Manage PCB Trace Lengths for Optimal System Timing
Once switching speeds in TTL logic circuits became fast enough to process data at 10 Gbps, high speed design techniques became critical for every PCB designer. If you didn’t have to worry about high speed design, then you weren’t designing to a professional level. Chief among these considerations are propagation delay, crosstalk, and timing skew. Any one of these effects can corrupt signals and lead to data desynchronization in different areas of your board.
Manually compensating timing differences requires implementing trace length matching throughout your signal nets. This technique is easy when you work with design software that offers the best CAD tools in a single package. Only Altium Designer gives you these design features and many more in a single package.
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A unified PCB design package with automated PCB trace length matching tools.
Any electronic signal requires a specific amount of time to travel through a conductor from its source to its destination. An electronic signal, whether digital or analog, travels along a trace with a velocity that equals the speed of light in the conductor. This speed, in turn, depends on the effective refractive index of the trace. The dielectric constant of the board, the geometry of the trace, and the dielectric constant of the conductor all determine the speed of signals in your traces.
Although propagation delay is normally used in digital electronics textbooks to refer to the amount of time a signal requires to propagate through an integrated circuit, propagation delay in a PCB refers to the time required for the head of a digital signal or the rising edge of an analog signal to reach its destination. As computer peripherals and other digital systems require successively faster operating speeds, the propagation delay in a computer network places tight tolerances on the allowed trace length in a conductor carrying digital signals.
The same idea applies to analog signals. The first quarter oscillation of an analog signal can be viewed in a similar manner as a digital signal. The wave requires a specific amount of time to travel from the source to the load, thus timing issues can arise when working in an analog signal that requires precise phase matching among signals. Timing issues also become important in mixed signal devices that require precise timing between an analog signal and digital signals or clock pulses.
So what is the best way to ensure your all-digital, all-analog, or mixed signals remain synchronized? There is no clear cut answer as the best solution depends on your application. Most of the time, different functional blocks on your board will require different solutions, and you will have to judiciously devise methods to keep your signals synchronized throughout your board. While this can be difficult to account for, it becomes much easier to keep signals synchronized when you use design software that makes it easy to address any signal timing issue.
When you’re working with parallel data in a signal net, the simplest solution to keeping your signals synchronized is to implement trace length tolerance matching throughout your PCB. Keeping your parallel data synchronized requires taking consideration of the data transfer rate in your system, rather than the rise/fall time of the digital signals in your board. If the traces carrying your digital data are slightly mismatched from each other, you’ll soon find that parallel signals are arriving at a load at different times, producing bit errors and causing downstream blocks to malfunction.
Instead of working with the velocity of signals in a trace, most PCB designers and electrical engineers convert the signal velocity to a time to travel a certain distance. In PCBs on FR4, the common value used for trace timing is 0.16 ns/inch. Your PCB design software should include standard signal velocity values or allow you to define a specific value for your board material. Your design rules should also allow you to define the trace length tolerances you need to keep your signals in synchronized.
What About Analog Signals?
One should note that the term “high speed” has nothing to do with data rate, but it has everything to do with signal rise time. As a result, high speed signalling has been around since before oscillators were capable of producing the clock frequencies required to synchronize data at such high speeds. Trace length matching is one critical step to ensuring that your differential pairs and your parallel data traces remain synchronized throughout your board.
Analog signals are susceptible to the same signal integrity problems as high speed digital signals, but the transition to transmission line behavior depends on the quarter wavelength of the analog signal on a trace. Skew accumulates between analog signals due to phase mismatches between an analog signal and its reference, as well as due to trace length mismatch. Going further and compensating for phase mismatch between analog signals requires using phase-locked loops in your PCB.
- There are plenty of misconceptions around high speed design, and the name itself is misleading.Learn about modern high speed PCB design issues.
- Digital signals aren’t the only signals that suffer from desynchronization due to propagation delay. Analog signals can form standing waves due to reflection at impedance mismatchesLearn more about timing delay and analog signal resonance.
- Desynchronization between signals in your PCB leads to skew, which increases bit error rates in downstream components due to timing errors.See how your PCB 3D STEP models improves productivity and collaboration.
3D view of length-matched traces in Altium Designer
Propagation delay is notorious for producing signal synchronization problems, but there are other problems that arise in high speed or high frequency signalling. When the propagation delay is larger than approximately one-third the signal rise time, any impedance mismatch at the load can cause signal reflection, sending a digital signal back towards the source. This leads to a signal resonance effect called ringing, which appears as an underdamped oscillation superimposed on the propagating digital signal.
