Signal Integrity Tools for PCB Designers in Altium Designer
With switching speeds in TTL and newer logic families being fast enough to process data at Mbps and Gbps data rates, high speed design techniques are now critical for nearly every PCB designer. As rise times in digital signals have become faster, designers must either make their interconnects shorter or device some important design strategies to prevent signal integrity problems.
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A unified PCB design package that integrates signal integrity tools alongside advanced design features.
Unless you are working with a simple DC board, a small board that operates with relatively slow switching speed, or other simpler devices that do not operate at very high speeds, signal integrity will be a chief concern in your designs. The saying goes that every designer will eventually need to worry about signal integrity problems that come with high speed signalling. Even DC boards that run on a switching power supply will need to be designed to withstand electromagnetic interference.
As design software has developed capabilities to address these signal problems, designers need to be aware of the best practices for suppressing common signal integrity problems. The best design software will include signal integrity tools that can be used to quickly diagnose common signal integrity problems in signal nets and help you devise the best strategy to address them. In many cases, there are some simple strategies you can take to solve common signal integrity problems in a circuit board.
Depending on whether you are designing an all-digital, all-analog, or mixed-signal circuit board, different functional blocks on your board will require different solutions. Working with the right design package can help you address signal problems in any portion of your board. Only Altium Designer integrates important signal integrity tools alongside your standard design features in a single package.
With high speed devices, designers have plenty of signal integrity problems to contend with. Some important signal integrity problems arise thanks to Faraday’s law and parasitics in a PCB. Parasitic inductance in a circuit board allows signals in one interconnect to induce crosstalk in another interconnect. A similar effect occurs due to parasitic capacitance between neighboring interconnects. This leads to crosstalk between neighboring traces, where a signal train in one trace induces current in another trace due to repeated switching.
Another common signal integrity problem that arises due to parasitics is signal reflection. This arises due to impedance mismatches along an interconnect, which cause a propagating wave to reflect back towards its source. Common sources of impedance mismatch occur between traces and components, at vias, along length mismatched vias, and at via stubs.
These problems can be solved by designing the proper trace geometry, via arrangement, and ensuring proper coupling to a ground plane. Any designer that is serious about high speed design will use at least a 4 layer board, and designing the right layer stackup arrangement can help suppress or eliminate these common signal integrity problems.
Signal Integrity Starts with Your Stackup
Designing a layer stack requires choosing the appropriate arrangement of power, ground, and signal layers in a circuit board. The stackup determines the effective dielectric constant of traces in a signal layer, which in turn determines the impedance of a trace in a printed circuit. This then has an influence on the potential impedance mismatch along a given interconnect. The effective dielectric constant then affects the propagation delay between a source and load along an interconnect and determines whether a given trace will act like a transmission line.
Judicious placement of ground planes in a multilayer board can provide a number of advantages. First, placing a ground plane below traces on a signal layer will provide tight coupling for return signals. Placing signal traces closer to a ground plane will create a smaller loop area, which decreases parasitic inductance for a given interconnect and helps improve immunity to electromagnetic interference.
Using a power or ground plane to separate signal layers will help ensure that traces do not induce crosstalk between signal layers. Low level digital or analog signals can also be placed between two planes in order to provide greater shielding against electromagnetic interference. This also helps you pass EMC tests in devices with wireless communication capabilities.
- There are plenty of misconceptions around high speed design, and the name itself is misleading.Learn more about modern high speed PCB design issues.
- The right layer stack can help you implement impedance controlled design in your board, ensuring your traces have consistent impedance throughout your board.
- Working with high speed and high impedance boards requires implementing high speed design techniques and simulations into your PCB design workflow.
Designing a termination network in Altium Designer
Once a PCB stackup for your has been designed to support signal integrity in your printed circuit board, you will need to consider trace impedance and propagation delay during routing. Traces should be kept as short as possible to prevent signal reflection back along a trace. Ringing can be suppressed where necessary by using LC elements to increase the damping constant in a trace, which will keep bit error rates as low as possible.
One design strategy that is useful in a high speed printed circuit is differential pair routing. Differential pairs have strong immunity to common mode noise, but they require precise routing. Traces in differential pairs must be symmetric and precise length matching between the driver and receiver. Many signalling standards are based on differential signalling, and this design methodology will continue to be in use going into the future.
Transmission Line Termination
Solving problems related to signal reflection requires impedance matching throughout an interconnect. This can involve routing traces with a consistent geometry throughout your board, only using components with consistent input and output impedance, and designing termination networks where applicable.
In the even that a trace is long enough to exhibit transmission line behavior, impedance controlled design techniques can be used to prevent reflection from loads due to impedance mismatch. This requires explicitly defining the trace geometry as a function of the dielectric constant of the substrate PCB material. Using impedance controlled matching and/or termination where necessary will reduce insertion loss into the load, ensuring you have maximum power transmission to the load component.
- There are several options for termination networks for PCB transmission lines. These networks are essentially RLC circuits that can be designed iteratively.
- Determining the termination network you need for an interconnect doesn’t have to be difficult when you use the right signal integrity tools.See how to create a termination network for your interconnects.
- Impedance controlled design will help reduce your dependence on termination networks, which frees up board space for traces and other components on your board.
Impedance controlled routing in Altium Designer
An analog circuit is susceptible to the same signal integrity problems as a high speed digital circuit. However, transmission line effects arise once the quarter oscillation period is shorter than the propagation delay along a trace. Skew also accumulates between analog signals due to phase mismatches between an analog signal and its reference, as well as due to trace length mismatch. This underscores the importance of trace length matching in analog and digital circuits.
High frequency analog circuits are much more sensitive to electromagnetic interference. Once noise induced in an analog signal trace, and that noisy analog signal is converted to a digital signal, even low level noise can lead to significant bit rate errors. Suppressing noise in analog signals requires adopting many of the same signal integrity measures used in digital circuits, including designing the right PCB stackup, proper ground plane design, and keeping traces as short as possible.
The Signal Integrity Tools in Altium Designer
Altium Designer includes signal integrity analysis and power integrity analysis tools in a single program, and these tools integrate directly with standard CAD and layout features. All these tools use the same data format and are accessible within a single program. All the design and analysis features in Altium Designer are built on the same rules-driven design engine that allows you to check your design against standard and specific design rules as you create your board.
- The integrated design environment in Altium Designer unifies your important design features on top of a single rules-driven design engine. You can define impedance tolerances, analyze crosstalk and transients, and much more in a single program.Learn more about Altium Designer’s unified design environment.
- Altium Designer contains the best features for high speed layout, simulation, analysis, and verification in a single design program.Learn more about high speed design in Altium Designer.
- The interactive routing features in Altium Designer help you control your trace length to prevent a transition to transmission line behavior. You’ll also be able to keep your signals synchronized with trace length matching tools.Learn more about the interactive routing tools in Altium Designer.
If you’re not familiar with high speed design or working with signal integrity tools, Altium gives you access to the resources you need to become a leader in the PCB design industry. You’ll have access to the AltiumLive forum, an extensive knowledge base, podcasts and webinars from industry experts, and detailed feature tutorials. No other PCB design software company gives you this many resources for success.
Altium Designer includes plenty of other features beyond signal integrity analysis tools. The layout tools are built on the same rules-driven design engine and data model as the signal integrity features, allowing these tools to easily create and access your design data. You’ll also be able to prepare your board for fabrication, generate a bill of materials and other deliverables for assembly services, and manage your component sources. All these tools are present in a single program, allowing you to take your next idea from a simple design to a finished product.