EMC Question of the Week: June 11, 2018
Twisting a pair of wires, as opposed to routing them side-by-side,
- nearly doubles the characteristic impedance
- nearly halves the characteristic impedance
- typically increases the characteristic impedance about 5%
- has no significant impact on the characteristic impedance
Answer
The correct answer is "d." The characteristic impedance of a twisted wire pair is the same as the two wires routed side-by-side without twisting. It's only necessary to know the cross-sectional geometry and permittivity of the wire insulation to determine the characteristic impedance of a wire pair. Typically, the characteristic impedance of a wire pair is between 100 and 150 ohms. Wire pairs with very thin insulation can have slightly lower values and wire pairs with very thick insulation can have slightly higher values.
Note that twisting two wires in a wire harness can ensure that they remain side-by-side. One could argue that this lowers the characteristic impedance by eliminating extra space between the wires. In applications where it is important to maintain a constant characteristic impedance, wire pairs should be twisted or held together to keep the space between them constant.
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