EMC Question of the Week: June 9, 2025

A circuit board trace embedded in an FR-4 (ε_r = 4.3) dielectric between two planes forms a transmission line with a capacitance per unit length equal to 1.5 pF/cm. What is the characteristic impedance of that transmission line? 

1. < 40 Ω
2. 46 Ω
3. 54 Ω
4. > 60 Ω

Answer

The best answer is “b.” 1.5 pF/cm is 150 pF/m. The velocity of propagation is,

$v=\frac{c}{\sqrt{{\epsilon }_r}}=\frac{3\times {10}^8\text{m/s}}{\sqrt{4.3}}=1.45\times {10}^8\text{m/s}.$

Therefore, the characteristic impedance is,

$Z_0=\frac{1}{vC}=\frac{1}{\left(1.45\times {10}^8\text{m/s}\right)\times \left(150\times {10}^{-12}\text{F/m}\right)}=46\text{Ω}.$

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