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'A' filtering - mimics human ear's frequency response

ADC (analogue-to-digital converter) - electronic device which converts an analogue voltage to a digital number

Aliasing - undesired modulation which distorts the baseband signal, caused by under-sampling, eg by ADC's in oscilloscopes or telecoms applications

AM (amplitude modulation) - the amplitude of a carrier wave is modified by an analogue or digital signal which is to be transmitted (see also FM and PM)

AMI (alternate mark inversion) - a method of transmitting binary digits where successive marks, equal in magnitude, normally alternate between positive and negative polarity

AMPS (american mobile phone standard) - US analogue cellular radio standard

Analogue - direct representation of data, such as a human voice, by a continuously varying electrical signal

ANSI - American National Standards Institute

ASCII (american standard code for information interchange) - represents text by binary numbers

ASIC - application-specific integrated circuit

Asynchronous data - transmission of digital data without locking the sender and receiver's clocks

ATM (asynchronous transfer mode) - a flexible high-bandwidth networking standard which uses cell-relay rather than packet switching or direct connection and avoids the need to synchronise the sender and receiver

Attenuation - reduction of signal strength, measured in dB; negative gain

AU (administrative unit) - high order SDH virtual carrier with pointer attached, ready for STM-N transmission

AUG (administrative unit group) - SDH AU, provides compatibility with SONET

Autonomy Time - length of time that an uninterruptible power supply can maintain full output when the mains input is removed

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B3ZS/B6ZS (bipolar 3/6 zero substitution) - coding format where strings of 3 or 6 consecutive zeros are replaced by a shorter code

Bandwidth - the difference between the lowest and highest frequency of a communications channel, used as a measure of its data carrying capacity

Band III - UK trunked PMR using 420MHz frequency band

Baseband signal - the signal input or output from a modulated transmission system, such as a human voice or computer data stream

Baud rate - the data transmission rate of a serial communications link, expressed in bits per second (b/s)

BCCH (broadcast control channel) - GSM signalling feature

BER - bit error rate

Binary - base-two number system widely used in digital and computer systems; it uses two symbols, 0 and 1, often represented by two voltage levels

Bit (binary digit) - the smallest unit of information in a binary system; a bit can have value 0 or 1

BLER (block error rate) - BER modified to count errors in transmitted blocks of bits

BS EN ISO 9000 - quality systems standard (formerly BS 5750)

Buffer - storage device used to accommodate different rates of data flow or event timing when transmitting data from one device to another

Burst mode - transmission of a short stream of data, often triggered by handshaking; in a high-speed network, the address of the data is sent, followed by the data itself

Bus - multi-wire electrical connection between devices, used in computers (eg AT bus and EISA bus), test equipment (eg IEEE 488), and mains systems (ie 3 phase and neutral)

Byte - eight bits of data, which form a digital 'word'; computer memory is arranged as an array of one or more bytes

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C (container) - PDH data stream used for SDH transmission

Carrier - often a high frequency sine wave to which modulation is added to transmit information

Cat 5 - standard for structured premises wiring, with a frequency response up to 100MHz

CCITT - Renamed the ITU-TS

CDMA (code division multiple access) - radio access method where users are allocated a unique code sequence within a spread-spectrum communication system

CMI (coded mark inversion) - a more complex version of NRZ which relies on transitions to indicate binary 1 or 0, used for higher rate PDH links

CMRR (common-mode rejection ratio) - a measure of a circuit's ability to reject signals common to both input connections, usually expressed in dB

Compander (compressor-expander) - variable gain device which automatically adjusts a signal level to maintain it within defined limits

Coupler - an optical or electrical device which splits or combines signals

CRC (cyclical redundancy check) - an extra bit is added to a set of data so the sum of the digits is odd (or even); enables detection of single errors in storage or transmission

CRT (cathode ray tube) - display used in monitors and televisions

Crest factor - the ratio of the peak voltage of an AC waveform to its RMS value

Crosstalk - the unwanted transfer of a signal from one circuit to another

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DAC (digital-to-analogue converter) - electronic device which converts a digital number into a corresponding analogue voltage or current

