Capacitance

The property of a system of conductors, electronic components and dielectrics that permits the storage of electricity when potential differences exist between the conductors. It is measured in farads. For cables it is usually measured as picofarads per foot (pf/ft), which indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair, the insulation of the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that are represented by square waves. A cable with a high capacitance slows down these signals so that they come out of the cable looking more like “saw-teeth” rather than square waves. The lower the capacitance of the cable, the better it performs at higher frequencies.