transmission line should be and so forth so I copied it very deliberately exactly like a manufacturer said and it worked great now let's take a look at yardstick one this is a looks very similar to Hoover tooth one same form factor also a wireless transceiver chip on a USB dongle and if the this is we just started shipping this recently and it is for sub one gigahertz radio application so like nine or 300 megahertz garage door openers industrial control system smart meters all kinds of stuff like that and it's a really easy to use platform because it's something that lets you plug into your host computer and just control from your host computer it has CC 1111 we looked earlier this schematic from Assisi 1110 another one of those chip con parts from Texas Instruments this is a CC 1111 which is the same as the CC 1110 except it adds a USB port so this actually integrates USB device microcontroller and radio transceiver like a digital radio modem all on one chip the more integration the better and this one has a little bit more complex of a radio architecture so let's go through it a little bit because I think it's it's a it's instructive so everything that's everything that's our RF is from this CC 1111 all the way over here to the SMA connector let's take a look in there and take a look at what's going on so what's the first thing you see over near the radio chip this that's a balun again I used a monolithic transformer a single component instead of a mess of capacitors and inductors now one of the magic things about yardstick 1 is actually that balanced with and the only reason I knew about it was because I found it when I did the hack RF project so so this balint is not specifically designed for this this transceiver chip but it turned out to be very applicable to it so I take this differential interface and you see this a lot on these wireless transceiver chips or any chip that has a mixer in it you see these differential interfaces and you end up having to use a balun to convert them to single-ended so right here we have about a hundred ohm differential interface and then immediately it gets converted to 50 ohm single-ended everything's 50 ohm signal in it from there on out so what it goes through here this is a capacitor and notice it just goes through it in series that's called a DC blocking capacitor we'll talk about that a little bit but our rep goes right through it so the RF goes right through there and it hits this part which is a switch it is a it is an RF switch and now there are three paths to get from this switch to that switch one path is this s curve that goes directly from one to the other another path goes through this amplifier another path goes through this amplifier there's a receive amplifier a transmit amplifier or a path that bypasses the two amplifiers remember in uber tooth I had one chip that gave me transmit and receive amplification all in one chip that was super convenient if you have the luxury to have the more integrated solution go for it it's great but I didn't find a single component that did it for me and so I ended up with this huge section 12 huge we're looking at a few millimeters here ah we ended up with this more complex section that includes two different amplifier two RF amplifiers two RF switches and methods for supplying power to those amplifiers it's a lot you know a lot more went into the design but realize that every single component follows the rules that I was talking about all along I have 50 ohm interfaces between everything and I have as much integration as I can and so forth now on the other end here of this switch we have see this thing looks kind of like a ballon that we seen before it's not a ballon because it's single ended in single ended out it's actually bi-directional so which one is in and out is kind of up to you but this is a filter again I'm replacing what could be a mess of inductors and capacitors with a filter this is a low-pass filter design for the 900 megahertz band 900 megahertz is the highest frequency range that is supported by yardstick one so I use a low pass filter to cut out any harmonics that are above 900 megahertz which is quite optimal when you're using yardstick one in the 900 megahertz band it's somewhat less optimal when you're using yardstick 1 and or frequencies but it's a heck of a lot better than nothing so I have an RF filter I filter as much as I can of unwanted frequencies with one component one component that I found using my favorite filter design tool and then I have this little structure here that allows you to provide DC power to the antenna port through an inductor we'll talk about that in a little bit - ah that's the entire RF section all summed up here so this is a lot more complicated but it's still following all the same rules that I talked about earlier they're the rules I follow for all my designs now let's talk yes sorry oh why do I have different size videos that's a good question um so partly they I only have two sizes it may be deceptive you may think some of these beers are like you might see more than two sizes but it's probably just a how much of the solder mask filled the hole but mostly I'm using two sizes I'm using a big size for power so like for example here is a capacitor a big decoupling capacitor it has a it has ground via and a supply via and I want to have those vias be able to carry as much current as possible and use low induction inductance as possible so I use the biggest vias that I can and then for signals that I don't that that aren't I don't really worry about the inductance much or the current carrying capacity much like let's say this one right here this one is a control signal for that switch it simply take it high to have it switch one way take it load and how to switch another way or it's actually more complicated than that because it's a multi pin control mechanism but but that's the idea and and so those signals I use this I use a smaller via so I can squeeze things closer together that's the only that's the only reason is that I like to have a bigger vias for power and smaller vias where I can get away with smaller kids The MITRE tracers oh the corner is how I use 45-degree angles um so that's more tradition than anything uh uh there there are some myths about having signals like not go through 90-degree turns don't don't worry about that but um but it does I find make the layout a little bit easier to work with if you're in the habit of doing things in 45 degrees it makes it makes a some