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Glossary (1 - D)
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1 - 10

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3DES triple data encryption standard  Using the DES encryption three times to improve security.
2G cell phone system  Second-generation digital cell phone system. The systems are GSM, TDMA, and CDMA.
2.5G cell phone system  A generation of cell phone between the original second-generation (2G) digital phone and the newer third-generation (3G) phone. It brings data transmission capability to 2G phones.
3G cell phone system  Third-generation cell phones, which are true packet data phones with enhanced digital voice and high-speed data transmission capability.
3GPP third generation partnership project  An organization dedicated to developing and improving cell phone technologies that are standardized through the ITU.
4G cell phone system  Fourth generation of cell phones. Used to describe still-to-come advanced cell phone technologies that feature OFDM and packet data rates up to 100 Mbps in a mobile environment.
10Base-2 Ethernet LAN  A type of Ethernet local-area network system implemented with thin coaxial cable.
10Base-5 Ethernet LAN  An Ethernet system using thick coaxial cable.
10Base-T Ethernet LAN  The twisted-pair version of Ethernet. The most widely used version.
10-gigabit Ethernet  A new version of Ethernet that permits data speeds up to 10 Gbps over fiber-optic cable.
100Base-TX.  See Fast Ethernet.
1000Base-T Ethernet  See Gigabit Ethernet.
1496/1596 circuit  A typical IC balanced modulator that can work at carrier frequencies up to approximately 100 MHz and can achieve a carrier suppression of 50 to 65 dB.
3089 IF system  A receiver IC originally developed by RCA.
   

 

A

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Absolute field strength meter  Device used for accurate measurements of signal strength. Usually measured in microvolts per meter.
Absorption  In fiber-optic communication, the way light energy is converted to heat in the core material owing to the impurity of the glass or plastic.
Absorption wave meter  Variable tuned circuit with an indicator that tells when the tuned circuit is resonant to a signal coupled to the meter by a transmitter; provides a rough indication of frequency.
Access method  The protocol used for trans­mitting and receiving information on a bus.
Accumulator  The combination of a register and adder.
Acknowledge character (ACK)  A character that acknowledges that a transmission was received.
Advanced encryption standard (AES)  An encryption method developed by the National Institute of Standards and Technol­ogy (NIST) to be more secure than the older DES standard because it uses 128, 192 or 256 bit keys.
Advanced mobile phone system (AMPS)  The original cell phone system, based on analog radio technologies.
A-law compander  The type of companding used in European telephone networks.
Aliasing  A problem that occurs when the sampling frequency is not high enough. Aliasing causes a new signal near the original to be created.
All-pass filter  A filter that passes all frequen­cies equally well over its design range but has a fixed or predictable phase shift characteristic.
American digital cellular (ADC)  See IS-136 TDMA.
American Standard Code for Information Interchange (ASCII)  The most widely used data communication code. This is a 7-bit binary code.
Amplification  The boosting of signal voltage and power.
Amplitude modulation  Varying the amplitude of a carrier signal to transmit information.
Amplitude shift keying (ASK)  Amplitude modulation by square waves or rectangular binary pulses.
Analog multiplier  A type of IC that can be used as a balanced modulator. It uses differential amplifiers operating in a linear mode.
Analog oscilloscope  Device that amplifies the signal to be measured and displays it on the face of a CRT at a specific sweep rate.
Analog signal  A smoothly and continuously varying voltage or current.
Analog-to-digital conversion (A/D conversion)  The process of sampling or measuring an analog signal at regular intervals and con­verting it into a binary value.
Angle modulation  A collective term referring to FM and PM.
Angle of elevation  The angle between the line from the earth station’s antenna to the satel­lite and the line between the same antenna and the earth’s horizon.
Angle of incidence  An angle A between the incident ray and the normal at the reflecting surface.
Angle of inclination  The angle formed between the line that passes through the center of the earth and the north pole and the line that passes through the center of the earth and is also perpendicular to the orbital plane.
Angstrom (Å)  A unit of measure for light wavelength, equal to 1010 m or 104µm.
Answering machine  A feature on an electronic telephone that answers a call after a prepro­grammed number of rings and saves the voice message. More formally called voice mail.
