High Speed Digital Design Pdf

[Zareen Zapata]

Todaymost PCBs can be considered to be at risk of some type of signal integrity problem that is normally associated with high speed digital design. High speed PCB design and layout focuses on creating circuit board designs that are less susceptible to signal integrity, power integrity, and EMI/EMC problems. While no design is ever totally free of these problems, by following these high speed board design guidelines they can be reduced to the point where they are unnoticeable and do not create performance problems in the final product.

The PCB stackup you create for a high speed circuit board will determine the impedance, as well as ease of routing. All PCB stackups include a set of layers dedicated to high speed signal, power, and ground planes, and there are several points to consider when assigning layers in a stackup:

Before designing your PCB stackup, consider the layer count needed to accommodate all digital signals in your design. There are several ways to determine this, but these methods rely on a bit of math and some past experience in high speed board design. In addition to the points listed above for considering layer count, large high speed ICs with BGA/LGA footprints can dictate the required board size. When doing BGA fanout, you can generally fit 2 rows per signal layer, and make sure to include the power and ground plane layers in your layer count when building a stackup.

FR4-grade materials can generally be used in a high speed digital design as long as the routes between components are not too long. If routes do become too long, there will be too much loss in your high speed channel, and components on the receiver end of the channel may not be able to recover signals. The primary material property to consider when selecting materials is the loss tangent of your PCB laminates. The channel geometry will also determine losses, but generally opting for a lower loss tangent FR4 laminate is a good place to start in smaller boards.

If your routes are too long, then a more specialized material might be needed as the substrate for your high speed signals. PTFE-based laminates, spread glass laminates, or other specialized material systems are a good choice to support larger high speed digital boards, where routes are very long and low insertion loss is required. A good entry-level high-Tg set of laminate materials for small-sized high speed PCBs is 370HR. For larger boards, something like Megtron or Duroid laminates are good options. Check with your fabricator to make sure your material selection and proposed stackup are manufacturable before proceeding.

Impedance is normally calculated using a formula or a calculator with a field solver tool. The impedance you need in your design will determine the dimensions of your transmission line, and the distance to nearby power or ground plane layers. The transmission line width can be determined with some of the following tools:

Using a layer stack manager with a field solver will give you the most accurate results while accounting for copper roughness, etching, asymmetric line arrangements, and differential pairs. Once the impedance profile for your traces is calculated, it will need to be set as a design rule in your routing tools to ensure your traces have the required impedance.

Most high speed signal protocols, such as PCIe or Ethernet, use differential pair routing, so you need to design to a specific differential impedance by calculating trace width and spacing. Field solver tools are the best utilities for calculating differential impedance in any geometry (microstrip, stripline, or coplanar). The other important result from your field solver utility is the propagation delay, which will be used during high speed routing to enforce length tuning.

Once components are placed, you can set up your PCB design tools to help you start routing your design. This is a sensitive part of high speed digital circuit design as incorrect routing can ruin signal integrity. However, if earlier steps were completed properly, signal integrity is much easier to achieve. You should set your impedance profile in your PCB design rules so that any routes in the design are placed with the right width, clearance, and spacing to maintain controlled impedance during routing.

The design rules you set in your high speed design project will ensure you meet impedance, spacing, and length targets as you route your design. In addition, important rules in differential pair routing can be enforced in your routing, specifically minimized length mismatches to prevent skew and enforced spacing between traces to ensure differential impedance targets are met. The best routing tools will allow you to encode your trace geometry limits as design rules so you can ensure performance.

Power integrity is a broad topic that is highly relevant in high-speed PCB design. Ensuring stable power delivery to high speed components is critical in PCB design as power integrity problems can masquerade as signal integrity problems. Power integrity focuses on low-noise power delivery to components. The PCB stackup and the layout of the PDN are the major factors that determine the level of power integrity in a digital design. If done successfully, power will be delivered to fast digital components with low noise and very weak transient oscillations on power rails. Designing a high-speed PCB with good power integrity ensures the low emissions, low noise power delivery, and elimination of some SI problems seen in high-speed interconnects.

The best high speed PCB design software will bring all these capabilities together into a single application, rather than forcing you to use separate workflows to overcome different design challenges. High speed PCB layout designers must perform a lot of work on the front end to ensure signal integrity, power integrity, and electromagnetic compatibility, but the right high speed layout tools can help you implement your results as design rules to ensure the design performs as expected.

More advanced PCB design software will interface with simulation applications to help you perform industry-standard analyses. Some simulation programs are specifically geared towards evaluating signal integrity and power integrity in a new design, as well as examining EMI in a PCB layout. Simulations are very useful in high speed design as they can help users pinpoint specific SI/PI/EMI problems before a design is taken into manufacturing. Some examples include return path tracking, locating an impedance discontinuity in traces, and ideal placement of decoupling capacitors to prevent EMI.

When you need to build advanced high speed digital systems while ensuring you maintain signal integrity and power integrity, use the best set of high speed design and layout tools built on a rules-driven design engine. Whether you need to layout a dense single-board computer or a complex mixed-signal PCB, the best PCB layout tools will help you stay flexible as you create your high speed PCB layout.

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 2500+ technical articles on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA). He previously served as a voting member on the INCITS Quantum Computing Technical Advisory Committee working on technical standards for quantum electronics, and he currently serves on the IEEE P3186 Working Group focused on Port Interface Representing Photonic Signals Using SPICE-class Circuit Simulators.

This page presents key features and enhancements that will become available in the next update to Altium Designer, but which are currently still in beta. Also included are features that are in Closed Beta - strictly only available to the beta user group

High-speed digital standards are quickly evolving to keep pace with emerging technologies such as 5G, Internet of Things (IoT), artificial intelligence (AI), virtual reality (VR), and autonomous vehicles.

Faster networking speeds require faster memory and faster serial bus communications. Peripheral Component Interconnect Express (PCIe) expansion bus speeds are evolving from PCIe 4.0 to PCIe 5.0 to support these increased speeds. The same is true for memory, as double date rate (DDR) memory evolves from DDR 4.0 to DDR 5.0. Increasing speeds of serial data communications requires precision high-speed testing at every level. Testing at these faster speeds requires full compliance testing to the latest standards.

Data center speeds are moving quickly from 100 gigabits per second (Gbps) to 400 Gbps to support exploding computing and performance demands in the network. Multilevel signal modulation technologies, such as 4-level pulse amplitude modulation (PAM4), will enable 400GE. You need scalable, reliable, and high-performance interconnectivity both inside the data center (intra-DCI) and between data centers (inter-DCI). We can help you seamlessly reach the next speed class in your high-speed digital interconnects.

5G and IoT will enable billions of new devices, such as smartphones, tablets, personal computers (PCs), and home entertainment systems, to connect to the network faster than ever before. You need to design interfaces to your consumer electronics using next-generation high-speed digital standards. Each new generation introduces new test challenges. Whether you are using Mobile Industry Processor Interface (MIPI) for your mobile device interfaces, High Definition Multimedia Interface (HDMI) for digital audio or video, or Universal Serial Bus (USB) for a wide range of consumer electronics, we can help you ensure the quality and performance of your devices.

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