things kind of easier to fit together sometimes like over here I like using 45 degrees but I'm not religious about it so you use whatever English you want pretty much there was a good discussion of that on the amp hour actually if Howard Johnson gave a answered that question or kind of debunk that myth a little bit on the amp hour so check that out so let's talk about components that I like to use on circuit boards are app circuit design and let's have a pop quiz all right which of these amplifiers these are amplifier ICS which of these amplifiers should you use in your design what's that the one on the right why should you use the one on the right it's internally matched to 50 ohms what about the one on the left oh the output is internally matched 50 ohms the input is pre match to 50 ohms what does pre match mean I have no idea I think it I think it's a euphemism for not matched take it take a let's look at the the BGA 7777 n7 on the left here let's look at it a little bit more closely look at this description down here the device features blah blah blah blah blah matching off chip it features matching off chip you gotta love marketing people right like oh yeah this this apartment features a three-mile walk to the nearest bus stop think of all the exercise you'll get so uh right um so if you look further down look for the recommended schematic always look for this game attic and see how easy is this part to use really okay so here's the chip here's the input and yeah look at that that's an impedance matching network on the input you have to just you have to supply that impedance matching hey and guess what the output doesn't look like it's quite 50 ohms either even though it said internally matched output 50 on AH so here's another reason why you maybe shouldn't use this particular chip which is I think from the same manufacturer as the other one part manufacturer and part numbers like various they don't just they don't tell you much about which components you should use or how you should lay them out and stuff like this so they're not particularly helpful this isn't the most easy to use component now there may be other reasons why you want to use this component but ease of use is not one of them compared to that other chip let's take a look at these I recommend you schematic for the other one you have a power supply with a decoupling capacitor you've probably done decoupling capacitors before easy you have an input and an output both of which go through a DC blocking capacitor which I mentioned very briefly I have some of those in the yardstick one design and other design there's a one capacitor on the input one capacitor on the output is truly 50 ohm match both input and output all you have to do is pick those DC blocking capacitor and you're done now DC blocking capacitor is something we'll talk about just briefly here but in particular I want to show you another kind of structure that you'll often see this is from another RF amplifier datasheet just something I happen to have on my laptop and it shows you this structure called a bias t you see this a lot especially on the outputs of amplifiers this actually has one on the input and the output but what you see here is an inductor that carries DC power and delivers it to the amplifier that way and then you see a capacitor that blocks that do you see a DC blocking capacitor now that prevents that that DC from going anywhere else so the the inductor here is called an RF choke when I when an inductor is used for this purpose it is called an RF choke and when a capacitor is used for this purpose it is called a DC blocking capacitor now if you have not done much analog design this may not come totally natural to you but this is something that you need to get familiar with that inductors and capacitors can be used in these ways capacitors and this is a line that remember it personally capacitors you know like from there there's schematic signal there are two plates right two different conductors that are isolated from each other no DC can cross so capacitors block DC but pass higher frequencies like RF okay so you can just stick it the passer right in series in your RF signal and the RF goes right through it but it will stop DC inductors are the other way around and again I remember just by looking at this maduk signal like this is a coil of wire going from one end to the other it conducts DC from one end to the other but it blocks RF one of these things conducts RF and blocks DC the other one conducts DC and blocks RN so this is why you see this common structure called a bias T it biases this section of the RF signal with a DC component that comes from this power supply and that's how the amplifier actually gets its power and then this DC blocking cap prevents DC from going other places and doing unexpected things so those are two things and and you have to be a little bit careful how you select your inductors and your capacitors for these purposes but look at the documentation provided by a manufacturer you may not be used to looking at capacitor data sheets but they exist some manufacturers are better at publishing these things than others I happen to like Mirada a lot because they have a lot of documentation about their capacitors so you I can download very great detail about how their capacitors perform at different frequencies that's often what you want to know you want to know does it pass my frequency of interest really well and does it block other things that I don't want mode notably DC in a DC blocking cap what really matters normally is how much how well does it pass the frequency that I'm interested in same thing with RF chokes look at your frequency of interest how well does this inductor stop the frequency of interest you don't want your radio signal going into your power supply you want your radio signal going out the output and so that's how you select the inductor in the capacitor is simply by looking at how much they pass or how much they block of your expected frequency here's another example that uses DC blocking capacitor this is an RF switch remember I mentioned I had a couple of RF switches on your stick one these components are typically pretty easy to use RF switches now they do tend to come in small packages the ones on half our F are particularly hateful because they're one millimeter by one millimeter 6-pin qfn but ah but you can solder them so and this is one of the things like when you get into RF you you just have to start to get used to doing