Fundamentals of Signal Integrity - why is it important?

JULIANO MOLOGNI: OK, good afternoon, everyone, and thank you for your time. My name is Juliano Mologni. I'm a product manager working for Ansys. And today we're going to talk about the fundamentals of signal integrity-- what it is and why is it important.

So this is the agenda. First of all, I'm going to talk about the evolution of electronics. And then we're going to do a deep dive on the fundamentals of signal integrity and, of course, electromagnetic compatibility and interference. There's no way to decouple signal integrity from EMI and EMC. And of course, I'm going to talk about the benefits of simulation to drive innovation in improving signal integrity and electromagnetic compatibility, especially using a new extension that we are announcing now, during the Autodesk University, called Fusion 360 Signal Integrity Extension.

So let's get started and talk about the evolution of electronics and smart devices. I got this slide from one of our customers, Whirlpool in Brazil. But that reflects the evolution of most of the smart devices.

This is a washing machine. And if you take a look on the left side, you see a washing machine that is 20 years old. And you can see the electronics content, back at that time, is an electromechanical timer.

If you go some years-- if you move to today's, you take a look at the washing machines today, they have Bluetooth connectivity, Wi-Fi connectivity, they have touchscreen display, they have inverters, microcontrollers-- so we have a much higher content of electronics in the washing machine today. And that's also valid and applies to many of our smart products, like watches, appliances, and basically everything else.

The problem is that, when you have more electronics, when you add wireless connectivity, you're not only increasing the density of electronics but you're also causing some issues, electromagnetic-interference issues. Also, if you have a product like a washing machine and it does have Wi-Fi and Bluetooth connectivity, you have to go through some EMC certification tests. So it becomes more challenging to design electronics that has some wireless connectivity and high-speed channels.

So when you have those smart devices with microcontrollers, USB connectivity, you likely have signal-integrity issues I'm going to explain what's signal-integrity issues. But it's basically how we can make sure that the signals are transmitted within our electronics in a very reliable way.

Also, when you have more electronics, of course, you have more electromagnetic interference. So it becomes really critical, the way that you design your PCBs and where you're going to place your antennas. And of course, electromagnetic compatibility-- you need to make sure your product is compatible in your environment, so it does not interfere on other electronics and it's not interfered by external electromagnetic noise.

So let's start with signal integrity. Signal integrity is actually the integrity of the signals. Every time that you're trying to transmit inside an electronics-- could be image, audio-- we're using digital signals. So that's what we see here, down on the bottom side, on the left.

If you overlap all those digital bits, what you're going to create is what we call an "eye diagram." So that's what we're showing here, on the right side, is an animation showing the eye diagram. What we are doing is overlapping those digital signals.

So the signal integrity is actually making sure that, in the end, on the receiver, the receiver will be able to identify the logic-- zero or one. That's what signal integrity is all about. So, as you can imagine, if you have an open eye, that's good, because the receiver can identify what is zero and what is one. And if you have a closed eye, that's a really bad thing because we're not going to be able to reconstruct the data that we want.

So if you take a look at many electronics products, you have print circuit boards with vias. You have connectors. You have cables. And every time that a signal sees those 3D structures, part of the signal is reflected back to the source, part of the signal is radiated, and, of course, part of the signal goes towards the receiver. And that's what we want. We want most of the energy going towards the receiver.

So for that, we need to control a few things. We need to control the design of the traces of the PCB. We need to control the impedance. We need to control the design of the connectors. We need to choose very good connectors.

So if we take a look in here, on the point A, we're sending really perfect digital signals-- really perfect square signals. But of course, as I said, as the signal is traveling through those PCB traces the signal is distorted. And what we want to make sure-- that, in the end, on point B, the receiver is going to be able to recover all the signals-- the zeros and ones.

So this animation shows a path. The path here is comprised by vias, by traces, micro strips, strip lines, connectors. . And then we have our transmitter, and then we have a receiver.

So we're sending the signal, the digital signal, from the transmitter. And as the signal sees those vias, those traces, and connectors, you see that there is a distortion on the signal. And if you plot the eye diagram and the transmitter and anywhere in your channel, you'll see that the eye diagram is closing. And that's not a good thing. What we want to do is to make sure that the signal reaches the receiver and the eye is still open.

We do have several simulation technologies that enables the users to extract all the electromagnetic behavior of any 3D geometry. There are tools like HFSS, where you can simulate anything in 3D, like print circuit boards, connectors, antennas. There are tools that are really focused on print circuit boards and IC packets like Ansys SIwave. And there are some other tools that can extract electrical properties, like resistance, inductance, and capacitance function of frequency for any 3D geometry, like Ansys Q3D.

