Meaning
Resistance and reactance encountered by an electrical signal as it travels through a transmission line at frequencies exceeding one gigahertz. Accurate multi-gigahertz impedance control is fundamental for maintaining signal integrity in high speed digital interfaces like PCIe or 5G radio paths. At these frequencies, the physical dimensions and material properties of the circuit board behave as reactive components rather than simple conductors.
Trace Geometry
Width and thickness of the copper traces on a PCB define the inductive and capacitive characteristics of the path. A multi-gigahertz impedance value is extremely sensitive to small variations in the manufacturing process. Etching tolerances must be tightly controlled to keep the impedance within the standard five or ten percent limit.
Designers use software simulations to calculate the exact dimensions needed for a specific stackup and dielectric material.
Dielectric Influence
Insulating material between the signal layer and the reference plane acts as a capacitor that stores energy. The permittivity of the substrate directly affects the multi-gigahertz impedance and the speed of signal propagation. Standard materials like FR-4 often show inconsistent properties at very high frequencies, leading to signal loss and distortion.
High performance laminates with a low dissipation factor are selected to provide a more stable environment for sensitive signals.
Discontinuity Management
Changes in the path of the signal, such as vias or connectors, create points where the impedance deviates from the target value. These discontinuities reflect a portion of the signal back toward the source, which degrades the waveform and causes errors. Managing multi-gigahertz impedance requires careful transition design, including the use of back-drilling or specialized pad shapes.
Every transition must be modeled to ensure the cumulative reflections do not exceed the budget of the system.
