EMC Considerations for Cable Assemblies

In any electronic system, cables act as antennas — both transmitting and receiving electromagnetic energy. A well-designed PCB can still fail EMC testing if the cable assemblies connecting it to the outside world are poorly specified. The most common cable-related EMC issues are conducted emissions on power cables, radiated emissions from unshielded or poorly grounded signal cables, and immunity failures where external fields couple into sensitive signal paths.

A shielded cable is only as effective as its ground connection. The shield must make a 360-degree (circumferential) connection to the connector shell at both ends. Pigtail grounds — where the shield braid is gathered into a single wire and connected to a pin — are a common source of EMC failures. A pigtail creates inductance that reduces shielding effectiveness at higher frequencies. At 100 MHz, a 25 mm pigtail can reduce shielding by 20–30 dB compared to a proper 360-degree termination. For connectors that don't support backshell termination, use a shield clamp or ferrule that maintains circumferential contact.

Keep signal cables physically separated from power cables wherever possible. As a rule of thumb, maintain at least 50 mm separation between unshielded power and signal runs. Where cables must cross, route them at 90 degrees to minimise coupling. Inside enclosures, route cables along the chassis walls rather than spanning open air — the proximity to the grounded chassis provides additional shielding. Bundle related signals together and keep them away from switching power lines and motor drives.

Ferrite cores (snap-on or moulded) are a cost-effective way to suppress common-mode noise on cables. They work by increasing the impedance of the cable to common-mode currents at high frequencies. Choose the ferrite material based on the frequency range you need to suppress. MnZn ferrites are effective from 1–30 MHz; NiZn ferrites work better above 30 MHz. For broadband suppression, stack two different materials. Place ferrites as close to the noise source as possible — typically at the connector where the cable exits the enclosure. Multiple turns through the ferrite core increase impedance but reduce the cable's flexibility.

• Using unshielded cable for signals above 1 MHz — even short runs can radiate enough to fail emissions tests. • Mixing power and signal conductors in the same cable without adequate shielding between them. • Relying on the PCB ground plane to handle cable shield currents — provide a dedicated low-impedance chassis ground path. • Over-specifying cable length — every extra metre of cable adds antenna length. Keep cables as short as practical. • Ignoring connector shell conductivity — anodised or painted connector shells may not make good electrical contact. Specify conductive finishes on mating surfaces.