four layer boards you have to get used to using o4o tubes you have to get used to using QF ends because those are the best components for the job and some but you will get used to them you can do them you can drag solder qf ends sometimes I cheat a little bit actually often I cheat a little bit and just make my qfn pads a little longer just to make them easier to drag solder especially for prototyping that's really handy you'll learn these things as you deal with these components more but you'll see here we have a DC blocking cap on the input do you see blocking cap on this output DC blocking cap on this output and then just to decoupling caps on there control voltages and ground and that's all you have to supply so the only thing complicated about using this part is just selecting the DC blocking caps and usually in a design you will find a DC blocking cap that's good and then use that same part for most your whole RF chain because most of your chains probably going to be running at the same frequency anyway so all you have to do is that effort once you're good to go so these are all the different types of components that you'll probably use in your RF design even though I used amplifiers a lot because they happen to have some nice examples for impedance matching or lack thereof and some and like bias T's and stuff I don't actually recommend that you get right into using amplifiers don't don't use them if you can avoid but I do recommend and that you use Wireless transceivers that you use RF connectors capacitors and inductors are going to find their way into your design in the form of DC blocking caps and RF chokes switches are handy anytime you want to have a signal like like maybe you have two different antennas you want to be able to switch between the two that these RF switch ICS make that super easy balance and filters look for those as monolithic components integrated solutions instead of doing it and look for all these things so things that you want to look for in these components so am I mentioned already but like ease of use is number one for me personally 50 ohm interfaces look for that look for in filters look for the frequency response look in the datasheet and look for the graph that shows you how much it passes the frequency of interest and how much it blocks other frequencies switches let's see mostly that's just an ease of use issue but you also look at this the frequency response potentially of a switch you'll also look at its insertion loss how much loss there is when you you put that date on every every step along the way in your signal chain loses power how much depends on the particular components use so look for how much loss there is inductors and capacitors we talked about a little bit transceivers again it comes down to ease of use of a lot of the time what's the most integrated component for the job that you're looking to do power oh that's a good point how much attention do I pay to power ratings the components quite a lot uh but mostly I pay attention to power ratings of the components only if I'm using amplifiers wireless transceiver ICS tend to be quite low power and tend to never be too much to overpower any of these other components except maybe amplifier and when you when you start dealing with amplifiers you need to be careful of things like the linearity of the amplifier and look in particular for what's called the 1 DB compression points often called p1 DB that is the amount of power that you can put through it and have it it will kind of linearly amplify you put more power in it gives you more output you put more power in it gives you more output you put more empowering you gives you more output but then it sort starts to level out you put more power in and it gives you like 1 DB less output power than you expect it based on this curve this nice linear section that's the point at which it stops behaving linearly you generally don't want to exceed that point and so pay attention to that be very careful because sometimes the 1 DB compression point is specified for its input power and sometimes it specify for its output power and those are very different things also is sometimes specified as a third or third order intercept point instead of 1 DB compression point and it happens to be about 20 DB higher than the 1 DB compression point so there's bunch of things like that there are complications of dealing with amplifiers that you generally don't have to think about you're dealing with these other components also anytime you're dealing with amplifiers you run the risk of getting into trouble creating problems for other users of the spectrum which are a lot less likely if you're doing low power stuff without any special amplification parts also pay attention in the amplifiers they not only do they have a certain point over which you they start behaving nonlinearly they also have a point beyond which you just damage them so look at their absolute maximum input power that you can give them blood so that's them actually the most challenging part to select compared to all these on others now one thing before I go I want to mention software-defined radio is a great tool for you if you're getting into RF circuitry because you can use software-defined radios platforms as low cost test equipment use your software defined radio platform as a way to monitor the transmission that your device is producing use your software to grant defined radio platform to transmit a signal into your device test test signal instead of having a bench full of tens of thousands of dollars of equipment you can get away with a lot by having low-cost uncalibrated test equipment like software-defined radio platforms and software-defined radio is also a good educational platform for you to learn a lot of these things so like I today I've helped you learn about components and how you piece things together and how you make your circuit board but what about the architectural layer how do you decide when you want to use a mixer how do you decide when you want to use a filter all these different sorts of things like my structure of switches and amplifiers and filters and stuff that I had in yardstick one like how do I come up with that solution well the way that I came up with that solution was based on my experience with software-defined radio doing things in software that now I'm doing in hardware so use SDR as a way to learn about this stuff I have a video series it's free open content that I'm working on ten lessons are up already