Minimize Signal Loss in High-Speed PCBs
A Printed Circuit Board (PCB) provides a stable and conductive pathway for electrical currents to flow through plated holes to internal layers and the opposing side, connecting various components. High-speed PCBs require special consideration to ensure signals can be transmitted and received with sufficient integrity. The main factors influencing signal integrity in high-speed applications are signal reflections and crosstalk. Using proper routing techniques, attenuation control, and EMI-compliant design practices, these issues can be minimized to improve the performance of your final product.
Reflections occur when a signal encounters an impedance pcb mismatch, such as via stubs or imperfections in the trace. Incorrect impedance matching and higher frequencies increase the chances of these reflections. Unwanted interference, such as crosstalk or ground bounce, also affects signal integrity.
To prevent these signal loss and distortion issues, you should keep the signal traces as short as possible to avoid unnecessary inductance and capacitance. This can be accomplished by placing large ICs near the connectors, placing all the signal traces as close as possible to their respective pins, and routing all interconnections over the shortest paths between them.
How to Minimize Signal Loss in High-Speed PCBs
During transmission, signals suffer losses due to various factors, such as dielectric and resistive losses in the copper conductor, skin effect, and variations in amplitude. A high-speed signal has a very small rise time, so the changes in signal voltage or current are very fast. These fast voltage transitions can cause crosstalk between adjacent traces or switching noise in the return path planes.
The insertion loss of a signal is proportional to its frequency, whereas the attenuation is constant over its length. A high-speed signal experiences a significant amount of attenuation, reducing its power. Moreover, a significant portion of the energy from the signal is lost in the form of radiation into nearby traces or the air, which is known as the transmission loss.
Signal integrity is of great importance in high-speed PCBs, because they are characterized by very low rise times and long signal distances. To reduce these issues, designers must implement appropriate termination schemes, attenuation control, and a good grounding scheme. Additionally, they should use wide and uniform power and ground planes, avoiding split return paths.
Trace width and spacing, via size and placement, and the PCB stackup all impact the impedance of a signal line. To calculate the target impedance of a high-speed signal, designers can use field solver tools that provide results from Maxwell’s equations.
Other considerations include minimizing the size of vias, routing signals over inner layers, avoiding right angles (which increase capacitance), and utilizing blind vias to reduce inductance. Finally, buried vias are more effective at reducing impedance than stubs because they connect the inner layers to each other instead of passing through external layers. Also, designers should minimize the via stub, which is the unused via portion. To eliminate this issue, a stub can be back drilled, but that may negatively impact the circuit performance. Hence, it is better to avoid the stub altogether.