EMC Question of the Week: April 17, 2023
At MHz frequencies, which EM coupling mechanism tends to be the main contributor to crosstalk between matched parallel microstrip traces on a circuit board?
- common impedance
- electric field
- magnetic field
- both (b) and (c) equally
Answer
The best answer is “c.” For two matched transmission lines in a homogeneous dielectric, the electric-field coupling and the magnetic-field coupling are the same. However, in microstrip traces, the presence of a dielectric below the traces reduces the electric-field coupling (relative to an air dielectric), but it has virtually no effect on the magnetic-field coupling. So, in microstrip traces the magnetic-field coupling is stronger.
At MHz frequencies, the common-impedance coupling is negligible. This is mostly because the return currents in the plane are concentrated directly below their respective traces. It is also because the field coupling increases proportional to frequency, while the shared impedance (plane resistance) is only proportional to the square root of the frequency.
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