on great sky gadgets calm and you can learn about software-defined radio there's a there's an entire lesson on decibels so you know we're talking about the p1 DB point like it specified in decibels for the amplifiers if you are not well versed in decibels go watch my video on decibels in something that everybody learns as a learning STR so STR is a great resource for you that you can use both to test your systems and also to just learn more about how radio systems work and I think we're about out of time do we have any last questions before we go oh SEC approval okay oh all right so Chris tells me I have five minutes which might be enough to answer this question no of knowledge there very briefly of the the FCC process is complicated but and it depends very much on what kind of device you're dealing with now if you want to sell a device that is a radio device in the u.s. you probably need to be dealing with FCC certification you need to be specifically looking at the equipment authorization services of the FCC the equipment authorization that's what you want to look into and however there are there are exceptions for example test equipment exempt so that's how I'm able to legally sell a car f1 for example and other software defined radio platforms that's how you know an Ritsu an agilent didn't well okay keysight and everyone else is able to sell like benchtop RF signal generators it's because test equipment is exempt so if your project is test equipment then you have a lot less to worry about and there are similar exemptions in other countries but if your project is not test equipment and you're trying to get it to a point where you need to be able to sell it then you definitely need to be looking into the equipment authorization rules of the FCC generally speaking if you use those wireless transceiver IOC's that are recommended and you follow the manufacturers recommendation you're highly likely and you follow these rules that I've given you you're highly likely to have a design that can pass FCC compliance testing and there are labs that specialize in testing your product for you and providing the documentation that you need to get your equipment authorization from the FCC if you're rolling your own RF solution you're not using one of these wireless transceiver ICS you may have more trouble but you know filter filter filter and use use the recommendations that I gave you in this in this talk how do I decide between external antenna and PCB antenna set the question um so PCB antennas are a thing you can build an antenna structure into a printed circuit board and external antennas are easy to support by putting a you know 50 ohm connectors such as an SMA connector or a BNC connector or an in connector on your board something like that I like SMA connectors a lot just because they work well at high frequencies and I tend to like to do things over wide range of frequencies but there are other RF connectors that are perfectly good and most of them are 50 ohm and are intended to work with 50 ohm impedance antennas so when I choose to use an external antenna connector it's usually because I want to give the user the option I want to give the user the option to use whatever antenna they want and when I use a PCB antenna it's usually because I don't care about that option or I I'm trying to do things as simple and as low-cost as possible fewer moving parts so for example Conference badge that has a wireless part on it throw a wireless you know PCB antenna on it and that way everybody can use it without like we're an antenna which is more expensive typically less expensive to go with the PCB in phenyl but they're worse performing so it's mostly a performance versus cost or a flexibility versus cost you do clear the layers underneath a PCB antenna and if you follow the manufacturers recommendations on this because some of the wireless transceiver chip manufacturers will actually give you P CB antenna designs that they recommend to connect to their their device use that Texas Instruments has a good application note on where they tested a whole bunch of different PCB antennae designs and you can see the specific performance of these different designs and you can copy those designs directly so I'd recommend checking that out as well behind you oh that's a good antenna - sorry good question - on antennas that's a third option you have the option of putting an external connector on you have the option of having a PCB trace antenna you also have the option of using a chip antenna which is a single device that you solder on to your PCB that is an antenna and sometimes there's a hybrid solution where you have like in a chip antenna that requires some special PCB tail trace Tatel off of it or something like that generally speaking it's the same sort of thing where it's a cost versus performance issue at but it also is a size issue if you're looking at if you decide you don't want an external antenna you definitely want to go with chip or PCB trace antenna you're trying to decide between the two usually a chip antenna will give you better performance per board area than a PCB antenna well PCB antenna will tend to take up more board area and that is the trade-off usually in my experience is that it's it's performance versus board area all right all right thank you all for coming

Channel: HACKADAY

Views: 179,636

Rating: 4.928719 out of 5

Keywords: RF Circuit Design, Circuit Design, Wireless, Radio Frequency, EE, Workshop, Hackaday, Hackaday SuperConference, Michael Ossmann

Id: TnRn3Kn_aXg

Length: 66min 21sec (3981 seconds)

Published: Wed Mar 23 2016

Reddit Comments

[u/3dprint_the_world]

Bear in mind that Michael Ossman's designs have some received some criticism; the RF front-end for the HackRF is not very well designed and has poor sensitivity.

Not to discourage anyone from entering RF design, but if you want to try it, there are plenty of great references from actual experts on the topic; this article from Maxim Integrated might be a good launching point, and the University of Utah has a great beginner course on it.

This is a fairly advanced topic so expect to spend a lot of time learning theory. It's not black magic though; it's well-established science and don't let anyone tell you otherwise.

[u/[deleted]]

I was never really interested in RF design, but I am now. What a fantastic video!

[u/AnnabelHou]

If you want to try RF PCB design, it's recommended to check out those detailed guidelines for RF and microwave PCB Design.