Routing High-Frequency PCB Traces for Signal Integrity

Key Takeaways

• Time delays, reflections, electromagnetic interference, and crosstalk are some of the issues seen in high-frequency PCB traces.

• The length of a high-frequency trace should be designed so that the critical rise time of the circuit board is shorter than the rise time of the signals. 

• Following the 3W rule can alleviate crosstalk and interference in high-frequency PCBs. 

In most electronic circuits, there is a controller that controls output signals or maintains the normal operation of a power circuit. In cases of abnormalities, such as a high current or sudden rise in input voltage, the control circuit protects the power circuit by activating the crowbar circuit.

Generally, control circuits use high-frequency clock signals. While routing the path for these clock signals, they should follow the guidelines of high-frequency PCB traces. This is not just the case for clock signals; the circuits used in microwave and radiofrequency are also high-frequency circuits and need to be designed accordingly.

High-Frequency PCB Traces 

The clock signal paths in electronic circuits are designed as high-frequency PCB traces for reliability and signal integrity. The clock signals change from low-level to high-level quickly,  indicating that the rise and fall time of these signals is very short. PCB traces should be capable of supporting these sudden changes and microwave and radiofrequency circuit signal paths should also be routed as high-frequency traces. 

The Length of a High-Frequency PCB Trace

Time delays, reflections, electromagnetic interference, and crosstalk are some of the issues commonly seen in high-frequency PCB traces. These issues arise when the length of the high-frequency PCB trace is comparable with the wavelength of the signal. The length of the high-speed or high-frequency trace directly influences the performance of high-frequency circuits.

High-frequency PCB traces can be designed as either microstrip lines or striplines. The performance of these lines depends on the frequency of the signals passing through them and the length of the path. The length of the high-frequency trace should be designed so that the critical rise time of the circuit board is shorter than the rise time of the high-frequency signals.

The critical rise time of the circuit board depends on the critical length of the high-frequency PCB traces. As the length of the trace increases, the critical rise time of the PCB board increases and results in an impedance mismatch. In such cases, PCB traces of high-frequency need to follow impedance-controlled routing. 

The Shape of High-Frequency PCB Traces

In high-frequency PCB boards, the probability of routing traces as straight lines is low; usually, bends are encountered in PCB designs for high-frequency applications. These bends are crucial in creating impedance changes in the circuit. In most electronic circuits, the width of the trace on either side of the bend is different. This width change causes impedance variations and these locations are vulnerable to reflection. The worst-case bend is one with 90°. It is a standard procedure to route high-frequency traces as rounded with smooth, curved bends. 

Spacing Between High-Frequency PCB Traces 

Routing high-frequency traces close to each other can result in crosstalk and interference. To eliminate these effects, traces need to be placed with an appropriate amount of spacing between each other. 

It may be tempting to follow the 3W rule—traces must be separated by a distance equal to three times the width of a single signal trace. However, compact size requirements make using the 3W rule challenging. Designing multilayer PCBs or stacking up can help maintain signal integrity in compact-sized, high-frequency PCBs.

When designing the geometry and spacing of  high-frequency PCB traces, layout engineers must pay careful attention to limit crosstalk and interference. Cadence software offers a full suite of PCB layout and design tools to create high-frequency circuits with minimal crosstalk and interference issues. 

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