This is an animation where we're showing a signal propagating from a package through the bond wires. So here on the right-hand side, you see the signal and the magnetic field. And here you see the eye diagram.

So if you extract the diagram at different points on your channel, you see that the eye diagram is closing. We just need to make sure that the eye diagram's open enough so the receiver can actually identify the logic levels zero and one.

So, in the end, for many of the protocols that we have today, like USB, PCI Express, we do have those-- what we call those "masks," in here-- those red masks. So those masks, they're used to be a compliance for those protocols. So what we need to do is actually to plot the eye diagram on the receiver. And if the eye diagram is overlapping those masks, it means that your channel is failing. Your design is failing. You have to redesign your print circuit board or your package.

And there are many things that affect the integrity of the signals. One of them is the attenuation due to dielectrics and conductor materials. So usually we see fer-4s used as dielectric for PCBs. But, for very high-speed channels, we see low-loss dielectrics, like Meg 4, Meg 6.

There's always a reflection, due to 3D geometries, like a via. If it's not really well designed, you see that part of the signal will reflect back to the source. And that's something that we want to avoid.

And also there's dispersion, due to the material combinations and nonideal conductors. And of course, there is electromagnetic interference, what we call "crosstalk," which is the electromagnetic noise induced by nearby conductors. And of course, there is thermal noise heat generation, especially in chips-- and, of course, (EMPHASIS) of chips-- due to the high transient and DC currents.

We always have more and more problems, when we're talking about signal integrity. We also have manufacturing tolerances. So when you're designing your PCB in Fusion, you know, the cross section of the trace is like a rectangle. But due to the manufacturing process, you see that this is not an exact rectangle. And that changes the impedance. That can change the signal integrity, especially at much higher frequencies.

Also we have the copper roughness. That's actually what attaches the copper to the dielectric. That can also change and affects the signal integrity.

We also have material variation-- the electromagnetic properties both of the copper and the dielectric-- that can change, based on temperature and humidity. And we need to take that into consideration. Also we have cable termination. Depending on the cable and the connector that you're using, that can also affect the signal integrity.

This is one example of a small change on a via and how that impacts the signal integrity. If you-- via, it's a vertical interconnect axis. It's just basically a hole with solder. So if you have a PCB with multiple layers and you're routing the top layer and you want to make a connection to other layer, we're going to use a via.

And this is a PCB. If you look on the side of the PCB on the cross section, you see this via. What happens is that this stub, which is the excess of solder, is actually one of the reasons that it creates an impedance discontinuation. You see here the electric field. It radiates in additional layers.

And if you go here and you drill, you remove that excess of solder, you see that your signal integrity is going to improve a lot. So this technique is widely used, especially on SerDes channels. "SerDes" stands for Serializer/Deserializer-- you know, very high-speed channels. So in order to improve the signal integrity, even though you're using very low-loss dielectrics, sometimes you have to do something like this. You see that the eyes overlap in the mask, so it's violating the mask. But if you perform those back drilling, you can improve signal integrity.

And if you take a look at the PCBs and connectors, you can create what we call a "serial channel." You can simulate the entire channel, of course, but sometimes you want to optimize pieces of your channel-- like the width of the traces or the radius of the vias. So what we can do is actually extract models for all of these pieces of your channel and run what we call a "design of experiment."

So you can optimize your design and get what we see here, which is actually hundreds of eye diagrams showing the best design combination and, of course, the worst design combination. What you want, of course, for signal integrity, is a wide, open eye. So this one in the middle is not good, but the other ones in here, they show a really good signal-integrity performance.

This is one example from smart modules where they were using impedance calculation to improve a DDR print circuit board. So this print circuit board is connecting some DDR modules. And in the past, when they created the first revision of the PCB, they were not concerned about impedance and crosstalk. So they had an eye diagram which was not overlapping the masks.

But due to the manufacturing processes, they were really concerned about this. They say, hey, I want to improve the design so I don't have any issues in the future, even though we know all the manufacturing tolerances. So they were controlling the impedance. So they're redesigning all the lines that were sending data to the memory-- and of course, the clock and then address lines. So they could actually improve the eye diagram.

So if you take a look in here, the eye diagram is much more open. So this is a much better design. And they were doing this just by controlling the impedance and the crosstalk on the PCB that they were designing.

