Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

a service or system requiring transmission channels capable of supporting bit rates greater than 2 Mbit/s.
broadband integrated services digital network (B-ISDN)
a generic term that generally refers to the future network infrastructure that will provide ubiquitous availability of integrated voice, data, imagery, and video services.

See decibel
ratio of the signal power (p) to a reference signal power (pref), usually the modulation carrier signal, expressed in decibels referenced to a carrier (dBc). Thus a harmonic signal that is 1/100th of the power in a desired fundamental signal is at -20 dBc.

PdBd = 10 log10 ( p / pref )
Power ratio in decibels referenced to 1 milliwatt.
ratio of the signal power in watts (p watts)to a 1W reference power, expressed in decibels referenced to 1 W (dBW). Thus
1 watt signal power is equal to 0 dBW, and -30 dBW is equal to 0 dBm.
decibel (dB)
a unit of measure that describes the ratio between two quantities in terms of a base 10 logarithm. For example, the ratio between the power level at the input and output of an amplifier is called the power gain and may be expressed in decibels as follows:

G(dB) = 10 log10 ( Pout / Pin )

Terms such as dBm, dBuV, dBW indicate that the decibel measurement was made relative to an established standard. A common power measure reference is 0 dBm, which is defined to be 1 mW (milliwatt, 0.001W). A common voltage reference is 1 μV (1 microvolt).
(1) signal or data that is sent back to a commanding unit from a control process output for use as input in subsequent operations. In a closed-loop system, it is the part of the system that brings back information on the process condition under control.

(2) the provision of a path from the output to the input of a system, such that the output may be made a function of both the input and the previous outputs of the system.

(3) the technique of sampling the output of an amplifier and using that information to modify the amplifier input signal. A portion of the output is "fed back" to the input. Positive feedback occurs when the output is added to the input; negative feedback occurs when the output is subtracted from the input. Negative feedback, invented by communications engineer Harold Black in 1928, usually results in a gain - bandwidth tradeoff: decreasing and stabilizing the amplifier gain, while increasing the bandwidth. According to Norbert Wiener, feedback is a method of controlling a system by reinserting into it the results of its past performance.