Designing Circuit Board Layouts with Impedance Control through Altium Software

Designing Circuit Board Layouts with Impedance Control through Altium Software

In the realm of printed circuit board (PCB) design, maintaining signal integrity becomes crucial, especially when high-frequency signals are involved. One effective method to achieve this is through controlled impedance routing, a technique that ensures the characteristic impedance of a transmission line formed by PCB traces and its associated reference planes remains consistent. This article demonstrates the use of controlled impedance routing in Altium Designer, a popular PCB design software.

To create and set up net classes for controlled impedance routing, such as 100 ohms and 50 ohms, in Altium Designer, follow these key steps:

1. **Create Net Classes for Controlled Impedance Nets:** - Open the PCB editor and go to **Design » Classes** to open the Classes dialog. - Select **Net Classes** from the list and click **Add Class**. Assign a name (e.g., "Impedance_50ohm" or "Impedance_100ohm"). - Add the nets that require controlled impedance (such as data lines or differential pairs) to these classes.

2. **Set Up the PCB Stackup with Defined Dielectric Parameters:** - Define your layer stack with precise dielectric properties and layer thicknesses matching your fabrication specs. - Altium allows you to configure layer materials and thickness in **Design » Layer Stack Manager**.

3. **Define Impedance Constraints for Net Classes:** - Open the **Design Rules** dialog via **Design » Rules**. - Locate the **Routing** section, specifically **Routing Width** and **Routing Impedance** rules. - Create a new **Routing Impedance** rule scoped to your net class (e.g., "Impedance_50ohm"). - Set the target impedance value (e.g., 50Ω for single-ended or 100Ω for differential pairs). - For differential pairs, also define a **Differential Pairs** rule with the specific impedance (usually 100Ω differential).

4. **Set Trace Width and Spacing According to Calculated Impedance:** - From your stackup and impedance requirements, calculate the trace width and spacing. - Specify these widths directly in your routing rules or by using length-tuned differential pairs.

5. **Routing with Controlled Impedance:** - When routing, assign traces to the specific net classes. Altium will apply your routing constraints automatically. - Use **Differential Pair** routing for 100Ω differential impedance nets. - The software will alert you if routing violates impedance or length rules.

6. **Verify Impedance and Adjust:** - Use the **Layer Stack Manager's impedance calculator** or external tools to verify your trace widths and spacing. - Adjust your design rules or stackup accordingly.

In this demonstration, a differential pair with 100 ohms impedance and a single-ended line with 50 ohms impedance were shown, with the ethernet section, which includes an ethernet IC and RJ45 connector, being the focus. All connections labeled MDI0, one, two, and three were 100-ohm differential pairs, while all other components were single-ended 50-ohm traces.

It is essential to note that controlled impedance traces should be differentiated from non-controlled signal traces to allow manufacturers to clearly identify them. Altium Designer's impedance rules work best with a correctly defined stackup and dielectric parameters. Realize that impedance depends on stackup geometry (trace width, dielectric thickness, dielectric constant). While Altium sets the rules, precise impedance matching often requires interaction with the PCB manufacturer and iterative tuning.

By following this workflow, controlled impedance nets are defined clearly in Altium Designer, and routing follows the required parameters for 50Ω single-ended and 100Ω differential impedance structures in your PCB. This approach is particularly relevant when high-frequency signals are propagating on the PCB transmission line, ensuring signal integrity and minimizing distortion.

1. In Altium Designer, the process of creating and setting up net classes for controlled impedance routing involves opening the PCB editor, navigating to Design » Classes, adding new net classes (such as "Impedance_50ohm" or "Impedance_100ohm"), and assigning high-frequency nets like data lines or differential pairs to these classes.
2. To utilize the controlled impedance technology effectively in Altium Designer, the PCB Stackup should be configured with defined dielectric parameters that match the fabrication specifications, allowing for precise control of the impedance value for both single-ended and differential pair nets.

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