And as I said before, there is no way that we can talk about signal integrity without talking about electromagnetic compatibility and electromagnetic interference. Instead of going through the official overview and description of electromagnetic compatibility and interference, I'm going to show a simulation that probably is going to make you understand what EMI and EMC is.

So imagine that you have someone using a driller really close to a TV. Right? You will likely see this. This is what we call "electromagnetic interference."

So the driller is interfering on the performance of the TV. And there are basically two mechanisms for electromagnetic interference. One of them is radiated emissions. So the motor on this driller is actually generating electromagnetic noise that is transmitted from the driller to the environment, and it's reaching the TV.

And we also have conducted emissions, conducted noise. So that driller is generating some noise. That noise is going back to the power plug through the cables that is also powering up your TV. So we need to make sure that, all of our products, they are not generating enough emission so they can pass the EMC test. How do we control those emissions?

There are several tests. If you take a look at the list of standards in here, there are hundreds of them that controls those conducted radiated emissions and many more tests. And what we have seen is that, on the traditional workflow, usually we have someone inside a company that knows the best design rules on how to design a PCB. But the problem again is, as I was showing before, is that those rules now are not exactly valid anymore, because you're now adding antennas, you're adding high-speed memory devices, you're adding much more electronics.

But still, They know some rules. They design a PCB. They build their prototypes. They integrate the PCB usually in a case, in a housing.

And then they go to the lab, to perform the EMC test, to see if they're going to pass the certification so they can sell the product. At that stage, the only thing that they can do is pray, because changing the prototype at that stage becomes extremely expensive and you don't want to do that. Right?

So one way that we can actually help anyone designing electronics products is use simulation. Some of the tests, electromagnetic-compatibility tests, they can be performed in-house-- like we see here, on the left side, this is a [INAUDIBLE]. But some of them, like, you're trying to measure the electromagnetic noise that is being radiated from a product, like this microwave oven in the middle, or a car, that requires lots of expensive lab equipments. We're going to need an anechoic chamber, we're going to need some big equipments, and that becomes very expensive. And you don't want to fail on that test, because you're going to have to rebuild your physical prototype. And that's going to cost not only time but also money.

So we do have a technology today that can provide you some virtual compliance for many of these tests. So what we are seeing here is a laptop. And as you can see, it's on a turntable. And then we have an antenna.

That antenna, down here, is actually capturing the radiated emissions from the laptop. And since electromagnetic fields is something that is very difficult to see, we have to put those devices under tests on a turntable. So we need to make sure that we're capturing the maximum electric field generated by any device.

And on the right side, you see a plot showing some measurements and simulation. The difference in here is that the simulation was performed when you have your CAD and your PCB in Fusion. You take that information, and you run your simulation, and you get this result.

And measurements, of course, you need the physical prototype. You need the physical PCB. You need your physical laptop, and need to go to a lab. And if you take a look at the results, they're very comparable.

One of the reasons that I want to show electromagnetic interference is that EMI can cause signal-integrity issues. Imagining the cell phone. Here we have a cell phone, we have a USB connector, and then we have two antennas-- one GSM and a Bluetooth. So this is a very old technology.

If you take a look at, here, the eye diagram on the USB connector from this chip, it's good. It's not overlapping this green eye. But when we turn on both Bluetooth antenna and the GSM antenna, you see that we're going to generate some electromagnetic interference on those signals that is actually now overlapping the eye. So when you turn on the antenna, the USB here is failing. So that's one of the interesting EMI effects that we are seeing here in mobile devices.

Another technique that we use to decrease EMI is to use spread-spectrum clock generation. Actually we change the duty cycle of the digital signals so we can decrease the peak of your signals. But what happens is that, when you increase the duty cycle, you see that the eye diagram is also affected. So that's another way where we need to make a compromise between electromagnetic performance, electromagnetic-compatibility performance, and also signal integrity.

And let's talk about today. Let's talk about a new technology called "5G." All of these 5G devices, they have millimeter [INAUDIBLE] antennas. Which means that we don't have one antenna anymore. We have an array of antennae. But why do we want an array of antennae?

One of the advantages of using an antenna array is that you can control what we call the "radiation pattern." So instead of moving mechanically the antenna, you can change the excitation of each of these antennas individually, and you can steer the beam towards the user. So if you have a 5G millimeter wave cell phone and you are walking around the room, the micro cell on the other side is going to follow you. So we're going to make a lot more efficient use of the electromagnetic energy.

