Electrical termination
In electronics, electrical termination is the practice of ending a transmission line with a device that matches the characteristic impedance of the line. Termination prevents signals from reflecting off the end of the transmission line. Reflections at the ends of unterminated transmission lines cause distortion, which can produce ambiguous digital signal levels and misoperation of digital systems. Reflections in analog signal systems cause such effects as video ghosting, or power loss in radio transmitter transmission lines.
For other uses, see Termination.Transmission lines[edit]
Signal termination often requires the installation of a terminator at the beginning and end of a wire or cable to prevent an RF signal from being reflected back from each end, causing interference, or power loss. The terminator is usually placed at the end of a transmission line or daisy chain bus (such as in SCSI), and is designed to match the AC impedance of the cable and hence minimize signal reflections, and power losses. Less commonly, a terminator is also placed at the driving end of the wire or cable, if not already part of the signal-generating equipment.[1]
Radio frequency currents tend to reflect from discontinuities in the cable, such as connectors and joints, and travel back down the cable toward the source, causing interference as primary reflections. Secondary reflections can also occur at the cable starts, allowing interference to persist as repeated echoes of old data. These reflections also act as bottlenecks, preventing the signal power from reaching the destination.
Transmission line cables require impedance matching to carry electromagnetic signals with minimal reflections and power losses. The distinguishing feature of most transmission line cables is that they have uniform cross-sectional dimensions along their length, giving them a uniform electrical characteristic impedance. Signal terminators are designed to specifically match the characteristic impedances at both cable ends. For many systems, the terminator is a resistor, with a value chosen to match the characteristic impedance of the transmission line and chosen to have acceptably low parasitic inductance and capacitance at the frequencies relevant to the system. Examples include 75-ohm resistors often used to terminate 75-ohm video transmission coaxial cables.
Types of transmission line cables include balanced line such as ladder line, and twisted pairs (Cat-6 Ethernet, Parallel SCSI, ADSL, Landline Phone, XLR audio, USB, Firewire, Serial); and unbalanced lines such as coaxial cable (Radio antenna, CATV, 10BASE5 Ethernet).
Types of electrical and signal terminators[edit]
Active[edit]
Active terminators consist of a voltage regulator that keeps the voltage used for the terminating resistor(s) at a constant level.
Signal Termination Applications[edit]
SCSI[edit]
All parallel SCSI units use terminators. SCSI is primarily used for storage and backup. An active terminator is a type of single-ended SCSI terminator with a built-in voltage regulator to compensate for variations in terminator power.
Controller Area Network[edit]
Controller area network, commonly known as CAN Bus, uses terminators consisting of a 120 ohm resistor.