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| Sample-and-hold (S/H) circuit (track/store circuit) | A circuit that accepts an analog input signal and passes it through, unchanged, during its sampling mode. In the hold mode, the amplifier remembers or stores a particular voltage level at the instant of sampling.
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| Sampling | The process of “looking at” an analog signal for a brief time. During this interval, the amplitude of the analog signal is allowed to be passed or stored.
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| Satellite | A physical object that orbits, or rotates, around some celestial body.
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| Satellite period (sidereal period) | The time it takes for a satellite to complete one orbit.
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| Satellite receiver | A special subsystem designed to work with a TV set, consisting of a dish antenna, a low-noise amplifier and down converter, an IF section with appropriate demodulators for both video and sound, and a method of interconnecting to a conventional TV set.
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| Satellite TV | The distribution of TV signals via satellite.
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| SatNav receiver | A communication receiver designed to be used as part of a satellite navigation system.
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| SAW filter | A solid-state filter used to obtain bandpass selectivity in RF or IF amplifiers.
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| Scanning | A technique that divides a rectangular scene up into individual lines.
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| Scattering | Light lost due to light waves entering at the wrong angle and being lost in the cladding of a cable because of refraction.
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| Scatternet | A linking of piconets.
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| Schmitt trigger circuit | A level detector that switches between specific voltage levels.
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| Schottky diode | A diode made with N-type silicon with a thin metal layer. Also known as a hot carrier diode.
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| Secure socket layer (SSL) | A process involving encryption and authentication to provide for secure transactions like credit card purchases over the Internet.
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| Selective fading | A phenomenon that occurs when the carrier and sidebands arrive at a receiver a slightly different times, causing a phase shift that can make them cancel one another instead of adding up to the original AM signal.
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| Selectivity | The ability of a communication receiver to identify and select a desired signal from the thousands of others present in the frequency spectrum and minimize all others.
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| Self-oscillation | Output voltage that finds its way back to the input of the amplifier with the correct amplitude and phase. The amplifier sometimes oscillates at its tuned frequency, and at other times at a higher frequency.
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| Semiconductor diode | A unidirectional current junction formed of P- and N-type semiconductor materials.
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| Semiconductor noise | Noise created by electronic components such as diodes and transistors.
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| Sensitivity | The ability of a communication receiver to pick up weak signals. This is mainly a function of overall gain.
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| Serial data transfer | Data transfer in which each bit of a word is transmitted one after another.
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| Serial-parallel conversion | A technique for converting between serial and parallel transmission and vice versa. It is usually done by shift registers.
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| Serializer/deserializer (SERDES) device | A circuit used for serial-to-parallel or parallel-to-serial data conversion.
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| Series modulator | An amplitude-modulating scheme that eliminates the need for a large, heavy, and expensive modulation transformer.
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| Series resonant circuit | A circuit made up of inductance, capacitance, and resistance used in filters.
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| Settling time | The amount of time it takes for the output voltage of a D/A converter or S/H circuit to stabilize to within a specific voltage range after a change in binary input.
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| Shadow fading | Fading caused by objects coming between a transmitter and receiver, for example, when a large building comes between a vehicle containing a transceiver and a base station transceiver.
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| Shannon-Hartley theorem | The relationship between channel capacity, bandwidth, and noise. This is expressed as CB log2 1 1 S/N2 .
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| Shape factor | The ratio of the pass bandwidth to the stop bandwidth of a bandpass filter. The smaller the ratio, the greater the selectivity. The steepness of the skirts or the skirt selectivity of a receiver.
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| Shielding | The process of surrounding EMI-emitting circuits or sensitive receiving circuits with a metal enclosure to prevent the radiation of pickup of signals.
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| Shift register | A sequential logic circuit made up of a number of flip-flops connected in cascade.
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| Shot noise | Noise produced by the random movement of electrons or holes across a PN junction. Also, the most common type of semiconductor noise.
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| Sideband | A new signal generated as part of the modulation process. Sidebands are at sum and difference frequencies directly above and directly below the carrier frequency.
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| Sidereal period | See Satellite period.
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| Sigma-delta () converter | A variation of the delta converter, also known as a delta-sigma of charge balance converter. This circuit provides extreme precision, wide dynamic range, and low noise in analog-to-digital conversion.
