EMC Question of the Week: February 19, 2018
Which of the following cables should NOT be used to convey a high-frequency differential signal?
- ribbon cable
- shielded twisted wire pair
- unshielded twisted wire pair
- coaxial cable
Answer
The correct answer is "d". Differential signals should be conveyed on balanced transmission lines. The first three choices are all balanced. Coaxial cables are unbalanced. When a differential signal propagating on a balanced transmission line (such as a trace pair) transitions to an unbalanced cable, a common-mode voltage is induced. That voltage drives the unbalanced cable relative to the balanced transmission line resulting in common-mode current. As little as 5 microamps of common-mode current flowing on the shield of a 1-meter coaxial cable can result in unacceptable radiated emissions.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at