What are twisted pair cables?

The twisted pair cable consists of a pair of insulated wires twisted together. It is a cable type used in telecommunication,audio,DMX for very long time. Cable twisting helps to reduce noise pickup from outside sources and crosstalk on multi-pair cables.

Twisted pair cable is good for transferring balanced differential signals. The practice of transmitting signals differentially dates back to the early days of telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission bring are particularly valuable in wide bandwidth and high fidelity systems. By transmitting signals along with a 180 degree out-of-phase complement, emissions and ground currents are theoretically canceled. This eases the requirements on the ground and shield compared to single ended transmission and results in improved EMI performance.

The most commonly used form of twisted pair is unshielded twisted pair (UTP). It is just two insulated wires twisted together. any data communication cables and normal telephone cables are this type. Shielded twisted pair(STP) differs from UTP in that it has a foil jacket that helps prevent crosstalk and noise from outside source. In data communications there is a cable type called FTP (foil shielded pairs) which consists of four twisted pair inside one common shield (made of aluminium foil).

When cable twisted at constant twist rate over the length of the cable, a cable with well defined characteristic impedance is formed. Characteristic impedance of twisted pair is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance. Because the impedance of a cable is actually a function of the spacing of the conductors, so separating the conductors significantly changes the cable impedance at that point.

When many twisted pairs are put together to form a multi-pair cable, individual conductors are twisted into pairs with varying twists to minimize crosstalk. Specified color combinations to provide pair identification.
The most commonly used twisted pair cable impedance is 100 ohms. It is widely used for data communications and telecommunications applications in structured cabling systems. In most twisted pair cable applications the cable impedance is between 100 ohms and 150 ohms. When a cable has a long distance between the conductors, higher impedances are possible. Typical wire conductor sizes for cables used in telecommunications 26AWF, 24AWF, 22AWG or 19AWG.

Here are some common impedances related to twisted pair lines:
1、100 ohms（Ethernet Cable）: This impedance is the standardized impedance to be used in the twisted pair wiring used in structured wiring systems standardized EIA/TIA 568 standard. Both unshielded and shielded “CAT5，CAT6，CAT7   and better” cables used on this kind of applications have 100 ohms impedance (usually at +-15% or better accuracy). Nowadays the most common LAN standard, Ethernet, is designed for 100 ohms twisted pair cable.

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2、110 ohms（DMX Lighting Cable）: 110 ohms shielded twisted pair cable is standardized as the cable type to be used for digital AES/EBU sound interface.

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3、120 ohms (RS485 Communication Cable): 120 ohms shielded cable is generally used for for RS485 communications in industrial networking. There are many industrial “control and data” cables which have impedance of around 120 ohms. Also some telecom cables (both shielded and unshielded) have impedance of 120 ohms, and there are digital telecom systems matched to this impedance.
4、150 ohms (Control and Data” Cable): This was the impedance used in shielded twisted pair wiring IBM cabling system and Token Ring network. There are also many shielded “control and data” cables that has impedance of around 150 ohms in use nowadays. Some modern microphone cabling (shielded twisted pair) has impedance of around 150 ohms at high frequencies and you can sometimes hear 150 ohms impedance mentioned in analogue audio applications (typical dynamic professional microphones have impedance of 150-200 ohms usually).
5、300 ohms (Antenna Cable): The twin lead wire used in some antenna applications has impedance of 300 ohms. This is a very low loss antenna cable type. 300 ohms is generally not used for anything else than some antenna applications.
6、600 ohms(Telephone Cable): 600 ohms is a standardized impedance used in telephone world. The first long telephone air lines (two wires on the poles separated from each other at some distance) used to have impedance of around 600 ohms. In practice the modern telephone cable do not have impedance of 600 ohms, but for historical reasons this impedance is spoken often and many telephone equipment are still matched to this impedance. You can sometimes (quite rarely nowadays) hear 600 ohms matching also in audio world.

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