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| Signal bandwidth | The frequency range occupied by a signal.
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| Signal bias | A bias method used in Class C amplifiers involving adjusting the time constant of R1 and C1 so that an average DC reverse-bias voltage is established. The applied voltage causes the transistor to conduct, but only on the peaks.
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| Signal generator | A device that produces an output signal of a specific shape at a specific frequency and, in communication, usually with some form of modulation.
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| Signal injection | A common way to troubleshoot receivers by using a signal generator of the correct output frequency to test stages of a receiver or transmitter for the correct output response.
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| Signal plus noise and distortion | See SINAD.
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| Signal processing | The manipulation of a signal by amplification, filtering, modulation, or other mathematical operations.
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| Signal-to-noise ratio (S/N) | A ratio indicating the relative strengths of a signal and noise in a communication system.
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| Signal tracing | A common way to troubleshoot equipment by using a signal detection device to follow a signal through various stages of the equipment.
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| Silica | Another name for glass.
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| Simplex communication | Communication that flows in one direction.
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| SINAD (signal plus noise and distortion) | A way of expressing the quality of communication receivers. SINAD equals the composite signal plus the noise and distortion divided by noise and distortion contributed by the receiver.
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| SINAD test | A method of sensitivity measurement that determines the input signal voltage that will produce at least a 12-dB signal-to-noise ratio.
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| Single-conversion down converter | A circuit used to translate the frequency of a transmitted or received signal to a lower frequency. A mixer with an output that is at a lower frequency than its input.
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| Single-conversion transponder | A device that uses a single mixer to translate the uplink signal to the downlink frequency.
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| Single data message format (SDMF) | A message format used in caller ID.
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| Single-mode step index cable | A cable used for long-distance transmission and maximum content. It eliminates modal dispersion by minimizing the paths through the core.
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| Single-sideband suppressed carrier (SSSC) or single-sideband (SSB) modulation | A form of AM in which the carrier is suppressed and one of the sidebands is eliminated.
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| Single-sideband transceiver | A transceiver that utilizes single sideband signals. Like AM and CW transceivers, in this transceiver the transmitter and receiver can share many circuits.
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| Singly balanced mixer | A mixer circuit using two diodes.
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| Skin effect | The tendency of electrons to flow near and on the outer surface of a conductor at frequencies in the VHF, UHF, and microwave regions.
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| Skip distance | The distance from the transmitting antenna to the point on earth where the first refracted signal strikes the earth and is reflected.
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| Skirt selectivity | The steepness of the sides of a tuned circuit response curve.
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| Sky wave | A signal that is radiated by an antenna into the upper atmosphere, where it is bent back to earth because of refraction in the upper atmosphere.
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| Slope detector | The simplest frequency demodulator. It makes use of a tuned circuit and a diode detector to convert frequency variations into voltage variations.
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| Slot antenna | A radiator made by cutting a half-wavelength rectangular opening in a conducting sheet of metal or into a waveguide. It is used on high-speed aircraft.
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| SMA connector | A coaxial cable connector that is characterized by the hexagonal shape of the body of the male connector.
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| Small-signal diode | A diode used for signal detection and mixing. Two examples are the point-contact diode and the Schottky barrier, or hot-carrier, diode.
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| Small-signal microwave amplifier | An amplifier made of a single transistor or multiple transistors combined with a biasing circuit and any microstrip circuits or components as required. Most of these amplifiers are of the integrated circuit variety.
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| Smith chart | A graph that permits visual solutions to transmission-line calculations. Snap-off varactor
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| Snell’s law | A formula that gives the relationship between the angles and the indices of refraction.
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| Software-defined radio | A receiver in which most of the functions are performed by a DSP.
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| Solar panel | A device that converts the light energy of the sun into a voltage. It is the most common source of power used in satellites.
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| SONET | See Synchronous Optical Network.
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| Space segment | The part of a satellite communication or navigation station that is in space.
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| Space wave | A wave that travels in a straight line directly from the transmitting antenna to the receiving antenna. It is also known as line-of-sight communication.
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| Spam | Unwanted and unsolicited email messages.
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| Spatial isolation | A technique used to increase the bandwidth and signal-carrying capacity of a satellite.
