4S6TMP.com — PCB transmission line impedance for RF and high-speed designs
Controlling impedance on PCB traces is critical for high-speed digital and RF signals. Mismatched impedance causes reflections, signal degradation, and EMI. Common target impedances are 50Ω (RF, most digital) and 75Ω (video, some RF).
Microstrip: Trace on outer layer with ground plane below. Simple but exposed to air, so εeff is lower than the substrate. Use for external traces, RF, and antenna feeds.
Stripline: Trace embedded between two ground planes. Better shielding, more consistent impedance, but requires more layers. Use for high-speed digital and sensitive RF.
Coplanar waveguide: Trace with ground planes on the same layer. Good for surface-mount RF components where vias to ground would be problematic.