How To Measure PCB Trace Impedance: A Practical GuideHow To Measure PCB Trace Impedance: A Practical Guide

How To Measure PCB Trace Impedance: A Practical Guide

The PCB trace impedance plays an important role in high-speed electronic design ensuring there is signal integrity and system reliability. It is essential to control appropriately the trace impedance and if not managed properly it can lead to issues including signal reflection, ringing, and data corruption. Hence, this makes the trace impedance critical for engineers and designers to ensure there is appropriate measure and maintained trace impedance during the design and manufacturing phases. Therefore, this blog from Sunstream serves as a practical guide on how to measure PCB trace impedance significantly.

What Is PCB Trace Impedance?

The trace impedance refers to resistance that a copper trace over a PCB gives due to the fast changing electrical signals. These are more than assess how wide or long the trace is as they include the information of PCB material and how these copper layers are arranged also impact. In the high-speed circuits including the systems that have radio frequency (RF) or any digital devices, it is important to maintain the impedance at the specific value. Also, in maintaining such impedance resistance will help the signals to travel smoothly without any loss or distortion. Hence, the engineers and designers usually aim for,

  • 50 ohms in the case of single signal lines called single-ended
  • 100 ohms for the pair of lines which is used together called differential pairs

However, the values can change slightly based on the circuit needs. 

How To Measure PCB Trace Impedance?

Time-Domain Reflectometry (TDR)

The Time-Domain Reflectometry or TDR is one of the methods to measure the PCB trace impedance by sending the fast pulse and analysing its reflection to determine the impedance variation. The frequency range used for this method is operated in the MHz to GHz range providing fast, seamless, and high-resolution measurements ensuring it meets the industry standards of the PCB and cable testing. One of the challenges in this method is the equipment is quite expensive and needs guidance from professionals.

Vector Network Analyzer (VNA):

The Vector Network Analyzer or VNA is another used to measure the trace impedance by measuring S-parameters across the wide frequency range. The frequency range includes from kHz to GHz making it align with the industry standard for RF and microwave application. This method offers high accuracy and best for frequency-dependent microwave application.

Impedance Analyzer:

The impedance analyzer is another method used to measure the trace impedance where an AC signal is applied and the voltage and current response is measured to calculate directly with high accuracy. This method supports the wide frequency range right from Hz to GHz making it ideal for testing the passive components and analyzing the power distribution networks (PDNs). However, this method is typically used within the confined environments and may not be appropriate for the practical field usage.

LCR Meter: 

The LCR meter is used to measure the trace impedances by using the low to mid frequency range (Hz to MHz) with the bridge circuits. This is commonly used for testing the passive components including the capacitors, inductors, and resistors. However, the LCR meter is simple, affordable, and widely used for high-frequency applications. 

Field Solver (Sigrity X):

Finally, the field solver or Sigrity X works on the advanced numerical simulations which helps in calculating the impedance based on the PCB stack-up, trace geometry, and material properties. This tool is based on the industry-standard for the pre-layout design and material modeling. Hence, this approach offers accurate predictions without any physical testing making them applicable for the early design stages.

OrCAD X To Measure Trace Impedance:

OrCAD X is used to measure and manage the PCB trace impedance through its in-built impedance analysis tools. These tools are incorporated into the design environment making it easier for the engineers and designers to control and analyze during the design phase.

OrCAD X help with this:

Cross-Section Editor – This tool lets you define the PCB stackup, including layer materials, trace widths, and dielectric thickness. It calculates impedance based on these inputs and allows adjustments to meet target impedance values across all layers.

  • Constraint Manager – This feature helps enforce impedance rules during routing. Designers can set specific impedance targets (e.g., 50 ohms), and OrCAD gives real-time feedback while routing traces, making sure they stay within the specified range.
  • The constraint manager is a feature in the OrCAD X tool which enforces impedance rules during the routing. With this feature, designers can also set the specific impedance targets and OrCAD X offers real-time feedback based on the inputs and allows you to meet the target impedance values across the layers.

Sunstream’s end-to-end PCB design services ensure your design is meeting its impedance goals and yields strong and reliable performance. Sunstream is the leading channel