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| Spatial multiplexing (frequency reuse) | The transmission of multiple wireless signals on a common frequency in such a way that the signals do not interfere with one another.
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| Spectrum analysis | The process of examining a signal to determine its frequency content.
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| Spectrum analyzer | An oscilloscope-like test instrument used to display received signals in the frequency domain.
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| Speech processing | The way the voice signal used in communication is modified before being applied to the modulator.
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| Speed of light | Approximately 300,000,000 m/s, or about 186,000 mi/s, in free space. Light waves travel in a straight line.
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| Splatter | A type of harmonic sideband interference, so called because of the way it sounds at the receiver.
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| Splicing | Permanently attaching the end of one fiber-optic cable to another. Also connecting electrical conductors.
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| Split pair | A wiring error often made when a cable contains multiple twisted-pair lines.
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| Spread spectrum (SS) | A modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth.
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| Spyware | Software attached to your computer unknowingly that monitors your activity, computer usage, and collects data and reports it back to its initiator without your knowledge or permission.
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| Square-law function | A current variation in proportion to the square of the input signals.
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| Squelch (muting) circuit | A circuit used to keep the receiver audio turned off until an RF signal appears at the receiver input.
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| SSB | See Single-sideband.
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| Standards | In communication systems, specifications and guidelines that companies and individuals follow to ensure compatibility between transmitting and receiving equipment.
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| Standing wave | A composite of forward and reflected voltage distributed along a transmission line, not matched to the load, which indicates that the power produced by the generator is not totally absorbed by the load.
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| Standing wave ratio (SWR) | The ratio of maximum current to minimum current, or the ratio of maximum voltage to minimum voltage along a transmission line.
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| Standing wave ratio (SWR) meter | A meter that measures forward and reflected power and therefore can display SWR.
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| Star topology | A network configuration consisting of a central controller node and multiple individual stations connected to it.
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| Start and stop bits | Binary levels that indicate the beginning and ending of a word in asynchronous data transmission.
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| Start frame delimiter (SFD) | The portion of a packet protocol which announces the beginning of the packet.
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| Start of header (SOH) character | The character that begins a frame. It is an ASCII character which means that the transmission is beginning.
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| Start of text (STX) character | An ASCII character indicating the start of text.
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| Static | Electrical disturbances that occur naturally in the earth’s atmosphere. Also called atmospheric noise.
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| Step index cable | A cable with a sharply defined step in the index of refraction where the fiber core and the cladding interface.
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| Step-recovery diode | A PN-junction diode made with gallium arsenide or silicon.
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| Stop band | The range of frequencies outside the passband, that is, the range of frequencies that are greatly attenuated by the filter. Frequencies in this range are rejected.
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| Storage area network (SAN) | A network of disk drives used to access massive amounts of data.
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| Straight-through processor | A device that picks up TV signals from a local station and amplifies the signal before multiplexing it onto the main cable.
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| Subcarrier | In an FDM system, each signal to be transmitted feeds a modulator circuit. The carrier for each modulator is on a different frequency and is called a subcarrier.
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| Subcarrier oscillator (SCO) | A modulated oscillator used in a telemetry system.
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| Submillimeter wave | An electromagnetic wave at a frequency below 30 GHz.
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| Subsatellite point (SSP) | The point on the earth that is directly below a satellite and through which the satellite’s location is specified.
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| Subscriber interface (subscriber line interface circuit, SLIC) | A group of basic circuits that power the telephone and provide all the basic functions such as ringing, dial tone, and dialing supervision.
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| Subscriber loop | See Local loop.
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| Subsidiary communication authorization (SCA) signal | A separate subcarrier of 67 kHz that is frequency-modulated by audio signals, usually music, and transmitted with an FM broadcast signal.
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| Successive approximations converter | A type of A to D converter in which the bits driving a DAC are turned on one at a time from MSB to USB to estimate input voltage level.
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| Successive approximations register (SAR) | A special register that causes each bit in a register driving a DAC to be turned on one at a time from MSB to LSB until the closest binary value is stored in the register.
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| Superheterodyne | A receiver that converts signals to the intermediate frequency and for which a single set of amplifiers provides a fixed level of sensitivity and selectivity.
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| Superheterodyne RF spectrum analyzer | A widely used type of RF spectrum analyzer.