Antenna  An electromechanical device used to transmit or receive radio signals.
Antenna array  Two or more antenna elements combined to create an antenna with directiv­ity and gain.
Antenna bandwidth  The range of frequencies over which an antenna operates efficiently; the difference between the upper and lower cutoff frequencies of the antenna, which acts as a resonant circuit or bandpass filter.
Antenna directivity  The ability of an antenna to send or receive signals over a narrow hori­zontal or vertical directional range.
Antenna Q  The ratio of inductive reactance to resistance. Although it is difficult to calculate the exact Q for an antenna, the higher the Q, the narrower the bandwidth. Lowering Q widens bandwidth.
Antenna radiation pattern  The geometric shape of the signal strength around an antenna.
Antenna reciprocity  The condition that exists when the characteristics and performance of an antenna are the same whether the antenna is radiating or intercepting a signal.
Antenna resonance  The frequency at which an antenna has peak voltage or current.
Antenna subsystem  An assemblage of devices used for both transmitting and receiving signals.
Antenna tuner  A configuration consisting of a variable inductor, one or more variable capacitors, or a combination of these components connected in various ways.
Antenna tuning  ­A technique used to maximize power output by impedance matching.
Anti-aliasing filter  A low-pass filter usually placed between the signal source and the A/D converter to ensure that no signal with a frequency greater than one-half the sample frequency is passed.
Antilog  The number obtained when the base is raised to the logarithm that is the exponent.
Anti-transmit-receive (ATR) tube  A type of spark gap tube. It effectively disconnects the transmitter from the circuit during the receive interval.
Apogee  The highest point of a satellite above the earth.
Arbitrary waveform generator  A signal gen­erator that uses digital techniques to generate almost any waveform.
Array waveguide grating (AWG)  An array of optical waveguides of different lengths made with silica on a silicon chip that can be used for both multiplexing and demultiplexing.
Asynchronous data transmission  A transmis­sion method in which each data word is accompanied by start and stop bits that indi­cate the beginning and ending of the word.
Asychronous digital subscriber line (ADSL)  The most widely used form of digital sub­scriber line (DSL). It permits data rates up to 8 Mbps and upstream rates up to 640 kbps using the existing telephone lines.
Asynchronous protocol  The simplest form of protocol for ASCII-coded data transmission, using a start bit and a stop bit framing a single character, and with a parity bit between the character bit and the stop bit.
Asynchronous transfer mode (ATM) system  A packet-switching system used in Internet backbones. ATM systems break data into 53-byte packets and transmit them over fiber-optic networks at speeds up to 10 Gbps.
Atmospheric noise  The electrical disturbance that occurs naturally in the earth’s atmos­phere. Also known as static.
Atomic clock  An electronic oscillator that uses the oscillating energy of a gas to provide a stable operating frequency.
Attachment user interface (AUI)  Device in which inputs and outputs from the NIC terminate when thick Ethernet network cable is used.
Attenuation  The reduction in signal amplitude over distance.
Audio  The voice or sound portion of a commu­nication system.
Authentication  An electronic method of verifying who a person is for the secure exchange of information.
Automatic frequency control (AFC)  A feed­back control circuit that is used in high-frequency receivers and keeps the local oscillator on frequency.
Automatic gain control (AGC)  A feature of receivers that ensures that the output signal remains constant over a broad range of input signal amplitudes.
Automatic power control (APC) circuit  In the analog AMPS cellular phone system, a circuit that sets the transmitter to one of eight power output levels.
Automatic splicer  A device used to achieve highly accurate alignment and splicing of fiber-optic cable.
Automatic voice level adjustment  The process of having a circuit sense and adjust the gain of an amplifier used to amplify voice signals in a radio or telephone system.
Autoranging  A feature of frequency counters that selects the best time-base frequency for maximum measurement resolution without overranging.
Avalanche photodiode (APD)  A widely used, sensitive, and fast photosensor. It is reverse-biased.
Azimuth  The compass direction in which north is equal to 0°.
   

 

B

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B - Y signal  I and Q color signals are also called R-Y and B-Y. Combining the three color signals causes Y to be subtracted from R or B signals.
