Coupling Attenuation vs. TCL
These both parameters are defined in the new international cabling standard ISO/IEC 11801 ed. 2.1 and define the electromagnetic characteristics of a cabling system.
TCL (= Unbalanced Attenuation au)
Balance is characterized by attenuation unbalance, i.e. the ratio between the unwanted common mode signal power and the injected differential mode signal power. This common mode signal which arises from imperfections in the cabling system, such as asymmetry, causes electromagnetic emission and affects noise immunity. Attenuation unbalance is characterized for components, including cables and connecting hardware. Limits for unbalance attenuation are also given for cabling. Attenuation unbalance test methods for components are well established for frequencies up to 100 MHz.
Screening effectiveness (= Screening Attenuation as)
Screening effectiveness is characterized for components including cables, connecting hardware and patch cords. At frequencies up to about 30 MHz. the effectiveness of component screening can be characterized by transfer impedance. Transfer impedance is the ratio of the longitudinal voltage developed on the secondary side of a screen to the current flowing in the screen. This unwanted current causes radiation and affects immunity. At higher frequencies screening effectiveness may be characterized by screening attenuation, i.e. the ratio between the common mode signal in the conductors enclosed in the screen and the radiated signal outside the screen.
Coupling Attenuation (= ac)
Balance and screening effectiveness properties may be combined in one parameter, coupling attenuation, which is the ratio between the wanted signal power and the unwanted radiated power from the cabling. Coupling attenuation is normally measured from 30 MHz to 1000 MHz. According to Annex D (electromagnetic characteristics) of ISO/IEC 11801 ed. 2.1 coupling attenuation can be applied to screened and unscreened cables, connecting hardware and cabling. This facilitates a direct comparison of the EMC characteristics of unscreened and screened systems.
What does this mean for cabling systems?
The following diagram shows the interaction between the frequency-depending unbalanced attenuation au (TCL) achieved by the pair twist and the frequency-independent screening attenuation as, which favours EMC.
For screened balanced cabling systems, coupling attenuation ac is computed according to the following formula:
ac = au + as
In the case of unscreened systems, interfering signals are merely suppressed by unbalanced attenuation, which decreases as the frequency increases (since screening attenuation equals zero), which results in more sensitive transmission behavior with increasing transmission rate:
ac = au (since as = 0)
The coupling attenuation of high quality screened systems may be higher by 20 to 40 dB than that of unscreened systems. This means that if for example such screened systems are used, external disturbance can be ten to one hundred times higher than for the unscreened cabling system used in the comparison.