Question
Which of the following can cause shielded cables to radiate or receive interference?
Answer Options
- A) Low inductance ground connections at both ends of the shield
- B) Common-mode currents on the shield and conductors
- C) Use of braided shielding material
- D) Tying all ground connections to a common point resulting in differential-mode currents in the shield
Correct Answer: B
Explanation
A shielded cable is designed to prevent signals inside the cable from radiating out, and signals outside from getting in. This is achieved when the signal current flows equally and oppositely on the center conductor and the shield (differential mode current). However, external interference can still occur if currents flow incorrectly on the shield.
Shielded cables can radiate or receive interference if common-mode currents flow on the shield and conductors. Common-mode current flows equally and in the same direction on all conductors and the shield itself (acting as an antenna). This unwanted current is decoupled from the desired signal path and makes the entire cable assembly an efficient, unintended radiating or receiving element.
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