Backbone LAN  A central network with nodes that are other LANs. This provides a convenient way to interconnect a variety of different LANs for organizationwide communication.
Backward wave oscillator (BWO)  A type of TWT in which the wave travels from the an­ode end back toward the electron gun, where it is extracted.
Balanced modulator  A circuit that generates a DSB signal, suppressing the carrier and leav­ing only the sum and difference frequencies at the output.
Balanced transmission line  A transmission line with neither wire connected to ground. Instead, the signal on each wire is referenced to ground.
Balun  A device used to convert transmission lines from balanced to unbalanced operation.
Bandpass filter  A filter that passes frequencies over a narrow range between lower and upper cutoff frequencies.
Band-reject, bandstop, or notch filter  A filter that rejects or stops frequencies over a nar­row range but allows frequencies above and below to pass.
Bandwidth  The narrow frequency range over which the signal amplitude in a circuit is highest. The portion of the electromagnetic spectrum occupied by a signal. The range of frequencies a cable will carry. It determines the maximum speed of the data pulses the cable can handle.
Barrel connector  A variation of the BNC con­nector that allows two cables to be attached to one another end to end.
Base station  A cell site in a cellular telephone system, the most visible feature of which is its antenna on a tower.
Baseband LAN  A local-area network in which the data to be transmitted is placed directly on the transmission medium, usually a twisted pair cable, as binary voltage levels.
Basic rate interface (BRI)  A type of ISDN connection using one twisted pair. Two bearer (B) voice channels and one data con­trol (D) channel are multiplexed on the line.
Baud rate  In digital communication systems, the number of signaling elements or symbols that occur in a given unit of time.
Baudot code  Rarely used today, this binary data code was used in early teletype machines.
Beam antenna  An antenna that is highly directional and has very high gain.
Beamwidth  The measure of an antenna’s directivity.
Beat frequency oscillator (BFO)  An oscillator that is built into receivers designed to receive SSB or CW signals.
Bessel filter  A filter that provides the desired frequency response but has a constant time delay in the passband.
Bessel function  A mathematical process used to solve the FM equation.
Bicone antenna  A widely used omnidirec­tional microwave antenna. Signals are fed into the antenna through a waveguide or coaxial cable ending in a flared cone.
Bidirectional antenna  An antenna that receives signals best in two directions.
Bidirectional coupler  A variation of the direc­tional coupler in which two sections of line are used to sample the energy in both direc­tions, allowing determination of the SWR.
Binary check code, block-check character, or block-check sequence (BCC or BCS)  The logical sum of the data bytes in a block used as a method of error detection.
Binary code  A type of code consisting of pat­terns of 0s and 1s. Each pattern represents number a letter of the alphabet, or some special symbol such as punctuation or a mathematical operation.
Binary phase-shift keying (BPSK)  A type of PSK in which a standard lattice ring modula­tor or balanced modulator is used for gener­ating DSB signals.
Binary phase-shift keying (BPSK) or phase-shift keying (PSK)  The process of modu­lating a carrier with binary data by changing the carrier phase 180.
Biphase encoding  See Manchester encoding.
Bipolar transistor mixer  A special mixer that consists of a single transistor biased into the nonlinear range to produce analog multiplication.
Birdies  Random “chirping” or “tweeting” sounds at the audio output of some receivers due to intermodulation products.
Bisync protocol  An IBM protocol, widely used in computer communication. It usually begins with the transmission of two or more ASCII sync (SYN) characters.
Bit error rate (BER)  The number of bit errors that occur for a given number of bits transmitted.
Bit error rate (BER) analyzer  A device that compares the transmitted and received data on a bit-by-bit basis to point out every bit error made.
Block  A group of data which represent hun­dreds or even thousands of 1-byte characters.
Bluetooth  A wireless PAN system. One of its applications is to provide hands-free cell phone operation.
BNC connector  Connector that is widely used with coaxial cables to attach test instruments to equipment being tested. Often used in LANs and some UHF radios.
BORSCHT  Seven basic functions provided by the subscriber line interface circuit (SLIC): Battery, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test.
