Is it true that an EMI source is always external to the cable containing the affected circuit?

The assertion that an EMI (Electromagnetic Interference) source is always external to the cable containing the affected circuit is false. EMI can originate both from external sources and from within the cable itself, affecting the signals carried by it. Internal EMI can be caused by the cable's own components or by other signals running in parallel within or near the same cable.

For instance, in twisted pair cables commonly used for networking, the twisting helps to minimize unwanted EMI effects, both from external and internal sources. Additionally, equipment connected to a cable can also emit EMI that affects the signals traveling in that cable. This understanding highlights that EMI considerations are not solely based on external sources but involve a broader context, including the characteristics of the cable and the environment in which it is installed.