DASS 2 (digital access signalling system) - UK predecessor to ISDN

DAT - digital audio tape

dB (Decibel) - logarithmic ratio of two signals (see Formulae & Conversion)

dBc/Hz - noise density in a 1Hz equivalent bandwidth of a modulation system, relative to the carrier amplitude

dBm - absolute measure of signal power, where 0dBm equals 1mW

DCS1800 (digital cellular standard at 1800MHz) - GSM at twice the frequency

DDCMP (digital data communications message protocol) - proprietary computer-to-computer communications protocol

DECnet - proprietary network architecture that works across all Digital Equipment computers; uses a peer-to-peer approach

Digital - representation of data by the nearest binary number

Disassembler - software tool which allows on-line evaluation and debugging of computer hardware and software

Distortion - undesired changes to a waveform

DOS - PC disk operating system

DPNSS (digital private network signalling system) - UK telecoms protocol for private networks

DSP (digital signal processor) - digital implementation of a signal filter

DS1 (digital signal 1) - T1 transmission standard (1.544Mb/s)

DS3 (digital signal 3) - T3 transmission standard (44.736Mb/s)

DTMF (dual-tone multiple frequency) - signalling system generated by pushbutton telephones

DVB - digital video broadcasting

Duplex - simultaneous, two-way independent data transmission

Dynamic range - the ratio of the largest signal level a circuit can handle to the smallest signal level it can detect (usually its noise level), usually expressed in dB

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EBCDIC (extended binary coded decimal interchange code) - eight bit character code used primarily in IBM computers

EEPROM (electrically erasable programmable read-only memory) - ROM that can be erased with an electrical signal and reprogrammed

EFS (error free seconds) - similar measure to BER and BLER

EIA (Electronic Industries Association) - a consultative group of manufacturers for standardising electronic equipment

EISA (extended industry standard architecture) - 32-bit adaptation of the 8/16-bit buses originally developed by IBM

EMC (electromagnetic compatibility) - definition of the acceptable limits for electromagnetic emissions by an electronic device, and how much electromagnetic interference it should tolerate

EPLD - electronically programmable logic device

EPROM (erasable programmable read-only memory) - ROM that can be erased, usually by exposure to ultraviolet light, and reprogrammed

ESD - electrostatic discharge

Ethernet - widely used LAN transmission network, based on a bus network topology, runs at 10 or 100Mb/s

ETSI - European Telecommunications Standards Institute

External trigger - a voltage pulse from an external source that triggers an event

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FDDI (fibre distributed data interface) - ANSI standard for fibre optic LAN, based on 100Mb/s logical ring topology, using token-passing access control

FDM (frequency division multiplexer) - a device which divides up the available bandwidth of a transmission path into separate communications channels

FDMA (frequency division multiple access) - radio access method where users are allocated an available frequency channel on request

FFT - fast fourier transform - mathematical method of analysing the frequencies in a measured waveform

FM (frequency modulation) - the frequency of a carrier wave is modified by an analogue or digital signal which is to be transmitted (see also AM and PM)

Frequency - the number of times a signal recurs during a set period, measured in Hertz (Hz)

FSK (frequency shift keying) - a form of FM in which the frequency of the carrier is modulated, or keyed, by a digital signal

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Gain - the factor by which a signal is amplified, sometimes expressed in dB

GAL - gate array logic

Glitch - a transient pulse in a digital circuit, narrower than the system's clock width

GMSK (gaussian minimum shift keying) - digital modulation scheme designed to minimise required bandwidth, used in GSM systems

GPIB (general-purpose interface bus) - alternative name for IEEE 488

GSM - pan-European digital cellular standard

GUI - graphical user interface

G.703 - cabling electrical specifications and line codes used by digital transmission interfaces

G.704 - specification for the functional characteristics of inter-network interfaces

G.732 - characteristics of A -law PCM multiplex equipment, specifying frame alignment loss, jitter, alarm indications and power loss

G.821 - performance criteria for digital connections, defines allowable error rates and locations

G.823 - specification for jitter and wander allowable on international digital connections using A-law systems

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Handshaking - method by which data is exchanged between devices only when each device has signalled that it is ready

