Meaning
Transmission lines placed on the outer layer of a printed circuit board consist of a conductor separated from a ground plane by a dielectric. This microstrip routing is the standard method for carrying radio frequency signals between antennas and transceivers. The width of the trace and the thickness of the board determine the characteristic impedance of the line.
Impedance Control
Maintaining a consistent 50-ohm environment is necessary to prevent signal reflections. Any variation in trace width or dielectric height causes an impedance mismatch that degrades the signal. Fabricators use controlled impedance coupons to verify that the manufactured board meets the design specifications.
Tight tolerances on copper etching are required for high frequency performance.
Field Distribution
The electromagnetic wave travels partly through the board material and partly through the air above the trace. This dual medium affects the effective dielectric constant and the velocity of propagation. Most of the energy is concentrated in the space between the trace and the reference plane.
External objects placed near the trace can disrupt this field and alter the signal. Component placement must avoid crossing the fields of high power traces. Solder mask thickness also contributes a small shift in the overall impedance.
Shielding Necessity
Open structures are susceptible to picking up noise from nearby circuits. Crosstalk occurs when two parallel traces are placed too close together. Designers often use ground pours or metal shields to isolate sensitive signals.
The absence of a top ground plane makes these lines more prone to radiation than stripline configurations.