Bow-tie antenna  A popular and effective variation of the dipole antenna which uses two-dimensional cones or triangles.
Bridge  Electronic equipment connected as a node on the network that performs bidirec­tional communication between two LANs.
Bridge circuit  An electrical network consist­ing of four impedances connected to form a rectangle, with one pair of diagonally oppo­site corners connected to an input device and the other pair to an output device.
Broadband connection  A fast Internet con­nection provided by a local telephone com­pany or other organization. A broadband connection can provide speeds up to several megabits per second.
Broadband LAN  A type of LAN in which the binary data to be transmitted is used to mod­ulate a carrier, which is then placed on the transmission medium.
Broadband system  One of two basic multi­channel architectures in use in communica­tion satellites.
Broadband transmission  The process of changing a baseband signal using a modula­tor circuit.
Broadside array  A stacked collinear antenna consisting of half-wave dipoles separated from one another by one-half wavelengths.
Brouter  A combination of a bridge and a router that can switch, perform protocol conversion, and serve as a communication manager between two LANs.
Browser  The software that makes it possible to navigate and explore the Web and to access and display information.
Bus topology  A topology in which all of the nodes in a LAN are connected to a common cable.
Butterworth filter  Filter with maximum flatness in response in the passband and uniform attenuation with frequency.
   

 

C

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C band  The frequency under which older satellite systems operate. This frequency requires large dish antennas.
Caller ID (calling line identification service)  A feature on many electronic telephones. The calling number is displayed on an LCD readout when the phone is ringing, permit­ting the recipient of the call to identify the caller.
Cap code  Special code assigned to some types of paging receivers.
Capacitor  A circuit component made up of two parallel conductors separated by an insu­lating medium. A capacitor is capable of storing energy in the form of a charge and an electric field.
Capture effect  The effect caused by two or more FM signals occurring simultaneously on the same frequency. The stronger signal captures the channel, eliminating the weaker channel.
Capture range  The range of frequencies over which a PLL will capture an input signal.
Carrier  A high-frequency signal usually a sine wave whose characteristics are changed by having a baseband signal impressed upon it.
Carrier frequency  The frequency generated by an unmodulated radio, radar, carrier commu­nication, etc., or the average frequency of the emitted wave when modulated by a symmet­rical signal.
Carrier generator  An oscillator.
Carrier recovery circuit  A term applying to a number of techniques that can be used in a receiver to generate a carrier signal that has the correct frequency and phase relationship to the original transmitting carrier.
Carrier sense multiple access with collision detection (CSMA/CD)  The access method used in Ethernet.
Carson’s rule  A method of determining the bandwidth of an FM signal.
Cassegrain feed  A method of feeding a parabolic antenna in which electromagnetic radiation from the horn strikes a small reflector that reflects the energy toward a large dish which then radiates it in parallel beams.
Cathode-ray tube (CRT)  A TV picture tube, or vacuum tube.
CATV (cable TV)  A system of delivering a TV signal to home receivers by way of a coaxial and/or fiber-optic cable rather than by radio wave propagation.
CATV converter  Known as a cable TV box, this tuner can select the special cable TV channels and convert them to a frequency that any TV set can pick up.
Cauer filter  A filter producing more rapid attenuation than a Chebyshev filter. Though Cauer filters provide greater attenuation out of the passband, they do so with an even higher ripple.
Cavity resonator  A waveguide-like device that acts like a high-Q parallel resonant circuit.
CB synthesizer  Often used in citizens band ra­dios, this is usually a PLL with one or more mixers that generate the carrier with the local oscillator frequencies for the receiver.
CDMA  See Code division multiple access.
Cells  In the cellular radio system, the small service areas.
Cellular radio system  Standard telephone ser­vice provided by two-way radio at remote locations.
Central office (local exchange)  A facility to which each telephone is directly connected by a twisted-pair cable.
Centrex  An alternative to private branch ex­change (PBX) that uses special equipment whereby most of the switching is carried out the local exchange switching equipment over special trunk lines.
Ceramic filter  A filter made with a manufactured crystal-like compound with the same piezoelectric qualities as quartz. Like a crystal filter, the ceramic filter provides high selectivity but is less expensive.