Hayes command set - de-facto standards for controlling modem dialling operations and setting operational parameters

HDB2/3 (high density bipolar of order 2/3) - coding system for providing AMI with extra transitions to allow timing recovery

HDLC (high-level data-link control) - set of ISO protocols for carrying data over a link with error and flow control

HDTV - high definition television

HP-GL (Hewlett-Packard graphics language) - set of easy-to-understand mnemonic commands for controlling functions such as pen movement, character generation, and axis printing

HP-IL (Hewlett-Packard interface loop) - serial interface designed for low cost battery operable systems

HP-IB (Hewlett-Packard interface bus) - alternative name for IEEE 488, a parallel interface designed by HP

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IC - integrated circuit, also known as a 'chip'

IEEE (Institute of Electrical and Electronic Engineers) - US professional body also involved in electronic standards

IEEE 488 - high-speed parallel communications interface, used extensively in Test & Measurement equipment

IEEE 802.3 - IEEE standardisation of Ethernet LAN protocol

IEEE 802.5 - IEEE standardisation of IBM's Token-Ring LAN protocol

IP- internet protocol

IrDA-1 - infra-red computer interface

I/O - input/output

IRIG WB1&2 (inter range instrumentation group wideband 1&2) - standard for recording, WB1 is DC to 20kHz and WB2 is DC to 125kHz

ISA (industry standard architecture) - 8/16-bit bus standard used in PCs

ISDN (integrated services digital network) - public telecoms network standard for transmitting voice, data and digital images such as videoconferencing

ISO - International Standard Organisation

ITU (International Telegraphic Union) - part of the United Nations which deals with telecoms

ITU-TS (ITU - Telecommunication Standards) - global telecoms standards body, formerly known as CCITT

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Jitter - instability of a signal, effectively a form of FM or PM

JPEG (Joint Photographic Experts Group) - standard for digital compression of graphic-image files

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LAN (local area network) - datacoms network that links computers into a network over a limited geographical area, allows users to communicate and share resources

LCC - leadless chip carriers

LO - local oscillator

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Microlease plc - exceptional service organisation used to satisfy electronic test equipment rental needs

MNP 4, 5, 10 - data compression standards for modems

Modulation - controlled variation of amplitude, frequency, and/or phase of a carrier wave in order to transmit a message

MPEG (Motion Pictures Experts Group) - standard for digital compression of video recordings

MPT1327 - UK analogue trunked radio standard

Multiplexer (mux) - device which outputs one of a number of input signals as commanded by a control input

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NAMAS (National Accreditation of Measurement and Sampling) - the standard to which UKAS accredits calibration and testing laboratories

NATEL - Swiss analogue radio standard

NEXT (near-end crosstalk) - interference between conductors of a cable when used in a LAN application

NICAM (near instantaneous companded audio multiplex) - stereo TELEVISION audio broadcasting standard

Noise - undesirable electrical signal

NRZ (non-return to zero) - two-state code which represents binary symbols simply by high and low signal levels

NTSC (National Television System Committee) - US standard for analogue colour television broadcasting

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O/C - open circuit

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PAL (phase alternation by line) - European standard for analogue colour television broadcasting

Parallel interface - datacoms interface in which data is transmitted simultaneously along several wires

Parity bit - a bit that is set at 0 or 1 in a data word to ensure that the total number of bits is even or odd, as desired

PBX (private branch exchange) - private telephone switch connected to a public network

PCL5 - printer language

PCM (pulse code modulation) - quantisation of an analogue signal, such as a voice link, for digital transmission

PCM30 / 31 - PCM framing standard

PCS (personal communication system) - US digital cellular standard similar to DCS1800

PCMCIA (Personal Computer Memory Card International Association) - physical and electrical standard for PC cards, such as memory, modems and interfaces

PDC - personal digital cellular

PDH (plesiochronous digital hierarchy) - a form of TDM where dummy information is added to synchronise data streams of slightly different speeds, ready for transmission over a higher-speed link

Period - a defined length of time

PF (power factor) - a measure of the phase alignment of voltage and current in an AC power system

PGA - pin grid array

Phase noise - short-term, random phase variations of an RF signal resulting from electrical noise within the signal generator, usually expressed in dBc/Hz