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| Superhigh frequency (SHF) | The frequency range from 3 to 30 GHz.
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| Supervision | The group of functions in the subscriber line interface that monitor local loop conditions and provide services.
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| Surface acoustic wave (SAW ) filter | A special form of crystal filter. It operates like a fixed tuned bandpass filter designed to provide the exact selectivity required by a given application.
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| Surge impedance | See Characteristic impedance.
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| Surveillance satellite | A satellite used for purposes such as military reconnaissance, map making, and weather forecasting.
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| Sweep generator | A signal generator with an output frequency that can be linearly varied over some specific range.
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| Switch | A hublike device used to connect individual PC nodes to the network wiring.
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| Switch hook | A double-pole mechanical switch that is usually actuated by a telephone handset.
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| Switched capacitor filter (SCF) | An active IC filter made of op amps, capacitors, and transistor switches. It can be designed to operate as a high-pass, low-pass, bandpass, or band-stop filter.
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| Switched integrator | The basic building block of a switched capacitor filter (SCF).
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| Switching amplifier | A transistor that is used as a switch and is either conducting or nonconducting.
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| Sync pulse | A pulse applied to one of the input channels at a transmitter to synchronize the multiplexed channels at the receiver.
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| Synchronizing circuit | A circuit needed to keep the sweep of a TV receiver in step with the transmitted signal.
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| Synchronous data link control (SDLC) protocol | A flexible and widely used synchronous protocol, it is used in networks that are interconnections of multiple computers.
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| Synchronous data transmission | The technique of transmitting each data word one after another without start and stop bits, synchronized with a clock signal, usually in multiword blocks.
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| Synchronous detection | A method in which an internal clock signal at the carrier frequency in a receiver switches the AM signal off and on, producing rectification similar to that in standard diode detectors.
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| Synchronous detector | A circuit that uses an internal clock signal at the carrier frequency to switch the AM signal off and on.
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| Synchronous (geostationary or geosynchronous) satellite | A satellite that appears to remain in a fixed position because it rotates in exact synchronism with the earth.
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| Synchronous Optical Network (SONET) | An optical network developed to transmit digitized telephone calls and data over fiber-optic cable at high speed.
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| T-carrier system | A digital telephone system used throughout the United States.
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| T connector | A coaxial connector accessory that provides a convenient way to attach an additional node to an existing coaxial cable.
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| T junction | See T section.
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| T network | A type of impedance-matching network. The configuration of circuit elements resembles the letter T.
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| T-1 system | A PCM system developed by Bell Telephone for transmitting telephone conversations by high-speed digital links. It multiplexes 24 voice channels onto a single line using TDM techniques.
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| T section | A device used to split or combine two or more sources of microwave power.
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| Tactical Air Navigation (Tacan) | The higher-frequency system of air navigation used by the military.
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| Tank circuit | A parallel-resonant LC circuit.
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| TCP/IP protocol | A software protocol, at the heart of the Internet, that ensures that data is properly partitioned, transmitted, received, and reassembled.
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| TDD | Telecommunication device for the deaf.
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| TDR tester (cable analyzer or LAN meter) | An instrument used to make tests and measurements of items such as opens or short circuits, cable attenuation, or cable miswiring.
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| Technician | A person working in the electronics industry with some kind of postsecondary education in electronics—usually 2 years of formal, post–high school training usually involved with equipment service, maintenance, repair, installation, or operation.
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| Technologist | In the field of electronics, a technologist usually holds a bachelor’s degree in electronics technology from a technical college or university usually employed as an engineer.
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| Tee switching configuration | In a PIN diode, a combination of two series switches and a shunt switch.
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| Telemetry | A communication system used for remote monitoring or measurement.
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| Telemetry, command, and control (TC&C) subsystem | Equipment consisting of a receiver and the recorders and indicators that allow a ground station to monitor and control conditions in a satellite.
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| Telephone hierarchy | The organization of the path in which a call travels from the person placing the call to the person receiving the call.
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| Telephone set | An analog baseband receiver with a microphone and speaker, better known as a transmitter and receiver. It also uses a ringer and dialing mechanism.
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| Telephone system | An analog system originally designed for full-duplex communication of voice signals. It now employs digital techniques.
