Fellesmodusstøy

Fellesmodusstøy, often translated as common-mode noise, is an unwanted electrical signal that appears identically on two or more conductors. In signal transmission, this typically refers to noise that affects both the signal line and its reference or ground conductor in the same way. This type of noise is particularly problematic in differential signaling, where the signal is represented by the voltage difference between two conductors. Because common-mode noise affects both conductors equally, it is not cancelled out by the differential receiver, leading to a degradation of the signal-to-noise ratio.

Sources of fellesmodusstøy are varied and can include electromagnetic interference (EMI) from external sources like motors, switching power supplies, or radio frequency transmitters. Internally, noise can be generated by digital switching within the same circuit board or adjacent equipment. Ground loops, where multiple ground connections create unintended current paths, are a common cause of common-mode noise. Poor shielding or inadequate filtering can exacerbate the problem.

Mitigating fellesmodusstøy involves several techniques. Differential signaling, when implemented correctly with twisted-pair cables and proper termination, inherently rejects common-mode noise. Isolation transformers can block common-mode signals from passing between circuits. Common-mode chokes are passive components specifically designed to attenuate common-mode currents while allowing differential signals to pass. Careful circuit design, including proper grounding strategies and shielding, is also crucial in minimizing the generation and propagation of this type of interference.