Channel  The amount of bandwidth that a signal occupies in the radio frequency spectrum.
Channelization process  The process of dividing a range of frequencies into multiple bands of frequencies called channels over which transmissions will be made.
Characteristic (surge) impedance  Impedance that is a function of the inductance, resis­tance, and capacitance in a transmission line.
Charged coupled device (CCD)  Often used in fax machines, this is a light-sensitive semi­conductor device that converts varying light amplitudes into an electrical signal.
Chebyshev filter  A filter with extremely good selectivity. The attenuation rate is much higher than that of the Butterworth filter. The main problem with this filter is ripple in the passband.
Choke  See Inductor.
Choke joint  Two flanges connected to a wave­guide at the center that is used to intercon­nect two sections of waveguide.
Chromatic dispersion  In a multimode step index cable, a type of dispersion that occurs when multiple wavelengths of light are used.
Chrominance signal  Color portion of a color TV signal. I and Q signals transmitted with luminance information in the bandwidth allotted to the TV signal.
Circular polarization  When electric and mag­netic fields rotate as they leave an antenna.
Circulator  A three-port microwave device used for coupling energy in only one direc­tion around a closed loop.
Cladding  A plastic sheath surrounding a fiber-optic cable with an index of refraction that keeps light waves in the core.
Class A amplifier  A linear amplifier. This is not an efficient amplifier and thus makes a poor power amplifier.
Class AB amplifier  Similar to a Class B amplifier but biased with a small amount of con­duction to eliminate crossover distortion.
Class B amplifier  An amplifier that is biased at cutoff so that no collector current flows with zero input. This type of amplifier is used mainly as a push-pull amplifier.
Class C amplifier  This amplifier is very effi­cient because it conducts for even less than one-half of the sine-wave input cycle.
Class D amplifier  An efficient switching amplifier that uses a pair of transistors to produce a square-wave current in a tuned circuit.
Class E amplifier  An efficient switching am­plifier in which only a single transistor is used, either bipolar or MOSFET.
Class F amplifier  A variation of the class E amplifier. It contains an additional resonant network in the collector or drain circuit.
Class S amplifier  This amplifier uses switch­ing techniques with a scheme of pulse-width modulation. It is found mainly in audio applications.
Cleaving  The first step in cutting fiber-optic cable, so that it is perfectly square on the end.
Client-server configuration  The general LAN configuration in which one of the computers in a network runs the LAN and determines how the system operates.
Clock recovery circuit  A circuit used to gener­ate clock pulses from a received signal.
Coarse wavelength division multiplexing (CWDM)  A method of multiplexing data on fiber-optic cable. Seealso Dense wave­length division multiplexing.
Coaxial cable  A widely used type of transmis­sion line that consists of a solid center con­ductor surrounded by a dielectric material. In cable TV systems, it is usually 75- RG-6/U cable.
Coaxial connector  An electromechanical ca­ble termination device designed to provide a convenient way to attach and disconnect equipment and cables and maintain the in­tegrity and electrical properties of a cable.
Code division multiple access (CDMA)  A digital cell phone system using direct sequence spread spectrum (DSSS). Seealso Direct-sequence SS; IS-95 CDMA; Time division multiple access.
Codec  A single, large-scale IC chip that takes care of all A/D and D/A conversion and related functions.
Coding  Also known as analog-to-digital (A/D) conversion.
Coil  See Inductor.
Collector modulator  A high-level AM modu­lator that takes a low-level signal and ampli­fies it to a high-power level.
Collinear antenna  An antenna consisting of two or more half-wave dipoles mounted end to end.
Color burst  In color signal generation in TV, a sample of the 3.58-MHz subcarrier signal is added to the composite video signal. This synchronizes color demodulation at the receiver and is called colorburst.
Color signal generation  The generation of color signals using three simultaneous signals (R, G, and B) during the scanning process by the light-sensitive imaging devices.
Colpitts oscillator  An oscillator in which the feedback is derived from a capacitive voltage divider.
Common mode rejection  In a balanced trans­mission line, any external signal induced into the cable appears on both wires simultane­ously but cancels at the receiver. This is called commonmoderejection, and it significantly reduces noise.