Similarly, if the propagation delay on an interconnect is larger than one quarter of the oscillation period for your analog signal, the trace will behave as an analog transmission line. It will be susceptible to the same signal reflection problems as a digital signal. However, ringing in analog signals is much more severe than for digital signals as analog signals can form standing waves if the trace length is an integer multiple of the analog signal wavelength. This type of ringing in an analog signal causes much stronger radiated EMI in nearby signal traces.
If the amplitude of a ringing signal is very large, it can cause involuntary switching at the load. Solving the problem of ringing requires impedance matching, or it requires adding LC elements in order to overdamp the reflected signal. Analog ringing signals are strong radiators as they form standing waves and act like an antenna, producing strong interference in nearby traces. Analog differential pairs can overcome this problem, but their trace length matching must be extremely precise.
Trace Length Tolerance Matching in High Speed PCB Design
Impedance controlled routing and trace length matching really go hand-in-hand. The easiest way to ensure traces within a signal net remain matched, is to increase the length of all traces in a net so that they match the length of the longest trace. Trace length matching in your PCB is much easier when you use the right design software. Built-in trace length matching should allow you to easily meander traces in a serpentine pattern as they travel from source to load.
There is always a danger that a trace makes a transition to transmission line behavior, but you can prevent reflection from loads due to impedance mismatch when you use impedance controlled design techniques. This requires explicitly defining the trace geometry with respect to the dielectric constant of the substrate material. This helps ensure that your microstrip trace impedance and stripline trace impedance will match to a desired value within a predefined tolerance interval.
- Whether you are using standard or custom components, ultra-accurate MCAD design requires including 3D component models in your component libraries.See how semi-automated routing tools make trace length matching easy.
- Working with impedance controlled board requires building the proper layer stack for your PCB.
- Working with high speed and high impedance boards requires implementing high speed design techniques and simulations into your PCB design workflow.
Altium Designer integrates your design and signal analysis features
Nowadays, every PCB designer needs tools that make high speed and high frequency design easy. You’ll need routing features that help you quickly match trace lengths in order to prevent skew in parallel data or timing jitter in serial data. You’ll also need tools that allow you to implement any strategy for compensating phase mismatch between analog signals and a reference timing signal.
Because trace, source, and load impedance mismatches are a critical concern in high speed and high frequency design, you need design features that allow you to determine the right stackup for impedance controlled routing or the best termination scheme for impedance matching. The signal analysis and simulation tools will give you an immediate view of any signal integrity problems in your board and help you iteratively determine the best components to use in a variety of termination schemes.
Altium Designer contains all this and much more in a single program. The deliverable generation, documentation generation, supply chain management, and data management tools ensure that you and your team can take your design from idea to product. Altium Designer also offers plenty of add-ons that will help you take your designs to the next level.
Work With the Best High Speed Design Features in Altium Designer
Altium Designer makes trace length tolerance matching and impedance controlled design easy by giving you the best high speed design tools in a single package. The auto-interactive routing tools make it easy to quickly route signal nets around your board and match trace lengths within your specified tolerance intervals. The layer stackup manager also includes an impedance equation feature that defines your desired trace impedance as a design rule. Controlling for signal integrity has never been easier.
- Altium Designer contains the best high speed layout, simulation, analysis, and verification tools in a single design interface.Learn more about the high speed design tools in Altium Designer.
- Matching trace lengths within a predefined tolerance interval is easy when you work with the best auto-interactive routing tools.Learn more about trace length matching with interactive routing in Altium Designer.
- Altium Designer is the only integrated PCB design platform that unifies all your features on top of a single design rules engine. You can define trace length tolerances and your desired controlled impedance by defining these values directly in your design rules.Learn more about rules-driven PCB design Altium Designer.
If you use command line based CAD tools or layout software that was not built to the demands of the professional PCB designer, you’ll spend far too much time manually measuring and compensating trace lengths in your high speed designs. Instead of fiddling with outdated design tools that can’t meet the demands of modern PCB design, you need to use the only design platform that integrates the best high speed design tools in a single package: you need Altium Designer.
If you’re not familiar with high speed design or with working in an integrated design environment, Altium Designer offers the tools you need to become leaders in the PCB design industry. When you switch to Altium Designer, you’ll have access to the resources you need to get started working in an immersive, unified design environment. Between an extensive knowledge base, the AltiumLive forum, podcasts and webinars from industry experts, and detailed feature tutorials, you’ll have access to the resources you need to enter the world of modern PCB design.
Altium Designer has something too for the professional PCB design, electrical engineers, mechanical engineers, and hardware startups. The integrated high speed design, simulation, analysis, and design features help you remain on the cutting edge of PCB design. Only Altium Designer offers the best high speed design tools you need in an integrated design environment.