How to Design PCB Transmission Lines with Controlled Impedance
Presented by: Sierra Circuits
Featuring: Brian Walker, Senior RF Engineer SME
When designing circuits in the GHz range, ensuring uniform transmission line impedance becomes critical to maintain signal integrity. These traces play a key role in managing propagation delay, which is essential for synchronized data transfer.
In this webinar, you’ll learn the best practices for designing PCB transmission lines with controlled impedance.
How to design PCB transmission lines with uniform impedance
To design efficient high-speed traces, first, you need to evaluate the critical length (lc). It is the threshold value beyond which a trace behaves as a transmission line. Typically, lc is 1/6th of the signal’s operating wavelength. When signal lines exceed this length, you need to implement appropriate design rules to avoid impedance mismatches.
For achieving uniform impedance throughout the transmission line, select laminates with a low dielectric constant (Dk<4). Ensure the material’s Dk is consistent over the operating frequency and temperature ranges. To reduce signal attenuation, choose materials with a lower dissipation factor (Df<0.005).
A well-planned stack-up is important for minimizing dielectric loss. Place high-speed layers adjacent to solid ground planes to lower parasitic inductance. Incorporate symmetrical stack-ups to avoid noise coupling.
Implement serpentine traces and meandering techniques to minimize propagation delay. Keep the spacing between serpentine loops at least three times the trace width to avoid coupling.
To ensure consistent impedance and reduce signal reflections, terminate the high-speed tracks correctly. A series termination involves placing a resistor close to the driver. Whereas, in shunt termination, a resistor is connected in parallel with the receiver. The choice of termination depends on the application and the characteristics of the signal.
To reduce conductor loss in PCB transmission lines, opt for ENEPIG and ENIG surface finishes as they minimize conductor resistance and help maintain uniform impedance.
Use time-domain reflectometry (TDR) and vector network analyzer (VNA) to detect any impedance discontinuities in your PCB transmission lines.
