Question
Which of the following would reduce RF interference caused by common-mode current on an audio cable?
Answer Options
- A) Place a ferrite choke on the cable
- B) Connect the center conductor to the shield of all cables to short circuit the RFI signal
- C) Ground the center conductor of the audio cable causing the interference
- D) Add an additional insulating jacket to the cable
Correct Answer: A
Explanation
Common-mode current occurs when unwanted radio frequency (RF) energy flows equally down the outside of a cable’s shield and also along the center conductor, causing the entire cable to radiate or receive the RF. For audio cables, this unwanted RF is often rectified into audible interference.
The most effective way to eliminate common-mode current is to place a ferrite choke on the cable. The ferrite material introduces a high impedance (resistance and reactance) to the common-mode current flow without significantly affecting the desired audio signal that flows in the differential mode (between the shield and center conductor). This blocks the RF from entering the audio equipment.
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