Incorporating Impedance Calculations Into Your Design Rules
Once you start designing high speed or high frequency PCBs, impedance in your traces becomes a critical parameter to consider during design. Impedance mismatches between signal sources, loads, and the traces that connect them can ruin signal integrity and cripple your PCB. Instead of working out the impedance of each trace in your PCB by hand, Altium Designer can incorporate your PCB trace impedance calculations as part of your design rules. This streamlines your design process and ensures that signal problems remain under your control.
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A fully unified, heavily rules-driven PCB design package with PCB impedance calculator.
Just like any other area of engineering, a number of equations are used to define and calculate various design parameters in PCBs. These equations specify everything from the substrate dielectric properties to the electrical characteristics of your traces. Despite the number of equations involved when designing a new PCB, your design software can play a unique role in streamlining these calculations and using them for design validation.
Rather than reinvent the wheel, your PCB design software should integrate these equations as part of the design process. You won’t need to have every single equation memorized, and you won’t have to work out calculations by hand. Your design rules should then check your design against requirements that you specify with these equations. The best PCB design software will integrate these features with your design tools, helping you maximize your design productivity.
Your design rules aren’t there to constrain your creativity. They’re built to ensure that your printed circuit board will work as you intended and that all its functions will play nice with each other. If you’re unsure of how best to route those traces between components, where to place components with respect to ground/power planes, or whether you’ve properly designed your pads and vias, your design rules are there to keep you in check.
Your design rules are also meant for more than just checking component placement. Certain features in your layout like traces and vias have important rules and constraints on their geometries and spacing that should be followed in order to maintain signal integrity. If your PCB design software has a powerful rules checking engine, you can avoid design errors that can hamper signal integrity and ensure that your device works properly as soon as it leaves the production line.
How Design Rules Ensure Signal Integrity
Issues like impedance mismatches between components and traces, via placement, trace length matching and spacing, and many other design parameters are specified with the goal of ensuring signal integrity in your device. Great PCB software will include these standard design rules and allow you customize design rules that are specific to your application.
- Your PCB software should make it easy to work with design rules and constraints within a single interface.
Learn more about working with design rules and constrations.
- A number of methods can be used to terminate traces, depending on the particular application for your device. Your PCB software should be adaptable and allow you to apply any termination method in mismatched traces.
Learn more about termination methods for your high speed/high frequency PCBs.
- When working with differential pairs, your trace spacing and geometry will determine your impedance in an interconnect. This is one of many parameters you can specify in your design rules.
The design rules editor in Altium Designer
In some cases, impedance calculators can be very useful as they allow you to calculate the impedance of a trace arrangement rather quickly. The equations that define the impedance in a trace are specific to the geometry of the trace, its arrangement in the board (microstrip vs. stripline), whether the trace arrangement is asymmetric, and the substrate material surrounding the trace. Keeping a trace impedance calculator around can be useful as long as you know when to use them.
It’s not that trace impedance calculators are difficult to use, but rather that they have limitations as far as their applicability. For example, the equation for the trace impedance in a differential pair of asymmetric striplines includes an approximation on the material parameters and geometry. Other calculators will use different equations for the same system, depending on the form of the approximation they invoke. Most calculators will allow you to place invalid values that fall outside the approximation range, which then produce inaccurate results.
Accurate Impedance Calculations
Instead of trying to rely on an external calculator, write down calculation results, and design traces around this particular geometry, your PCB design software should integrate these capabilities into your design rules and check your impedance values automatically during design. This gives you a major productivity boost and allows you to automatically check your layout against these critical design parameters.
Since PCB design rules need to be adaptable and trace impedance values will have some tolerance level, your design rules editor should make it easy to set the appropriate constraints for your trace impedance. The equations your program uses to determine trace impedance should also be editable, as this allows you to account for any deviations from the typical approximations used in trace impedance equations.
- Impedance matching and trace termination are just two aspects of high speed design that help ensure signal integrity.
Learn more about maintaining signal integrity in high speed design.
- Trace impedance and resulting signal integrity problems can also be diagnosed using powerful simulation and analysis tools.
- Your layer stack will affect your trace impedance, and this should be accounted for when defining design rules and routing traces.
See how your layer stackup can affect your trace impedance.
The layer stackup manager in Altium Designer
Whether you’re working through differential or controlled impedance in your circuit board, or trying to design an adequate ground plane with necessary trace width and impedance control, you’ll be better off with software that can be taught to read the problems you’re looking for. Whether a differential pair or trace thickness in your circuit, you should have the best design environment.
Design rules in your PCB design software need to integrate with the rest of your design tools, helping you ensure your device remains functional. Your layout and routing tools need to access your design rules to ensure that your design will be functional. Signal integrity is vital in high speed design, and your PCB design software should contain the best simulation tools for verifying the functionality of your device. Design rules can only anticipate so many problems, and signal integrity simulations are vital for verifying the functionality of your device.
Unified Design in Altium Designer
All of this communication between design tools is only possible in a fully unified design environment. Altium Designer places rules checking and compliance at the core of the design process, and these verification tools are built to work with design tools in a single interface. The rest of the PCB design industry still segregates all of these features into separate programs with different workflows. Instead of working under the same old paradigm, it’s time to take the plunge into unified design.
- The environment in Altium Designer is heavily rules driven. This ensures that your design choices guarantee functionality without sacrificing creativity.
Learn more about rules driven design in Altium Designer.
- Examining signal integrity allows you to diagnose problems that can’t be anticipated with design rules. A great simulation package is the best choice for verifying signal integrity in your design.
Learn more about signal integrity analysis in Altium Designer.
- Interactive routing features in Altium Designer interface directly with your design features, ensuring that your trace impedance will always remain within tolerance.
Learn more about interactive routing in Altium Designer.
A unified design platform brings your design features together in a single interface. You’ll only find integration between your design rules and accurate impedance calculations in a unified PCB design environment. This important area of analysis helps ensure that your next high speed device won’t be crippled by problems like ringing, noise coupling, or EMI.
Now you can get access to all the design support you need directly from Altium. Everyone that uses Altium Designer can access the AltiumLive forum, podcasts with design experts, a comprehensive knowledge base, and webinars provided by industry experts. Altium Designer never wants you to design alone and provides all the support you need.
The best design, simulation, and rules checking features will ensure that your device does not fall victim to signal integrity problems and will work as designed. Only Altium Designer supplies you with all these tools and many more in a single software package. These tools work together as part of a single platform that is governed by a rules-driven engine. Instead of guessing at the trace geometry, Altium Designer can help you get it right the first time.