Impedance Challenges in PCB Design
As a PCB design engineer, maintaining impedance continuity is crucial. However, there are instances where impedance discrepancies may arise, requiring strategic solutions.
Understanding Impedance
Impedance, characteristic impedance, and instantaneous impedance are key concepts in PCB design. While distinct, they often overlap in practice, all relating to the fundamental definition of impedance:
- The input impedance at the start of a transmission line is known as “impedance.”
- The impedance encountered by a signal at any moment is termed “instantaneous impedance.”
- When a transmission line maintains a constant instantaneous impedance, it is referred to as the “characteristic impedance.”
Characteristic Impedance
Characteristic impedance is the transient impedance encountered by a signal traveling along a transmission line, crucial for signal integrity. It is influenced by factors like dielectric properties, trace dimensions, and copper thickness.
Impedance Discontinuity Solutions
Corners: Sharp turns in RF signal traces can cause impedance mismatches. To reduce reflections, corners can be chamfered or rounded with a radius greater than 3 times the trace width.
Large Pads: Large pads on microstrip lines can disrupt impedance continuity. Increasing dielectric thickness and hollowing out the ground plane beneath the pad can mitigate this issue.
PCB Vias: Vias are common sources of impedance discrepancies. Factors like via dimensions and anti-pad design can impact impedance. Strategies to minimize via-induced discontinuities include via-less processes and optimizing anti-pad diameter.
PCB Through-Hole Coaxial Connectors: Similar to vias, through-hole coaxial connectors can introduce impedance challenges. Mitigation strategies include via-less processes, appropriate outlet techniques, and anti-pad diameter optimization.
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