Communication  The process of exchanging information.
Communication satellite  Type of satellite that makes long-distance communication possible by serving as a relay station in the sky.
Commutating filter  A variation of a switched capacitor filter made of discrete resistors and capacitors with MOSFET switches driven by a counter and decoder.
Commutator  A form of rotary switch used in multiplexers in early TDM/PAM telemetry systems.
Companding  A process of signal compression and expansion that is used to overcome prob­lems of distortion and noise in the transmis­sion of audio signals.
Compression  A process that reduces the num­ber of binary words needed to represent a given analog signal.
Computer branch exchange (CPX)  See Private branch exchange.
Conductor  A wire or cable that carries current from one place to another.
Cone of acceptance  The area external to the end of a fiber-optic cable and defined by the critical angle. Any light beam outside the cone will not be internally reflected and transmitted down the cable.
Conical antenna  A dipole antenna that uses two cone-shaped elements.
Connector  A special mechanical assembly that allows fiber-optic cables to be joined to one another or used to connect a transmis­sion line to a piece of equipment or to another transmission line.
Constant-k filter  A filter that makes the prod­uct of the capacitive and inductive reactances a constant value k.
Continuous phase frequency-shift keying (CPFSK)  A type of frequency modulation that eliminates phase discontinuities.
Continuous tone-control squelch system (CTCS)  A system activated by a low-frequency tone transmitted along with audio to provide some communication privacy on a particular channel.
Continuous-wave (CW) radar  A type of radar using a constant-amplitude continuous microwave sine wave. The echo is also a constant-amplitude microwave sine wave of the same frequency but of lower amplitude and shifted in phase.
Continuous-wave (CW) transmission  A code transmission such as Morse code in which the signal is turned on and off to transmit a message.
Control segment  The part of the GPS system consisting of the various ground stations that monitor the satellites and provide control and update information.
Converter  See Mixer.
Copper digital data interface (CDDI)  Standard wire cable version of the FDDI system. It is less expensive than FDDI because twisted pair is used instead of fiber-optic cable.
Cordless telephone  Full-duplex, two-way radio system made up of two units, the portable unit or handset, and the base unit.
Core  The glass fiber in a fiber-optic cable that is usually surrounded by a protective plastic cladding.
Corner reflector  A type of high-frequency antenna in which a reflector is placed behind the dipole to improve gain and directivity.
Correlated noise  Another name for intermod­ulation distortion. Correlated noise is pro­duced only when signals are present. It is manifested as the signals called birdies.
Cosmic noise  Noise generated by stars outside our solar system.
Counterpoise  The entire ground-plane collec­tion of radials.
Coupled circuit  A circuit linked by way of a magnetic field through transformer action.
Critical angle  The angle between the incident light ray and the normal to the glass fiber surface. This value depends upon the index of refraction of the glass.
Critical coupling  The degree of magnetic coupling at which the output reaches a peak value.
Crossed-field amplifier (CFA)  Similar to a TWT, this amplifier has a lower gain but is somewhat more efficient.
CRT  See Cathode-ray tube.
Crystal diode  See Point-contact diode.
Crystal filter  A filter made from the same type of quartz crystal normally used in crystal oscillators. It is used in communication receivers in which superior selectivity is required.
Crystal oscillator  An oscillator in which the frequency of the AC output is determined by the mechanical properties of a piezoelectric crystal.
Crystal radio receiver  An early radio receiver that had weak reception because no active amplification was used.
Cutoff frequency  The frequency at which the output of a frequency-selective device, such as a filter or amplifier, drops to 70.7 percent of its maximum output. Also known as the half-power or 3-dB down point.
Cyclical redundancy check (CRC)  A mathe­matical technique used in data transmission that catches 99.9 percent or more of trans­mission errors.
   

 

D

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D layer  The layer of the ionosphere that is far­thest from the sun. This layer is weakly ion­ized and exists only in daylight.
Daisy chain topology  A variation of a ring topology that can be described as a ring that has been broken.
Data  Information to be communicated.
Data bus  Multiple parallel lines that carry binary data.
