EMC Question of the Week: August 14, 2023
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
- radiate less
- are less lossy
- take up less space
- all of the above
Answer
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 with 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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