Ping - TCP/IP function for confirming connection to a remote IP address

PLCC - plastic leaded chip carriers

PM (phase modulation) - the phase of a carrier wave is modified by an analogue or digital signal which is to be transmitted (see also AM and FM)

PMR (private mobile radio) - wide area mobile radio system used by closed group organisations such as utilities and emergency services

POCSAG - pager encoding format

Post Script - printer language

ppm - parts per million

PQFP (plastic quad flat pack) - integrated circuit package

PRBS - pseudo-random binary sequence

PROM (programmable read-only memory) - non-volatile memory chip which can be programmed (once) with an electrical signal

PSTN (public switched telephone network) - otherwise known as the dial-up telephone network

PVC (permanent virtual circuit) - ATM optimisation standard

PWM (pulse width modulation) - the pulse width of a square wave is modified by an analogue or digital signal which is to be transmitted (see also AM, FM and PM)

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RAM (random access memory) - fast memory chip which retains stored information only while power is applied

Raster - constant scanning pattern of an electron beam on a CRT display which forms images by turning the beam on and off at appropriate times

Real time - data is processed as it is acquired, rather than being stored and processed later

Refractive index - ratio of the speed of light in a vacuum to the speed of light in a material

Resolution - the smallest signal increment that can be detected by a measurement system, expressed in bits, proportions, or as percentage of full scale reading; resolution in a video system is the amount of detail in a graphic image

RF - radio frequency

RISC (reduced instruction set computing) - computing architecture in which fewer processor instructions are defined so that most can be performed in a single processor cycle, improving computing efficiency


Rise time - time required for the leading edge of a pulse to rise from 10% to 90% of its final value

RMS (root mean square) - a form of averaging

ROM (read only memory) - memory chip used for permanently recorded computer programs and data

RS232 - serial interface standard

RS423 - serial interface for cables longer than the RS232 24m limit

RS449 - physical interface standard for RS422 and RS423 signals

RZ (return to zero) - two-state code in which binary 1 symbols are represented by a pulse rather than a level, ie the signal returns to zero for the second part of the system clock cycle

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SACCH (slow associated control channel) - GSM signalling feature

SCM - suppressed carrier modulation

SCSI (small computer standard interface) - interface standard for connecting intelligent peripherals such as hard disks, printers, and optical disks; pronounced 'scuzzy'

SDH (synchronous digital hierarchy) - ITU-TS standard for transmission of synchronously multiplexed tributary signals; aligned with SONET from 155Mb/s (STM-1) and higher

SDLC (synchronous data link control) - the primary data link protocol used in SNA networks

SECAM (séquential couleur à mémoire) - French standard for analogue colour television broadcasting

SEL - sound exposure level

SELCALL (selective calling) - analogue PMR feature which uses tone encoding/decoding

Serial interface - datacoms interface in which data is transmitted bit by bit down a single pair of wires

SIM Card (subscriber identity module) - used in digital cellular mobile phones to program data such as the unit's telephone number and home network operator

SINAD - ratio of combined signal plus noise and distortion, to noise and distortion, expressed in dB

Single-mode fibre - an optical fibre that supports only one mode of light propagation

Slew rate - maximum rate of change of an amplifier's output when operated within its linear region

SMD - surface mount device

SNA (systems network architecture) - IBM's proprietary standard for communication between hardware devices

SNAP (subnetwork access protocol) - an IP which implements logical link control and enables non-standard higher layer protocols

SNMP (simple network management protocol) - originally designed to manage TCP/IP networks, now used more widely

SNR (signal-to-noise ratio) - the ratio of overall RMS signal level to RMS noise level in dB

SONET (synchronous optical network) - US standard for transmission of synchronously multiplexed tributary signals

SPL - sound pressure level

SSB (single side band) - radio communication where half of the modulator output is transmitted to reduce channel bandwidth and transmitted power

SS7 (signalling system number 7) - ITS digital transmission protocol for interconnection of telephone exchanges

Strain gauge - a transducer which changes its resistance in proportion to the applied force

STM (synchronous transport module) - SDH frame into which a 'payload' of data is multiplexed for transmission; 'overhead' space is also allowed for network control

