Controlled impedance refers to maintaining uniform characteristic impedance throughout a signal line on a PCB. This is critical to ensure signal integrity in high-speed designs.
Factors affecting controlled impedance
- Trace geometry: Width (ΔW), thickness (T), and distance from reference planes (H1).
- Dielectric material: Dielectric constant (Dk) and substrate thickness.
- Transmission line model: Microstrip, stripline, and coplanar waveguide.
- PCB stack-up: The arrangement of layers in the PCB, including signal layers, ground, and power planes.
Types of controlled impedance traces
- Single-ended: It is the impedance of an individual trace relative to a ground plane. Typical values range from 40 to 70 Ω.
- Differential: Used for pairs of traces carrying complementary signals (e.g., differential signaling in USB, HDMI, PCIe, SATA, or Ethernet). Typical values range between 90 and 100 Ω.
Standard impedance tolerance is typically ±10%, but tighter tolerances can be achieved with advanced design and manufacturing techniques.