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| Termination | A resistive load connected to a transmission line or waveguide to prevent reflections on the line.
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| Terminator | A special connector containing a resistor whose value is equal to the characteristic impedance of the coaxial cable.
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| Test signal | A special tone placed on the local loop by the phone company to check the status and quality of lines.
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| Thermal agitation | Random variation of electrons in an electronic component due to heat energy.
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| Thermal noise | Noise resulting from the random motion of free electrons in a conductor caused by heat.
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| Thermistor | A resistor whose resistance varies inversely with temperature.
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| Third-order intercept test (TOI or IP3) | A measure of the linearity of amplifiers, mixers, and other circuits.
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| Time delay (transit time) | The time it takes for a signal applied at one end of a transmission line to appear at the other end of the line.
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| Time division multiple access (TDMA) | A special form of time division multiplexing that provides multiple voice channels per satellite or telephone spectrum.
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| Time division multiple access (TDMA) cell phone system | See IS-136 TDMA.
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| Time division multiplexing (TDM) | A timesharing or sampling technique that makes it possible for each signal to occupy the entire bandwidth of a channel. However, each signal is transmitted for only a brief period of time.
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| Time domain display | A display of variations in voltage, current, or power with respect to time as on an oscilloscope screen.
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| Time domain reflectometry (TDR) | A test for all types of cables and transmission lines. It is widely used in finding faults in cables used for digital data transmission.
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| Tip | A terminal on a telephone at which wires end.
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| Token passing | An access method used by To-ken-Ring systems in which a binary word is passed around the ring to communicate when a node desires to send data.
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| Token Ring | A LAN configuration developed by IBM. It uses twisted pair.
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| Tone dialing | A dialing system used in most modern telephones that uses pairs of audio tones to create signals representing the numbers to be dialed. See DMTF.
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| Top hat | In a shortened antenna, an arrangement in which conductors are added at the top of the antenna.
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| Topology | The physical paths used to connect the nodes on a network.
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| Toroid | A doughnut-shaped core used in RF transformers, usually made of a special type of powdered iron.
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| Transceiver | A communication equipment package in which both transmitter and receiver are in a single unit.
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| Trunked repeater system | A system in which two or more repeaters are under the control of a computer system that can transfer a user from an assigned but busy repeater to another, available repeater.
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| TTY | Teletypewriter.
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| Tunable laser | A laser used in fiber-optic systems whose frequency can be varied by changing the DC bias on the device or mechanically adjusting an external cavity.
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| Tuned circuit | A circuit made up of inductors and capacitors that resonate at specific frequencies.
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| Tuned radio frequency (TRF) receiver | A receiver in which sensitivity is increased by multiple stages of RF amplification followed by a demodulator. The main problem with this receiver is tracking the tuned circuits.
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| Tuner | An RF unit with LNA, mixer and local oscillator used as the front-end in a receiver to produce an IF output.
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| Tuning synthesizer | A local oscillator set to frequencies that will convert the RF signals to the IF.
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| TV remote control | A small handheld battery-powered unit that transmits a serial digital code via an IR beam to a receiver that decodes it and carries out the specific action defined by the code.
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| TV signal | Voice and video signals that occupy a channel in the spectrum with a bandwidth of 6 MHz.
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| TV spectrum allocation | An allotted range. For example, using VHF and UHF frequencies, TV stations in the United States use the frequency range between 54 and 806 MHz. Sixty-eight 6-MHz TV channels are assigned frequencies within this range.
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| TVRO (TV receive-only) system | A satellite receiver containing circuitry for controlling the positioning of a satellite dish antenna.
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| Twisted pair | Two insulated copper wires twisted together loosely to form a transmission line.
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| Type-P display | A kind of radar display using a cathode-ray tube (CRT) that shows target reflections as vertical “blips” with respect to a horizontal time sweep across the face of the CRT.
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| Ultrahigh frequency (UHF) | The frequency range from 300 to 3000 MHz.
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| Ultraviolet range | The range of frequencies above violet visible light, which has a wavelength of 400 nm, or 0.4 mm.
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| Ultrawideband (UWB) radar | Radar in the form of pulsed radar that radiates a stream of very short pulses several hundred picoseconds long. The resulting spectrum is very broad.
