(1) - RF and Microwave PCB Design - Altium Academy

[Music] hi everyone its Ben Jordan here with the on-track whiteboard video series welcome back and today I want to talk about RF and microwave I like to use the lambda there hope you don't mind the pun RF and microwave PCB design this is actually part one of several episodes I'm going to be doing over the coming months talking in general about all the ins and outs of designing boards at radio frequencies and for microwave microwave applications cuz a lot of people are kind of intimidated by the idea of doing microwave PCB design and aren't really sure where to start and I was that guy you know I wanted to learn about it it was a little bit afraid to try it and I started doing some research and realized you know what with the right guidance any good PCB designer can do RF and microwave PCB design so that's what we're talking about and today I want to talk about a few of the absolute basic fundamental things to start with and some of those things are around materials some of those things around the actual physics of RF and microwave propagation but we'll go into more depth in the coming episodes so let's let's begin I've got a little example here of a piece of the you know section of PCB substrate and you'll note I've drawn it with some copper on the bottom there that's effectively a copper plain that's a reference plane and a trace a microstrip line trace on the top and this is pretty typical of any PCB trace and they're all they're connected to each other through the substrate the substrate is a combination of usually epoxy or some kind of thermosetting plastic material reinforced with glass fibers typically almost always some older or cheaper boards use paper like paper phenolic resin for fr1 and for the most time you know most times recently fr4 is the most common cheaply available material and you can actually do some RF design on a typical effort fr for PCB but you need to know what's going on so first let's talk about conductors at the frequencies of first of all let me actually say what is RF really RF or radio frequency is anything like 300 kilohertz are above technically and you can you can go on the FCC website for example or if you're in another country there'll be a government website that rep where they have information about laws that regulate the use of what we call the spectrum the radio frequency spectrum and but really as far as RF and PCB goes we're talking anything that's intentionally processing analog signals or amplifying or carrying analog signals from an antenna or to an antenna or at those at those frequencies kind of in the megahertz and above microwaves is when we get up around you know one one-and-a-half gigahertz and above and so at those frequencies of three things behave differently so first of all let's talk about the most important thing to know when you're dealing with different RF or microwave frequencies and this is wavelength so if if we're dealing with signals that propagate through the air from an antenna for example we're dealing with lambda zero this is the wavelength at the speed of radio frequency signal propagating through air or vacuum air is close to vacuum so it's basically the speed of right over the frequency so basic stuff right and anyone who studied or does ham radio would know like an easy way to calculate that is to say speed of light is three times 10 to the 8 meters per second but if you call it 300 you can put F in megahertz so if I have a 2.4 gigahertz which is the Wi-Fi or Bluetooth sort of range 2.4 gigahertz signal I'd say 300 over 2400 and the wavelength in air or vacuum of a signal at that at that frequency is approximately sixteen point six seven centimeters right so you're dealing with this is one full wave and half wave and a quarter wave and in future episodes we're going to talk a lot more about the significance of those multiple franck of multiples of a quarter wavelength are very important in the PCB design so sixteen point six seven centimeters now if you're in a PCB it's a bit different because in a PCB we have this dielectric and the dielectric is made up of all sorts of different molecules in a glassy kind of plastic and of course that we have the e glass fibers which are the reinforcements and if we this is an oversimplification but it's just to help understand we have all these domains of Kempe of the chemical which are insulators however those insulators like any material they're made up of protons neutrons electrons you know in some bigger structure and so they can hold an electric charge and they can't conduct but they can hold charges in different random orientations and when you put a voltage between two conductors so the trace and the the reference plane is zero and we have a pulse voltage on the tracer 3.3 volts what we're doing where that voltage you this we're forcing the molecules in the insulator to align their charged domains because like Pell like charges repel and dissimilar charges attract and so it having a voltage applied across a dielectric creates these these charge domains to orient themselves and that that's kind of a mechanical or physical it's an actual physical phenomena and that gives rise to capacitance and what we call displacement current and so as a signal travels down this trace and this is this is for any kind of digital signaling as well not just RF and microwave but also pulses in a digital system as the voltage travels down at a finite speed down this trace it has to charge those domains in the insulator between the trace and the reference plane ok so so that takes time unlike a vacuum that takes time and effort and so that time and effort to orient these charges gives rise to a different propagation velocity in the material of the substrate so in a PCB or any other material like a coaxial cable or some other kind of cable or any kind of insulator we have what's called an effective dielectric constant or decay as you may know it and this is typically an attribute on a PCB material datasheet DK it's also also sometimes called and the proper engineering term is epsilon epsilon R is the relative permittivity of the material and that gives rise to an epsilon effective which is recognizing that this material is not just one solid homogeneous material but a mixture of materials resin and glass and we kind of treat it like a homogeneous thing and it has an effective dielectric constant so let's say typical fr for effective dialogue dielectric constant decay for fr for decay would be approximately four point two and then what's the wavelength of a 2.4 gigahertz signal on an fr4 PCB with a effective dielectric constant then our formula for wavelength changes and we have what's called the guide wavelength lambda lambda sub G and that is the speed of light simply put speed of light over frequency times the square root of the epsilon right this is an oversimplification and in coming episodes we'll get into more detail not to dive too much into the maths but we'll explain it a little bit and then use some calculators because that's a lot faster okay so anyway in this case we'd have three times ten to the eighth over 2.4 gig square root 4.2 and that gives us an approximate wavelength in a PCB of about six point one centimeters right so frequent the frequency number one parameter you need to know when you get started with PCB design that microwave and RF frequencies obviously what's the frequency you're operating at that tells you what's one wavelength and from there we want to derive all our other work based on a wavelength or a fraction of a wavelength typically a quarter wavelength so when when is your PCB design going to be an RF PCB design it's when the length of your interconnects become some significant multiple of this or fraction of this I should say and the general