Signal Reflection
The reduction in the ratio of incident power to reflected power caused by impedance mismatches or physical defects in a high speed signal path leads to reduced data integrity. When return loss degradation occurs, energy that should have reached the receiver is instead reflected back toward the source, creating interference and jitter. This loss is measured in decibels, where a lower absolute value indicates a higher percentage of reflected power.
Impedance Inconsistency
Variations in the width of copper traces or the thickness of the dielectric layer are the most common causes of this phenomenon in printed circuit boards. Every time a signal passes through a via or a connector, a slight return loss degradation is inevitable, but sudden changes in the stackup can make the effect much worse. Manufacturers use time domain reflectometry to locate the exact point where the impedance deviates from the target value.
This allows them to identify whether the problem lies in the base material, the plating process, the etching chemistry, or the final assembly of the components.
Performance Constraint
High speed protocols like PCIe or Ethernet have strict limits on how much signal bounce the system can tolerate before errors occur. If the return loss degradation exceeds these limits, the link might drop to a lower data rate or fail to connect entirely. Controlling the moisture content of the laminate and the precision of the etching process are the primary ways to keep these reflections under control.
