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Net length evaluation for high-speed transmission lines takes place prior to layout. For electrically long traces, use xSignals and Design Rules to constrain net topology within a defined maximum length during layout.
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Make traces easier with the best PCB design software available.
Set length constraints in the New Rule Wizard
Trace lengths for high-speed signals are determined prior to layout during circuit design phase of a project. Maximum lengths are determined by analyzing a signal’s rise or fall times with respect to propagation delay. The signal must be able to travel from source to load, and back again, before the next edge transition. Otherwise, crosstalk, ringing, or reflections will occur leaving the design vulnerable to noise.
Establishing maximum lengths before crosstalk, ringing, or reflections defines the trace as electrically long. Electrically long traces require termination schemes to shunt or minimize noise to ground. Minimizing noise protects circuit designs from corruption and assures clean signal propagation along the circuit board trace. Should board layout require nets that exceed maximum length, termination circuits are required.
Before board layout, electrically long traces are defined and constraints set. This automates the layout process by including rules within the tool to guide routing. Techniques such as fly-by topology build continuous nets with included series components within the trace of interest. Rules for length matching are also set for high-speed data lines found in memory or CPUs to avoid noise due to skew.
Circuit designers analyze high frequency signal rise and fall times, along with propagation delay, to determine whether a transmission line is an electrically long trace. If so, there is a maximum length that signal may travel along a circuit board net before termination components are required in the design. Preventing a circuit board from generating noise due to transmission line net lengths is an important design step. Defining which signals are susceptible incorporates calculations of trace length using known rise and fall times for the signal.
Calculate maximum lengths for critical trace identification
Maximum trace length and length matching is an important design step for high-speed transmission nets. Although independent from trace width effects on impedance control, maximum length and length matching are necessary design steps for producing a quiet design. Maximum length and length matching consider the time domain when laying down traces in order to avoid crosstalk, ringing, and reflections.
Now that you’ve analyzed high speed effects and which traces may produce them, consider methods for noting your findings in EDA tooling.
Transferring design intent into schematic software requires attention to detail. Those design intent details need careful attention when communicating to PCB layout designers. Otherwise, layout misses important aspects of the design such as trace length limits or matched traces. Both trace limits and matched traces need to be identified early in software tooling, so design rules may be set up to propagate through the software design files. Rather than making intricate notes on the title page of your schematic, consider using EDA software with menus for inputting net rules.
Fly-by topology considers serial components to complete the trace length
You want EDA software that is able to incorporate rules, set in schematic capture, into the PCB layout. And you want the function to be automatic. Rather than keeping layout notes on the title sheet of your schematic, it is best to have EDA tools that incorporate rules throughout the design environment. In this way, design intent is automated and important details such as maximum trace lengths set in schematic capture are maintained in PCB layout.
Now that you’ve calculated trace length and analyzed your maximum net lengths for routing, let’s see how Altium Designer sets up rules and constraints for routing critical nets.
Altium Designer provides both a means for identifying and defining electrically long traces, and for setting up design rules that propagate through from schematic capture to PCB layout. You no longer need to keep intricate notes on the schematic title sheet for communication of details to your PCB layout designer. Instead, Altium Designer provides xSignals specification for circuit designers to classify nets. Once nets are classified, rules and constraints may be specified for application during routing.
Create xSignals for trace length calculated routing maximums
Altium Designer’s xSignals are traces defined by the circuit designer for consideration as electrically long traces. These traces include series termination and can encompass several net names. As such, Altium Designer routing tools are notified of designated xSignals when designing real estate on the board. There is a wizard for defining xSignals, or menu commands may be used to define pathways from pad-to-pad or component-to-component.
Designing for signal integrity includes calculating maximum trace lengths for electrically long traces. Once electrically long traces are identified, use Altium Designer’s xSignals feature to define which nets are considered when applying maximum length calculations. Altium Designer provides the Rules and Constraints Editor where xSignals receive maximum length and length matching characteristics that are used to automate routing when laying out the board. Calculating and applying trace length analyses to PCB layout is organized within Altium Designer’s unified environment.