## EMC Question of the Week: July 5, 2021

The crosstalk between two matched 50-Ω stripline traces plotted as a function of frequency has a maximum value of -30 dB. If the distance between the traces is increased, reducing the mutual capacitance between the traces by a factor of 2, the maximum crosstalk will be

- -30 dB
- -33 dB
- -36 dB
- < -40 dB

## Answer

The best answer is “c.” The crosstalk between two matched transmission lines in a homogeneous dielectric is proportional to the mutual capacitance. Reducing the mutual capacitance by a factor of two reduces the coupled voltage by a factor of 2 (6 dB).

Note that the crosstalk is also proportional to the mutual inductance between the two traces. In a homogeneous dielectric, the ratio of the mutual inductance to the mutual capacitance between two 50-Ω transmission lines is (50 Ω)^{2}. Any change is the trace separation that reduces the mutual capacitance by a factor of two also reduces the mutual inductance by a factor of two.

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