SVC (switched virtual circuit) - ATM optimisation standard

Synchronous data transmission - information is transmitted at predictable times usually defined by a clock signal

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TCP/IP (transmission control protocol / internet protocol) - set of protocols that allows sharing of applications among PC's, host computers and workstations in a high-speed communications environment

TDD (time division duplex) - bi-directional radio access method where the same frequency is used for both transmit and receive modes

TDM (time division multiplexer) - multiplexer which combines tributary channels by interleaving the data streams

TDMA (time division multiple access) - radio access method where users are allocated an available time slot within a frequency channel on request

Thermocouple - temperature transducer formed from two dissimilar metals, which produce a small voltage dependent on the temperature

Time base - calibrated sweep generator used to control displays, such as oscilloscopes, or the repetitive variation of a measurement parameter, for instance in network analysers and sweep generators

TIMS - transmission impairment measurement standard

THD (total harmonic distortion) - the ratio of the sum of harmonic distortion signals to the overall RMS signal, in dB or percent

Token-Ring Network - widely used LAN transmission network in which access is controlled by passing a token to each station around the network in a predetermined sequence

TOP (technical and office protocol) - Boeing proprietary teletext standard

Transconductance - the ratio of the incremental change in the output current of an amplifier to the incremental change of input voltage

Transducer - a device which responds to a physical stimulus (heat, light, sound, pressure, motion, flow, etc), and produces a corresponding electrical signal

Transceiver - a combined transmitter and receiver

Trigger - a stimulus which starts a measurement or display function which then continues under its own control

TU (tributary unit) - SDH virtual carrier with pointer attached

TUG (tributary unit group) - SDH tributary unit with several TUs interleaved ready for a higher order virtual carrier

Twisted pair - two insulated copper conductors that are wound around each other to reduce the effects of induced electrical noise

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UKAS - United Kingdom Accreditation Service

UNI (user network interface) - ATM standard

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VAR (volt-ampere reactive) - the out-of-phase component of an AC power system which does no useful work

VC (virtual carrier) - PDH container with path overhead ready for SDH TU or AU

VCI (virtual channel identifier) - ATM standard

VCO - voltage controlled oscillator

VDEW - German digital PMR signalling system

VPI (virtual path identifier) - ATM standard

VPT - teletext format

VSWR (voltage standing wave ratio) - the ratio of maximum to the minimum voltage along a transmission line, caused by reflections

V Series - ITU-TS recommendations governing data transmission over analogue telephone lines; the series includes:

V.21 - 300b/s duplex modem, similar to RS422

V.22 - 1,200b/s duplex modem

V.22bis - 2,400b/s duplex modem

V.23 - 600 or 1,200b/s FSK modem

V.24 - interchange circuits, similar to and compatible with RS232

V.32 - up to 9,600b/s modem

V.32bis - up to 14.4kb/s modem

V.35 - 34-pin 48kb/s interface

V.36 - 4-wire datacoms at speeds of 48kb/s and higher

V.42 - error correction protocol

V.42bis - data compression technique for use with V.42

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WAN (wide area network) - datacoms network covering a wider geographical area than a LAN using telecoms links

Waveguide - transmission line consisting of a hollow conductor used to guide microwaves from one end to the other

White Noise - broadband pseudo-random signal with uniform power in each frequency band

Word - the standard number of bits that a processor or memory manipulates at one time; computers and microcontrollers typically use 8, 16 or 32-bit words

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XNS (Xerox network systems) - a proprietary peer-to-peer protocol developed by Xerox, incorporated into several LAN schemes

X-ON/X-OFF (Transmitter On / Transmitter Off) - control characters used for flow control instructing a terminal to start transmission (X-ON) and stop transmission (X-OFF)

X Series - ITU-TS recommendations governing data transmission over public data networks; the series includes:

X.20 - asynchronous communications interface for use over PSTN

X.21 - synchronous communications interface for use over PSTN

X.25 - packet-switched data interface

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ZIF (zero insertion force) sockets - socket for an integrated circuit package which reduces the chances of damage to its pins

ZVEI1 - German digital PMR signalling standard

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