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| Ultrawideband (UWB) wireless | A type of wireless that transmits data in the form of very short pulses.
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| Unbalanced transmission line | A transmission line in which one conductor is connected to ground.
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| Transferred-electron device (TED) | See Gunn diode.
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| Transimpedance amplifier (TIA) | The input stage of a light receiver circuit during which the diode current is converted into an output voltage and amplified.
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| Transit system | A satellite navigation system used by the U.S. Navy.
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| Transit time | See Time delay.
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| Transit-time noise | Noise caused by the time it takes for a current carrier such as a hole or electron to move from the input to the output.
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| Transmatch circuit | A type of antenna tuner that uses a coil and three capacitors to tune the antenna for optional SWR.
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| Transmission line | A cable that carries radio signals, telephone signals, computer data, TV signals, etc. The two requirements of any transmission line are that it introduce minimum attenuation and distortion of the signal, and not radiate any of the signal as radio energy.
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| Transmission line transformer | A type of transformer widely used in power amplifiers for coupling between stages and impedance matching.
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| Transmit-receive (TR) tube | A special vacuum tube used as a fast high-power switch that permits both the receiver and a high-power transmitter to share a single antenna without damage to the receiver.
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| Transmit subsystem | In a satellite, collection of electronic equipment that takes the signal to be transmitted, amplifies it, and sends it to the antenna.
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| Transmitter | A collection of electronic components and circuits designed to convert an electrical signal into one that can be transmitted over a given medium.
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| Transponder | The transmitter-receiver combination in a satellite.
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| Transverse electric (TE) field | The electric field at a right angle to the direction of wave propagation.
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| Transverse magnetic (TM) field | The magnetic field transverse to the direction of propagation.
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| TRAPATT diode | A PN-junction diode made of silicon, GaAs, or InP. It is designed to operate with a high reverse bias that causes it to avalanche, or break down.
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| Traveling-wave tube (TWT) | A versatile microwave RF power amplifier. It can generate hundreds and even thousands of watts of microwave power.
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| Tree topology | A bus topology in which each node has multiple interconnections to other nodes through a star interconnection.
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| Trellis code modulation (TCM) | A special form of QAM that facilitates error detection and correction.
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| Triode | A type of vacuum tube with three ele-ments—the cathode, the grid, and the plate.
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| Tripler | A frequency multiplier that multiplies the frequency by 3.
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| Trunk cable | The main output cable in a modern cable TV system usually a fiber-optic cable.
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| Unidirectional antenna | An antenna that sends or receives signals in one direction only.
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| Uniform resource locator (URL) | A special address used to locate sites on the Web.
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| Universal asynchronous receiver transmitter (UART) | A special large-scale digital IC that performs parallel-to-serial and serial-to-par-allel conversion.
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| Up conversion | The process in which the original signal is generated at a lower frequency and then converted to a higher frequency for transmission.
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| Uplink | The original signal transmitted from an earth station to a satellite.
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| User datagram protocol (UDP) | A protocol used with TCP/IP.
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| Varactor | A diode designed to optimize the variable capacitance exhibited as reverse bias is changed.
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| Varactor diode | A voltage variable capacitor. When a reverse bias is applied to the diode, it acts like a capacitor.
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| Varactor modulator | A modulator that utilizes a varactor.
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| Varactor phase modulator | A simple phase-shift circuit that can be used as a phase modulator by using a varactor.
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| Variable frequency oscillator (VFO) | An oscillator used to provide continuous tuning over a desired range.
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| Varistor | A nonlinear resistance element whose resistance changes depending upon the amount of voltage applied across it.
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| Velocity factor | The ratio of the velocity of propagation of a signal in a transmission line to its velocity in free space.
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| Vertical-cavity surface-emitting laser (VCSEL) | A laser used in fiber-optic systems that is made on the surface of silicon wafer–like transistors and integrated circuits.
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| Vertical radiation pattern | The part of the doughnut-shaped radiation pattern of an antenna indicating its vertical response.
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| Very high frequency (VHF) | The frequency range from 30 to 300 MHz.
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| Very low frequency (VLF) | The frequency range from 15 to 20 kHz.
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| Vestigial sideband signal | An AM signal where a portion of one sideband is suppressed. Used in TV.
