Propagation Loss
Signal strength along a printed circuit board transmission line decreases as the distance from the driver increases due to physical losses in the conductors and dielectrics. The microstrip attenuation refers to this power loss along a trace that sits on the outer layer of a board, separated from a ground plane by a dielectric substrate. This loss increases with signal frequency, making it a critical design parameter for radio frequency and high speed digital circuits.
Layout engineers use simulation software to optimize trace width and dielectric thickness to control these losses.
Signal Reduction
Conductor roughness and dielectric dissipation constitute the two primary components of signal loss. When microstrip attenuation is dominated by conductor loss, the electrical current travels within a very thin surface layer of the copper due to the skin effect, making it highly sensitive to the roughness of the copper foil. Using smooth copper foils reduces this resistive component, while selecting a substrate with a low dissipation factor decreases the dielectric loss.
Both factors must be optimized to ensure the signal maintains enough power to be resolved by the receiver.
Dielectric Geometry
Geometry of the microstrip also influences the total signal loss. Narrower traces have higher resistance and therefore higher loss per unit length than wider traces. However, wider traces require thicker dielectric layers to maintain a fifty ohm characteristic impedance, which can increase the overall board thickness and fabrication cost.
