Host Level Radiated Spurious Emission Pre Scan Chamber Verification Protocol
Host level pre scan verification systematically isolates host radiated spurious noise before accredited certification testing to prevent market entry delays.
Common Mode Cable Noise
Meaning
Unwanted currents flowing in the same direction on all conductors of a cable assembly generate radiated electromagnetic fields that can disrupt electronic equipment. This common mode cable noise is often the primary cause of failures during radiated emissions compliance testing. It occurs when asymmetric signal return paths or ground loops drive currents onto the outer shield or the conductors of external cables.
Noise Mechanism
Imbalances in differential signal paths or poor grounding within the enclosure drive these parasitic currents. These currents act on the cable shield as if it were a linear antenna, radiating energy into the surrounding environment. Designing balanced drive circuits and optimizing the impedance of the ground planes helps suppress common mode cable noise.
Ferrite beads or common mode chokes are often added to external interfaces to introduce high impedance to these currents.
Diagnostic Method
Near field magnetic loops are used to scan along the cable run to identify current concentrations. High levels of radiation near the connector interface indicate poor shield termination to the chassis.
Shielding Technique
Solid termination of the cable shield to a metal enclosure offers a low impedance path to ground. Pigtail connections should be avoided because their high inductance increases the radiation efficiency of the shield. Product designers use metal backshells with 360 degree contact to shield the cable assembly properly.
The handover documentation for the cable assembly must specify these shielding and termination requirements.