EMC Question of the Week: August 14, 2023

photo of circuit board traces

Controlled impedance traces on printed circuit boards typically have a 50-Ω characteristic impedance. Traces without a controlled impedance are more likely to have a characteristic impedance of around 70-80 Ω. A key reason for using a lower characteristic impedance in controlled impedance traces is that these traces 

  1. radiate less
  2. are less lossy
  3. take up less space
  4. all of the above


The best answer is “b.” For a given layer spacing, 50-Ω microstrip traces are wider than traces with a higher characteristic impedance. This means that they have a lower resistance per unit length (i.e., lower conductor loss). This helps to reduce dispersion in high-speed digital signals.

Radiation directly from microstrip traces is generally negligible, so it would not be correct to suggest that radiated emissions are reduced by using 50-Ω traces. In fact, the larger trace widths and higher currents in 50-Ω traces can actually increase the coupled noise that ultimately produces significant radiated emissions.

Microstrip traces with uncontrolled impedance typically have a width that is approximately equal to the layer spacing. In an FR-4 dielectric this results in a characteristic impedance on the order of 70-80 Ω. The wider trace widths associated 50-Ω traces take up more board space. 

Note: Another advantage of making controlled impedance traces wider is that the characteristic impedance is less sensitive to small variations in the trace width and thickness.

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