EMC Question of the Week: November 14, 2022
Suppose the separation distance between two microstrip traces is increased, reducing the mutual capacitance between them by a factor of two. In this case, the mutual inductance between the two circuits will
- also be reduced by a factor of two
- be doubled
- increase slightly
- stay the same
Answer
The best answer is “a.” In a homogeneous dielectric, the ratio of the mutual inductance to the mutual capacitance between two transmission lines is equal to the product of the characteristic impedances of the two lines. Any geometry change, that does not affect the characteristic impedances of the lines, has the same effect on the mutual capacitance (electric-field coupling) that it does on the mutual inductance (magnetic field coupling).
This is not true for transmission lines with non-homogeneous dielectrics (such as microstrip traces), though it is often close enough to make rough estimates of the coupling.
In matched transmission lines with homogeneous dielectrics, this relationship between the mutual capacitance and the mutual inductance is the reason that the capacitive coupling and the inductive coupling are always equal. It's also the reason that the far-end crosstalk is zero and near-end crosstalk is not zero in these lines.
Signal integrity engineers are generally aware of this relationship between mutual inductance and mutual capacitance, while many EMC engineers are not. Nevertheless, most EMC engineers would recognize that increasing the separation between two microstrip traces reduces the magnetic field coupling between them. This recognition eliminates (b), (c) and (d), leaving (a) as the only viable option.
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