How to Get Your Controlled Impedance Right the First Time?

In any circuit used for high-speed digital signals, signal integrity has to be critically important. This includes one of the most important points — PCB design for controlled impedance. Mismatch in impedance can cause signal reflections, data loss, and harmful performance. Getting it right the first time is best. Rework, delay, and performance issues must be avoided. Let us explore the roadmap to help you get controlled impedance into your PCB from the beginning.

1. Understand Controlled Impedance

Controlled impedance refers to designing PCB traces to maintain a specific impedance value throughout the signal path. It ensures signal integrity in high-speed circuits by minimizing reflections and distortion.

Think of impedance as the combination of resistance, inductance, and capacitance in an AC circuit. At high frequencies, typically 100 MHz, PCB traces behave like transmission lines, so the impedance must be controlled to prevent unwanted reflections and signal degradation. Variations in impedance along the trace will distort the signal and lose data and performance.

By designing PCB traces to a constant and matched impedance, engineers ensure reliable signal transmission from source to destination.

2. Identifying Signals That Require Controlled Impedance

From here, one needs to start determining which signals within a particular design require controlled impedance. In general, these signals can be classified as high-speed digital signals in applications:

• RF communication?
• Telecommunications?
• High-speed signal processing?
• High-quality video (DDR, HDMI, Gigabit Ethernet)

Datasheets of integrated circuits usually specify which signal nets require controlled impedance. Therefore, read through the datasheets and application notes very carefully.

3. Optimise PCB Stack-Up for Controlled Impedance

PCB stack-up is important to achieve controlled impedance. Designers need to determine the stack-up for the board to determine the trace values. If the stack-up is well-defined, the impedance characteristics will be uniform. Important factors are:?

• Dielectric Materials: Try to select constant dielectric materials over a wide frequency range. FR-4 is common but has higher Dk variation, impacting impedance control. PTFE or ceramic-based materials offer stable Dk and low loss, ideal for RF, microwave, and high-speed designs.
• Copper Thickness: Impedance depends on copper thickness and must be considered.
• Trace Width and Spacing: The combination of trace width and spacing determines the impedance. Use impedance calculation tools to determine the appropriate dimensions based on the chosen stack-up.
• Reference Planes: Keep reference planes beneath signal traces for a uniform return path for signals. In flat microstrip structure construction, this translates to having an unbroken ground plane underneath the trace. In stripline construction, this means having ground or power planes above and below the differential pairs.?

4. Follow Best Practices for Trace Routing

Follow these routing guidelines to maintain controlled impedance:

• Symmetrically Differential Pairs: Route high-speed differential pairs in parallel on both sides with equal spacing to maintain the specified differential impedance.Minimise spacing variations at pads, vias, or bends.
• Trace Spacing: Traces shall be separated by a distance of at least 3W (spacing between traces to be at least three times the trace width) or 2W (minimum two times the trace width). This does not apply to differential pairs.?
• Avoid Split Plans: Never route high-speed signals over split planes. This interrupts their return paths and degrades signal integrity. Instead, surround split planes with traces.?
• Vias: Vias have to be properly designed so that they maintain uniform impedance throughout the PCB. Stitch vias connect ground planes to provide a continuous return path. Vias staggered to minimize the reference plane voids.

5. Ensure a Continuous Return Path with Reference Planes

All high-speed signals require a continuous reference plane for a proper return path. A common cause of noise coupling and EMI issues is an incorrect return path. In high-speed signals, the return current closely follows the signal path, whereas in low-speed signals, it takes the shortest route. Typically, high-speed signal return paths are provided by the nearest reference plane to the signal layer.

6. Verify Design with a Controlled Impedance Checklist

A checklist for controlled impedance design should be used to ensure that all critical aspects are covered:?

• Mark controlled impedance lines on the schematic.?
• Match the lengths of differential pairs of traces within a limited tolerance.?
• Use connectors of high data frequency.?

7. Measure and Validate Impedance Performance

The impedance will be tested and measured with the help of tools like Time-Domain Reflectometry (TDR) or Vector Network Analyzers (VNA). This ensures that the PCB design conforms to the impedance requirements. These measurements ensure the alignment of the fabricated board with your design calculations.

Conclusion

In PCB design, controlled impedance is essential for maintaining signal integrity in high-speed applications. Every possible aspect of impedance, from optimizing PCB stack-up and routing layout to preventing signal loss, reflections, and EMI problems, is considered important.

PCB Power manufactures impedance-controlled PCBs designed and engineered with precision stack-up configurations that fit your design parameters. Advanced fabrication techniques ensure impedance is rightly achieved in high-frequency applications, such as RF communications, high-speed digital processing, and telecoms.?

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