accepted industry Stan if any of you are electrical engineers many of you I you'll remember this from your element your transmission lines class and fields and waves the general rule is when when your length of an interconnect on a PCB is going to be greater than or equal to sorry less than or equal to one twentieth of the guide wavelength at if if you're dealing with links around one twentieth or shorter those notes it's greater sorry at that point you're dealing with an RF or microwave design because the length of the traces and features you're dealing with at that point have a profound impact on how how the circuit actually operates if you're going much beyond that then or if you're much smaller than a wavelength so if your frequency is much lower then the the characteristic length of traces on the board then the length of the traces is is so tiny compared to a wavelength that they're they can be treated as lumped elements okay so let's talk about so we've talked about insulation material god wavelength all of that let's talk about conductors a tiny bit here I have a cross-sectional view I've drawn if if you get a section of a small PCB trace and you look at it under a microscope and it's polished you'll see the surface of typical PCB copper is very rough it's actually intentionally made this way they take the copper and it comes off a drum and one side of it's smooth and the other side is rough it's called electrodeposited copper because of the plating buff that's used to get copper out of solution onto a drum and they make the foil out of that and they want it rough on at least one side because that helps it adhere this tooth this is called the tooth and that helps it adhere like grab into the insulation material during the lamination cycles but that causes a problem too because at very high frequencies particularly microwave frequencies this thing called the skin effect and again we'll talk about it more in a future episode causes most of the signal to travel just in the surface just in the extremely thin surface of the copper and that effectively increases that surface roughness effectively increases the length of the transmission line significantly and leads to pretty significant losses and so we we may decide doing RF PCB design if you're going very high frequencies like above 10 gigahertz you probably are getting into a realm where you start to need to use different types of copper and you need to specify you want rolled annealed instead of electrodeposited or you may even have an a specific manufacturer part number for the copper you want to use for under 10 gig if you're not going too far and you can you can design around the losses that you'll get you can use standard fr4 and standard electrodeposited copper you just need to know what to do and finally in this introductory episode the last thing I want to talk about I mentioned this whole concept of wavelength and why it's important just just one other thing that was going to lead to what we discussed in the future is this notion that people think of current and voltage and you think of it in a in a traditional circuit analysis way that is we have electrical conductors and current goes in loops around the conductor's and in in the PCB design profession particularly within the last five years we've had a lot of really good training from industry experts like dan Vika Rick Hartley Lee Ritchie Eric Baggesen you know you've all heard of these guys and they're they're fantastic and what they've been teaching us is what we really need to think about is the energy and the energy is traveling of the energy of a signal is traveling in the space in the form of the fields and waves and this for anyone who studied electrical engineering you know this it's from your fields and waves classes in third and fourth-year engineering school but but it's important to remember why and how that actually happens we we are in the electronics industry because we are still dealing with electrons and they're charged particles electrons are negatively charged and when an electron moves away from a nucleus of its parent atom the atom itself the nucleus is positively charged because of the protons in it so in conductors electrons can move around freely and be shared and jump around and this is this is why metals form nice crystal lattices is because they're sharing electrons and the electrons are mobile and they move around and magnetic metals like iron the electrons can all move in in sync together and give rise to a magnetic field and that's how permanent magnets work for example so in any conductor we can actually place a voltage source and have have the electrons at one time instant all go away from the middle so we have a positive net positive charge in the middle of the conductor and negative at the extremities and this would look if we could graph the voltage over the distance of this conductor we'd see this kind of wave form this would be a positive half wave for example of a sine wave on this conductor where we're at zero in the middle and we have so that's a sine wave they're just we're just looking at it phase-shifted we have negative at the extremities positive in the middle and zero like net zero charge somewhere a quarter of a wavelength in from each and now at another time instant all the electrons that it's just think of it it's like a trough and there's water and there's a wave of the water in it at one moment there's more water in the middle and that the next it's down in the middle and bigger at the eds and it can go back and forth like this right so here we have a positive goes to a trough and a peak at each side so we're positive over here and here and in the middle we're in a negative and this when we have this is one full wavelength right that's one wavelength if I can draw that upside down on that piece of conductor and when we start vibrating this around and around this gives rise to propagation and most people have heard of the term dipole a dipole is a piece of metal effectively or two pieces of metal what this connected to an oscillating voltage source and this whole thing is one half wavelength apart so at one instant with this voltage source pushes all the charge down to one end and it's very negative here and very positive here and then it reverses and pushes all the charge back up here so it's very negative here and then very positive here and that gives rise to propagation away from that and this is how antennas are built this is how antennas work and so you know here I've got RF drawn like antennas and we'll talk about PCB antennas in our final episode because that's the most complex sort of thing to get to but we'll just give a basic introduction of what kinds of antennas are out there and also these metal structures that are multiples of one what a wavelength they're very useful in designing the actual board so so we'll go into some detail in episode maybe three about what we can do to design an RF circuit using just pieces of copper on the board which is pretty cool I think that's pretty cool so anyway that's all I've got time I've probably spent way too much time today but I hope you find that useful if you really like this and you want to pay attention to the series and know when the next edition or when the next episodes coming up please subscribe if you liked it give us a thumbs up and share it please feel free to share with your colleagues and friends in the industry and comment below and I'll do my best to answer any questions and thanks very much for watching this is the on-track whiteboard series [Music]

Channel: Altium Academy

Views: 37,863

Keywords: Altium, PCB Design, electronics, engineering, technology, printed circuit board, EDA, electrical engineering

Id: bVdwu1IoX4k

Length: 21min 4sec (1264 seconds)

Published: Thu Apr 02 2020