Controlled impedance is a crucial factor in the design of high-speed printed circuit boards (PCBs). The impedance of a trace determines the signal’s behavior, and if the impedance is not controlled, it can lead to signal distortion, crosstalk, and electromagnetic interference. Therefore, it is essential to calculate controlled impedance accurately during PCB design. In this blog, we will guide you through the process of calculating controlled impedance for PCBs.
How to Calculate Controlled Impedance for PCB?
Step 1: Determine the Required Impedance
The first step in calculating controlled impedance is to determine the required impedance for the trace. The required impedance is usually specified by the system designer or is based on the transmission standard being used. For example, the required impedance for a USB 2.0 trace is 90 Ohms, and for a DDR3 SDRAM interface, it is 50 Ohms.
Step 2: Choose the Trace Geometry and Material
The next step is to choose the trace geometry and material. The geometry includes the width and thickness of the trace, as well as the distance between the trace and the ground plane. The material includes the dielectric constant (Dk) of the PCB substrate. The trace geometry and material affect the controlled impedance of the trace.
Step 3: Calculate the Trace Width
Once the required impedance and trace geometry are determined, the next step is to calculate the trace width. There are several formulas and online calculators that can be used to calculate trace width. The most commonly used formula is the microstrip formula, which is given by:
W = (Zo * t) / (8.85 * Er)^0.5
Where W is the trace width, Zo is the required impedance, t is the thickness of the trace, Er is the dielectric constant of the PCB substrate.
Step 4: Check for Tolerance and Adjustments
After calculating the trace width, it is important to check for tolerance and make adjustments if necessary. The tolerance for controlled impedance is usually specified by the system designer, and it varies depending on the application. If the calculated trace width is outside the tolerance range, adjustments must be made to ensure that the trace impedance is within the required tolerance range.
Step 5: Verify Impedance with Simulation Tools
Finally, it is important to verify the impedance of the trace using simulation tools such as a signal integrity simulator. These simulation tools help to ensure that the calculated impedance is accurate and that the trace will perform as expected.
Conclusion
Calculating controlled impedance is a critical step in the design of high-speed PCBs. By following the steps outlined in this blog, you can accurately calculate the controlled impedance of a trace and ensure that your PCB design will meet the required specifications. Always remember to check for tolerance and verify the impedance using simulation tools to ensure optimal performance.