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| VHF omnidirectional ranging (VOR) | A system of air navigation used by nonmilitary aircraft.
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| Video intermediate frequency | A standard 45.75-MHz frequency for TV pictures or 41.25 MHz for sound.
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| Video signal | A voltage variation representing variations in light intensity along a scan line.
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| Virtual private network (VPN) | A technique that uses software to establish a virtual network over a LAN or the Internet for the purpose of providing security only to authorized users.
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| Virus | An unwanted program that disrupts or destroys the software and hardware of a computer.
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| Visible light | The frequency in the 400- to 700nm range, or 0.4 to 0.7 mm, depending upon the color of the light.
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| Visible spectrum | A group of signals with frequencies ranging from 0.4 to 0.7 mm.
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| Vocoder | A circuit that digitizes voice signals. See also Codec.
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| Voice frequency (VF) | A group of signals with frequencies ranging from 300 to 3000 Hz. This is the range of normal speech.
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| Voice mail | See Answering machine.
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| Voltage-controlled oscillator (VCO) or voltage-controlled crystal oscillator (VXO) | A circuit in which the varying direct or alternating current changes the frequency of an oscillator operating at the carrier frequency. An oscillator often used in applications in which voltage to frequency conversion is required. Its frequency is controlled by an external input voltage.
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| Von Neumann architecture | The stored-pro-gram concept that is the basis of operation of all digital computers.
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| Von Neumann bottleneck | Term describing the fact that only one path exists between the memory and the CPU. Therefore, only one data or instruction word can be accessed at a time. This greatly limits execution speed.
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| Waveguide | A hollow metal conducting pipe designed to carry and constrain the electromagnetic waves of a microwave signal. It has a rectangular or circular cross section and is made of copper, aluminum, or brass.
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| Waveguide cutoff frequency | The frequency below which a waveguide will not transmit energy.
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| Wavelength | The distance occupied by one cycle of a wave (usually expressed in meters).
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| Wavelength division multiplexing (WDM) | Frequency division multiplexing (FDM) used on fiber-optic cable. It permits multiple channels of data to operate independently on different light wavelengths in a fiber-optic cable.
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| WCDMA- wideband CDMA | A version of CDMA that uses 5 MHz wide channels to implement 3G cell phone systems. A standard based on the 3GPP.
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| White (Johnson) noise | Noise containing all frequencies randomly occurring at random amplitudes.
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| Wide-area network (WAN) | One of three basic types of electronic networks in common use. The long-distance telephone systems linked together are also this type of network. The Internet backbone is a WAN.
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| Wide-wavelength division multiplexing (WWDM) | A variation of 10-gigabit Ethernet in which data is divided into four channels and transmitted simultaneously over four different wavelengths of infrared light near 1310 nm. It is similar to frequency division multiplexing.
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| Wi-Fi | The trade name of the wireless local area networking technology defined by the IEEE 802.11 standard. It uses the 2.4 and 5.8 GHz bands to send high speed data at arange up to 100 meters. It is widely used to access the Internet via hot spots with a laptop computer.
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| WiMAX | World interoperability for Microwave Access. The trade name for a broadband wireless technology based on the IEEE 802.16 standard. Use for fixed broadband access and for mobile access at high data rates with cell sites up to 3 miles away.
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| Wireless |
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| Wireless LAN | A network communicating through radio and infrared techniques. In this system, each PC must contain a wireless modem or transceiver.
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| World Wide Web | A special part of the Internet where companies, organizations, government, or individuals can post information for others to access and use.
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| xDSL modem | A specific digital subscriber line (DSL) standard. The “x” is one of several letters that define the standard.
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| Xmodem protocol | A protocol used for asynchronous ASCII-coded data transmission between computers via modem. These transmissions begin with a NAK character.
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| Yagi antenna | A directional gain antenna made up of a driven element and one or more parasitic elements.
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| Zero | A frequency at which there is zero impedance in a circuit.
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| Zero IF (ZIF) receiver | See Direct conversion receiver.
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| ZigBee | The trade name of a wireless technology that is based on the IEEE standard 802.15.4. It uses low speed data in the ISM UHF and low microwave bands to implement monitor and control functions over short distances in home, commercial and industrial applications. It is based on the concept of mesh networks.
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