Data communication  A technique that can be used to transmit voice, video, and other ana­log signals in digital form.
Data compression techniques  A digital data processing technique that looks for redun­dancy in the transmitted signal to speed up transmission.
Data conversion  In digital communication, the conversion of data in analog form into digital form and vice versa.
Data encryption standard (DES)  One of the first and still widely used encryption stan­dards created by the National Institute of Standards and Technology (NIST). It uses 56-bit keys that are no longer considered to provide the ultimate in security.
Data rate  The speed of the binary pulses in a communication system.
dBm  A power level expressed in dB referenced to one milliwatt.
Decibel  The unit of measure created as a way of expressing the hearing response of the human ear to various sound levels but now primarily used to express gain or loss in electronic cir­cuits, cables or communication links.
Decimation in time (DIT)  The processing that takes place with the fast Fourier transform.
Decimator  A digital filter.
Decryption  Deciphering an encrypted message to recover the original data.
De-emphasis  A process used to return the fre­quency response of a preemphasized signal to its normal level.
De-emphasis circuit  A simple low-pass filter with a time constant of 75 ms.
Definition  The resolution of detail in a TV picture.
Delay line  A circuit that delays a signal or sample by some constant time interval.
Delta modulation  A special form of A/D con­version that results in a continuous serial data signal being transmitted.
Demodulator (detector)  A circuit that accepts modulated signals and recovers the original modulating information.
Demultiplexer  A device with a single input and multiple outputs, one for each original input signal.
Dense wavelength division multiplexing (DWDM)  A light frequency multiplexing method that uses 8, 16, 32, 64, or more data channels on a single fiber.
Depletion region  A thin area with no free car­riers, holes, or electrons around a PN semi­conductor junction that acts like a thin insulator that prevents current flow through the device.
Detector  See Demodulator.
Detector probe  See RF probe.
Deviation meter  A device designed to mea­sure the amount of carrier deviation of an FM/PM transmitter.
Deviation ratio  The number of times per sec­ond the carrier frequency deviates above and below its center frequency.
Dial tone  In a telephone set, the audio note indicating that the telephone is ready to use.
Dialing circuit  A circuit that provides a way for entering the telephone number to be called. The system used is either a pulse or a tone system.
Dielectric  The insulating portion of a capacitor.
Dielectric (lens) antenna  An antenna that uses a special dielectric material to collimate or focus microwaves from a source into a narrow beam.
Differential amplifier modulator  An ampli­fier with high gain and good linearity that can be amplitude modulated 100 percent.
Differential phase-shift keying (DPSK)  A type of phase modulation that simplifies the demodulation process because the transmit­ted signal itself becomes the phase reference.
Diffraction  The bending of waves around an object.
Digital AMPS (DAMPS)  See IS-136 TDMA.
Digital cell phone system  The system that has superseded the original analog AMPS system.
Digital private branch exchange (DPBX)  A telephone switching system in which all switching is done by IC transistor switches that have replaced the mechanical relays in older PBXs and PABXs.
Digital satellite radio or digital audio radio service (DARS)  A satellite service that provides hundreds of channels of music, news, sports, and talk radio primarily to car radios.
Digital signal  A signal that changes in steps or discrete increments. Most digital signals use binary or two-state codes.
Digital signal processing (DSP)  The process­ing of analog signals by digital computing methods.
Digital storage oscilloscope (DSO; digital or sampling oscilloscope)  A device that uses high-speed sampling or A/D techniques to convert the signal to be measured into a series of digital words that are stored in internal memory.
Digital Subscriber Line (DSL)  The process of transmitting digital data over analog tele­phone lines using OFDM/DMT.
Digital-to-analog (D/A) conversion  The process of translating multiple binary num­bers back into the equivalent analog voltage.
Digital TV (DTV)  See High-definition TV.
Diode  A unidirectional semiconductor device used for rectification, signal detection, and mixing.
Diode detector  A simple and widely used am­plitude demodulator.
Diode mixer  A widely used mixer that uses a diode.
Diode modulator  A simple amplitude modulator.
Diode ring or lattice modulator  A popular and widely used type of balanced modulator.
