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Modern PCB design requires methods for achieving precise characteristic impedance calculations and impedance control. Digital circuits deliver desired performance because of short transition times and high clock rates. Devices and appliances have better capabilities because of the faster, sequential transfer of signals. Constantly increasing signal switching speeds requires another look at controlling the impedance of the transmission lines/PCB traces.
TOPICS IN THIS SOLUTION
PCB designs require strict values and tolerances for traces. Let’s consider several examples of factors that can cause impedance problems. Sudden changes in trace direction can cause changes in impedance or the dielectric constant can change across the length or width of a PCB. Changes in frequency and temperature also cause the dielectric constant to change. Each variance affects the characteristics impedance of an RF circuit.
Matching the impedances throughout the circuit yields a desired low voltage standing wave ratio (VSWR). Circuits with a low VSWR transfer the maximum amount of power from the source to the load. If you achieve a characteristic impedance of 50-Ω in your circuit design, RF signal power transfers efficiently from the source to the load. Few signal reflections occur. You can impact impedance by working with the thickness and width of trace conductors, the thickness of the dielectric substrate, and the dielectric constant of the substrate impact impedance.
The need for precision in controlling impedance becomes amplified when routing high speed designs that feature differential pairs. Differential signaling transmits a signal down a pair of tightly coupled carriers. One line carries the signal while the other line carries an equal, opposite image of the signal. From a design perspective, differential signaling minimizes electromagnetic interference generated from the signal pair and remains immune to common mode electrical noise.
Differential pair routing establishes a balanced transmission system that carries the equal and opposite differential signals across the PCB. Rather than require a specific differential impedance, PCB routing for differential signals has the objective of ensuring that the intact signal arrives at the target. The factors that affect the calculation of the differential impedance include:
Much of this need for controlling impedance for differential pairs occurs because of the dramatic impact that higher frequencies have on impedance. With impedance controlled routing, target input pins receive the correct signal from output pins. PCB designs that require the exact geometries needed for impedance matching rely on impedance calculators that provide the dimensions needed to control impedance on a high-frequency RF/microwave PCB or a high-speed digital PCB.
To assist design teams with controlling differential pair impedance, Altium Designer provides a suite of tools that include Interactive Length Tuning, Interactive Differential Pair Length Tuning, high speed design rules, the Gloss feature, and an Advanced Impedance Calculator within its Enhanced Layer Stack Manager. Why use a suite of tools? Unified stack-up planning and impedance calculations go past the capabilities of stand-alone impedance calculators because of the capability to check signal integrity, discover any vulnerabilities within the circuit, ensure correct trace routing impedances, and gain the capability for placing more circuits on the PCB.
In addition, analysis tools within the Enhanced Layer Stack Manager assist with avoiding potential signal integrity issues. Those tools detect nets have potential ringing and reflection issues. The capability to ensure correct trace routing impedances for circuits that have fast rise times protects against reflection. Depending on whether the circuit features a plane layer on one side or plane layers on both sides, Altium Designer uses either a microstrip characteristic impedance formula or a stripline characteristic impedance formula for differential pairs. Both show how the copper, dielectric thicknesses, routing width, and dielectric constant affect impedance.
Altium’s PCB Rules and Constraints Editor features the Matched Length rule and the Length rule within the High Speed category. The Matched Length rule applies to skew or signals arriving at the same time by specifying that target nets must route to the same length within an established tolerance. Every Matched Length rule uses the longest route in the target set of nets as a reference length. The Length rule applies to signal delay and sets the overall routed length of the net. The Length Tuning Tool works with both design rules and uses the rules to define the tightest constraints for the differential pair.
Altium Designer has automatic functions that speed and simply differential pair calculations. The unified design environment provided by Altium Designer eliminates the need for shifting out of the design, loading a third-party impedance calculator, performing the calculations, and manually feeding the calculations back into the design. Instead, your design team uses the Enhanced Layer Stack Manager to configure the PCB layer stack and input the dielectric constant, the copper widths, and the dielectric widths. Altium Designer’s integrated calculator automatically pulls this information from the board layer stackup, calculates the correct impedance values, and then pushes the results into the design rules.
The impedance calculations always refer to the configuration of the layers in the PCB design and utilize on industry standard microstrip and stripline formulas as defaults. After the impedance calculator performs the calculations, you will need to configure the design rules to accept the automatic values. The impedance calculator includes the capability to go beyond the default formulas and to use the impedance formula editor to change the calculations.
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Altium Designer utilizes software algorithms to provide the intuitive user control that powers Interactive Length Tuning. Intuitive user control allows you to add length tuning segments by wiping the cursor along the route path. Altium Designer relies on a length tuning algorithm to automatically calculate the dimensions and positions of the tracks and arcs used for building the tuning segments. You can base the properties used for length tuning on design rules, properties of the net, or specified values.
Altium Designer allows you to tune the length of one differential pair against the length of other differential pairs with the Interactive Differential Pair Length Tuning command. Altium recommends that you:
When using the command, you will need to use either the Length or Matched Length design rule or one of our Differential Pair query keywords. Tuning the length of differential pairs requires one matched length rule that defines pair-to-pair matching requirements and another higher priority matched length rule that establishes the within-pair length matching requirements.
Altium’s PCB Editor offers the Gloss and Retrace tools that design teams can use to improve the quality of existing routing. In terms of differential pairs, the Gloss command maintains the differential pair gap and the trace width. To improve the trace geometry, Gloss reduces the number of corners and shortens the route length.
The Retrace command allows designers to verify that the routing matches design rules. In contrast to Gloss, the Retrace tool does not change the trace geometry. While Gloss maintains the pair gap and trace width, Retrace changes those settings to Preferred. Design teams use the Retrace tool when a design rule change must apply to existing routing.
Find your design solutions with the right design software
Your design team can easily define differential pairs on the schematic with Altium Designer. From there, you can move the differential pair definitions to the PCB with design synchronization, transfer the differential pairs to the PCB editor, and then view and manage the differential pairs on the PCB. Working within the unified environment, the software can create any schematic, provide any trace routing capabilities, establish the layout, align ECAD and MCAD requirements with 3D design, and output the final drawing documentation.