But what happens is that, what if you change the radiation pattern towards the electronics? You see here, this eye diagram is from a DDR4 memory. So we have a communication from this chip to a memory. And depending how you're holding your cell phone in here, or depending where you're walking, this radiation pattern can change and can also interfere in the signal integrity. So this is a new EMI effect, electromagnetic-interference effect, that we have been observing a lot lately on devices that are using 5G technology.

So as you can see here, there are only two types of engineers-- the engineers that have EMI problems and signal-integrity problems, and the engineers that are going to have EMI problems and [LAUGHS] signal-integrity problems. We're using more and more electronics, we're using newer technologies, and we need to make sure that we're designing our products properly to support them.

So I think by now it should be clear, the benefits of simulation. I like to show this plot, because it shows the development phase of any product development and the cost of a change. So when you're here on the concept design, you know, you're in Fusion-- you're changing the layout of a PCB, you're changing mechanical CAD-- the cost is 1x. After you have a physical prototype, if you want to change that, it's going to cost at least 100x more.

So usually what we see is that we have companies-- they know how to design their products. But after they have the prototype, the physical prototype, they say, hey, maybe I should place this antenna somewhere else. You know, maybe we should change something. And that's when your cost is 100x more.

But if you bring all these changes-- the y-axis is the number of changes-- back to the concept design, when you're in Fusion, and if you can get the same outputs that you'd get from a physical test, that makes you very smart because your total design cost is going to be much less. Not only that-- when you go to the certification tests, you're likely going to pass, because you have lots of insight in your design. And that's valid not only for PCBs but also for mechanical CADs.

So one of the things that we'd like to show is that, with a certification test, usually you have a report that says, hey, you know, your product passed or your product failed. But you don't know where to change, where to fix, on your prototype if you fail. But with the simulation results, you can actually get more insight.

You can see electric fields. You can have scanners that can show you where you need to fix-- like, hey, you have to improve your impedance in here. Or maybe these two traces are generating too much crosstalk. Right?

So you can actually troubleshoot, so you know where to fix it. So that's the power of simulation. And with that being said, I would like to show the Fusion 360 Signal Integrity Extension that we are announcing now during the Autodesk University 2022. This is a mockup. This is not, of course, the final design and view of the Signal Integrity Extension. But you're going to be able to see how that extension is going to help you design products better.

So this is the Fusion 360. You see, under the Simulation tab, if you are designing the PCB, that you have Analyze Signal. And that extension can calculate and provide you insight on many things on your PCB design.

You can compute resistance, inductance, capacitance. You can compute coupling, crosstalk, and also characteristic impedance. So if you plot the characteristic impedance in some of the traces, you see a red region that is showing, hey, there might be something wrong in here. And at first glance, it doesn't seem that there is nothing wrong. Right?

If you take a look at, here, where we're displaying the same layers, but if you enable the visibility of the bottom layer you see that there is a void on the ground plane. And that generates an impedance discontinuation, and that can radiate a lot. So, with the extension, you can actually fix that right away. But if you want more information-- hey, I want to see the fields-- you can go to Ansys. So you can click on the Ansys button. And we're going to import, with a single click, all the design and information required to run the simulations.

So this is actually the batch that we are using as an asset for the Autodesk University 2022. And once you're in the Ansys environment, you can run thermo, mechanical, optical simulations and electromagnetic simulations. So in this case, we're using the same geometry that it was designed in Fusion 6. And now we're going to compute the radiated fields, and we're going to run a signal-integrity analysis to evaluate the eye diagram.

So this is the fields that we see on the surface of the PCB. And if you go back and compare with extension in Autodesk, you'll see exactly the places or the regions on your design where you have to improve. So here on the top, you'll see where we have a hole-- a void. And down on the bottom, we fixed that. There's no more void.

But of course, if you want to quantify the electric field, if you want to run really advanced signal-integrity analysis, you can always go to Ansys. So here you see where you have this hole. You have a hot spot of electric field. If you fix that, there is no electric field radiation from that hole. And you can also take a look at the signal-integrity analysis, the eye diagram itself.

So, to wrap up, the key takeaway that I think we have here, I think it becomes really clear the benefits of using simulation really early in the design cycle. And if you have access to that simulation capability within the Fusion environment, that makes it even better, because if you have to go to another tool, like Ansys, another environment, that could be time-consuming. But if you have part of the technology that can actually show you where you have to improve your design, that's extremely valuable-- especially now that we have electronics with antennas and high-speed digital signals.

So, with that, I would like to thank you all for your time. And I'll finish my presentation. Thank you very much. [LAUGHS]

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