Screened and Shielded Cabling - Noise Immunity, Grounding, and the Antenna Myth
The benefit of specifying balanced twisted-pair cabling for data transmission is clearly demonstrated by examining the types of signals that are present in building environments. Electrical signals can propagate in either common mode or differential (i.e. "balanced") mode. Common mode describes a signal scheme between two conductors where the voltage propagates in phase and is referenced to ground.
- Examples of common mode transmission include dc circuits, building power, cable TV, HVAC circuits, and security devices. Electromagnetic noise induced from disturbers such as motors, transformers, fluorescent lights, and RF sources, also propagates in common mode.
Virtually every signal and disturber type in the building environment propagates in common mode, with one notable exception:
- twisted-pair cabling is optimized for balanced or differential mode transmission.
Differential mode transmission refers to two signals that have equal magnitudes, but are 180º out of phase, and that propagate over two conductors of a twisted-pair. In a balanced circuit, two signals are referenced to each other rather than one signal being referenced to ground. There is no ground connection in a balanced circuit and, as a result, these types of circuits are inherently immune to interference from most common mode noise disturbers.
In theory, common mode noise couples onto each conductor of a perfectly balanced twisted-pair equally. Differential mode transceivers detect the difference in peak-to-peak magnitude between the two signals on a twisted-pair by performing a subtraction operation. In a perfectly balanced cabling system, the induced common mode signal would appear as two equal voltages that are simply subtracted out by the transceiver, thereby resulting in perfect noise immunity.
In the real world, however, twisted-pair cables are not perfectly balanced and their limitations must be understood by application developers and system specifiers alike. TIA and ISO/IEC committees take extreme care in specifying balance parameters such as TCL (transverse conversion loss), TCTL (transverse converse transfer loss) and ELTCTL (equal level transverse converse transfer loss) in their standards for higher grade (i.e. category 6 and above) structured cabling. By examining the performance limits for these parameters and noting when they start to approach the noise isolation tolerance required by various Ethernet applications, it becomes clear that the practical operating bandwidth defined by acceptable levels of common mode noise immunity due to balance is approximately 30 MHz. While this provides more than sufficient noise immunity for applications such as 100BASE-T and 1000BASE-T, Shannon capacity modeling demonstrates that this level provides no headroom to the minimum 10GBASE-T noise immunity requirements. Fortunately, the use of shielding significantly improves noise immunity, doubles the available Shannon capacity, and substantially increases practical operating bandwidths for future applications.
An effect of degraded twisted-pair signal balance above 30 MHz is modal conversion, which occurs when differential mode signals convert to common mode signals and vice versa. The conversion can adversely impact noise immunity from the environment, as well as contribute to crosstalk between pairs and balanced cables and must be minimized whenever possible. Shielding can decrease the potential for modal conversion by limiting noise coupled onto the twisted-pair from the environment.