TABLE OF CONTENTS Figure 11.1 on page 258 divided EMC control measures into three levels. This chapter deals with the secondary (interfaces and filtering) level. If your product has no interfaces let s say it is a hand-held, battery operated device then you already have a head start when it comes to EMC since only the enclosure port is relevant to you. But for the rest of us, power, signal and data interfaces are a fact of life and they offer a ready route for disturbances into and out of a product. The first thing that gets connected to an interface is a cable, so we need to consider how cables couple these disturbances, and if they are to be screened, how the screen connection needs to be dealt with in practice; and then if the interface is unscreened, how to specify and implement the necessary filtering.
Due to their length, external cables are more efficient at interacting with the electromagnetic environment especially in the HF and VHF range than are enclosures, PCBs or other mechanical structures. Cables, and their connectors which create the interface to the equipment, must be carefully specified. The main purpose of this is to ensure that differential mode signals are prevented from radiating from the cables, and that common mode cable currents are neither impressed on the cable by the signal circuit nor are coupled into the signal circuit from external fields via the cable.
In many cases you will have to use...