Dip oscillator  A tunable oscillator used to de­termine the approximate resonant frequency of any de-energized LC-resonant circuit.
Diplexer  A type of filter or circulator that allows a single antenna to be shared by a transmitter and receiver.
Dipole antenna  An antenna consisting of two pieces of wire, rod, or tubing that together are one-half wavelength long at the operating resonant frequency. Also called Hertz antenna or doublet.
Direct attached storage (DAS)  A method of communications between a computer and external storage like hard disks using a stan­dard parallel interface over short distances.
Direct Broadcast Satellite (DBS) service  A TV signal distribution system that uses K band signals to send cable-TV-like service to homes equipped with 18-in.-diameter satel­lite antennas.
Direct conversion (DC) or zero IF (ZIF) receiver  A special version of the superheterodyne that translates RF directly to baseband.
Direct digital synthesis (DDS)  A form of frequency synthesis in which a sine-wave output is generated digitally with a DAC.
Direct-sequence SS or code division multiple access (CDMA)  A type of spread spectrum in which the serial binary data is mixed with a higher-frequency pseudorandom binary code at a faster rate and then used to phase-modulate a carrier.
Directional coupler  A commonly used wave­guide component that facilitates the mea­surement of microwave power in a waveguide and the SWR.
Directivity  The ability of an antenna to send or receive signals over a narrow horizontal and vertical directional range.
Director  A kind of parasitic element in a Yagi that is shorter than the half-wave dipole-driven element and is mounted in front of the element to increase gain and directivity.
Discrete Fourier transform (DFT)  An algorithm that can be used in a DSP processor to analyze the frequency content of an input signal.
Discrete Multitone (DMT)  The name for OFDM used in DSL.
Discriminator  A circuit in a system that takes an FM signal and re-creates the original DC or AC signal produced by the transducer.
Dish antenna  Special antenna using a parabolic reflector which selects the signal from the de­sired satellite and provides very high gain.
Dispersion  Pulse stretching caused by the many different paths through a fiber-optic cable.
Distance measuring equipment (DME)  Equipment that permits accurate distance measurements between stations in aircraft navigation.
Distortion  A condition that causes a signal’s shape to be changed by a circuit. The intro­duction of harmonics into a signal.
Distributed feedback laser (DFL)  A laser made with a cavity that contains an inte­grated grating structure that acts like a selec­tive filter.
Diversity system  A way to minimize fading caused by multipath signals by using multi­ple transmitters, receivers, or antennas.
Doppler effect  A frequency shift that occurs when there is motion between a transmitting station and a remote target.
Dotted decimal  A way of simplifying an IP address by converting each byte or octet of the address into a decimal number and separating the decimal numbers with a dot or period.
Double-conversion down converter  One of two types of down converter used in earth station receivers. It resolves the problems of image rejection and tuning or channel selection difficulties.
Double-sideband suppressed carrier (DSSC or DSB) signal  A signal containing both up­per and lower sidebands, with the carrier suppressed.
Doubler  A frequency multiplier that multiplies the frequency by 2.
Doublet  See Dipole antenna.
Doubly balanced mixer  A balanced modulator.
Down conversion  The process used in radio receivers in which high-frequency radio sig­nals are converted to a lower, intermediate frequency.
Driven array  A directional antenna that has two or more driven elements.
Drooping radial  A wire or other conductor that serves as a part of the ground plane of a quarter-wave vertical antenna. A conductor that is positioned at an angle greater than 90° from the vertical element of the antenna.
Dual-conversion receiver  A receiver in which the first mixer converts the incoming signal to a relatively high intermediate frequency, for the purpose of eliminating images, and the second mixer converts that IF to a much lower frequency at which good selectivity is easier to obtain.
Dual-conversion transponder  A device that translates the uplink signal to the downlink frequency in two steps with two mixers.
Dual-tone multifrequency (DTMF) system  The tone dialing system used in most modern telephones.
Dummy load  A resistor that is connected to the transmission line in place of the antenna to absorb transmitter output power.
Duplex communication  Communication that flows in two directions.
Dynamic nonhierarchical routing (DNHR)  In long-distance telephone service, a method permitting long-distance connections with two or fewer switching centers.







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