Electronics glossary part2

Mega- I A prefix to a number, denoting a multiple of 106. 2 A prefix used in computing, to denote a multiple of 220 (i.e. I ,048,576). Symbol: M.

Megger A portable insulation testing equipment.

Memory Any device associated with a digital circuit (particularly a computer) which is capable of storing information in digital form. Synonymous: store.

Memory location A storage element with a unique address.

Meter Any measuring equipment.

Meter movement The part of an analog meter which indicates the measured value, typically constructed of a finely balanced moving­ coil in a magnetic field. The coil rotates when a current flows through it, the amount of rotation is proportional to the value of current.

Meter resistance The internal resistance of a meter.

MF Abbreviation for medium frequency.

Micro- A prefix, denoting a multiple of 10·6. Symbol: 1-1.

Microcomputer I A single integrated circuit which contains all the parts which can be combined to function as a computer i.e. central processing unit, memory, timing and control circuits. 2 A computer which comprises an integrated circuit microprocessor. 3 A home computer.

Microphone A transducer which converts sound energy into electrical energy.

Microprocessor An integrated circuit which contains the central processing unit of a computer.

Microwave An electromagnetic wave with a frequency between infra-red and radio waves in the electromagnetic spectrum. Microwave wavelengths range from about 3 mm to 1.3 m.

Mike Abbreviation for microphone.

Milli- A prefix, denoting a multiple of 10·3• Symbol: m.

Mismatch When a circuit's load does not have the same impedance as the load itself.

Mixer I An audio circuit to combine two or more signals. The output signal is merely the addition of the input signals. 2 A radio circuit which combines two or more signals to produce an output signal of a different frequency to the inputs.

Modem Acronym for modulator-demodulator. Any appliance which converts signals from one circuit or system to signals of another circuit or system. Typically modems are used to connect two computers via telephone circuits.

Modulation The alteration of a signal's parameter by another parameter. For instance, a carrier wave's amplitude may be modulated by a music signal. Other parameters which may be modulated include: phase, frequency, or a combination of more than one.

Monochromatic light Light of a single colour i.e. it has only one frequency.

Monochrome television Black and white television. Monostable multivibrator A circuit which has one stable state.

On application of a triggering pulse, the output of the monostable multivibrator assumes a second state for a defined period of time, after which it returns to the stable state. Synonymous: one-shot.

Morse code Internationally agreed code for the transmission of alphanumeric symbols, in which each symbol is transmitted as a combination of short and long pulses (dots and dashes).

Morse telegraphy Electric telegraphy transmitting alphanumeric symbols as Morse code.

MOSFET Abbreviation of metal oxide semiconductor field effect transistor. A type of field effect semiconductor.

MOST Type of FET with oxide insulating layer between the metal gate and semiconductor channel. It has a higher input impedance than the junction type FET.

Motorboating Term used to describe an oscillation arising in low or audio frequency amplifiers, resembling a motorboat engine.

Moving coil A device which relies on its motion due to current through a coil in a magnetic field.

Multiplex Combination of two or more signals, such that a single signal is obtained which may be transmitted and later demultiplexed back into the original signals.

Multiplexer A circuit which allows the multiplex process to take place.

Multivibrator A circuit which contains two inverters coupled so that the output of one forms the input of the other. Resistive coupling of the two inverters produces a bistable multivibrator, or flip-flop. Resistive/capacitive coupling produces a monostable multivibrator. Capacitive coupling produces an astable multivibrator.

NAND gate Logic circuit whose output is high if one or more of its inputs are low, and low if all its inputs are high.

Nano- A prefix denoting a multiple of 10·9. Symbol: n.

Natural frequency The frequency at which free oscillation occurs in a resonant electrical, electronic, or mechanical system.

N-channel The conducting channel of a field-effect transistor of N-type semiconductor material. The term is also used to refer to the transistor i.e. N-channel field-effect transistor.

Negative bias A voltage applied to an electrode of some electronic component, which is negative with respect to a fixed reference potential.

Negative feedback Type of control procedure in which all or some part of a system's output signal is fed back to the system's input terminal. Generally, by changing the amount of negative feedback

the system's gain is changed. The gain may thus be controlled by choosing the required amount of negative feedback.

Negative modulation Type of modulation procedure followed in the transmission of television signals, such that a black display results from a more positive signal and a white display results from a negative signal. This principle is followed to ensure that any noise

which a television receiver picks up produces a darker image and is thus less noticeable than it would be if positive modulation were used.

Neon indicator Type of indicator, relying on the gas-discharge properties of the inert gas, neon. A voltage of about 80 V is required to illuminate such indicators, and so they are typically used as indicators to display the presence of mains voltages.

Neper A dimensionless unit used to express the ratio of two signal powers. One neper equals 8.686 decibels. Symbol: Np.

Network Alternative term describing a circuit.

Neutral I One of the three lines of the domestic mains electric supply. 2 Descriptive term implying no overall positive or negative charge.

Neutralisation In radio frequency transistors there is a tendency for self-oscillation to occur due to the collector-base capacitance. In modem radio frequency transistors this capacitance can be made very small. To overcome the effect in early radio frequency transistors it was usual to use a small amount of capacitive negative feedback in each stage, this being known as neutralisation.

Nicad Abbreviation for nickel cadmium.

Nickel-cadmium cell A secondary cell, with a nickel-based anode, a cadmium cathode and a potassium hydroxide electrolyte. Abbreviation: nicad cell.

Node Any point on a transmission line, where standing wave is of zero value.

Noise Unwanted signals occurring in an electronic system, causing spurious output signals. Noise can be the result of man-made causes, or natural causes. Many different types of noise exist, named after their basic nature e.g. thermal noise, atmospheric noise, white noise, impulse noise.

Noise factor The ratio of a device's or circuit's input signal-to­ noise, to its output signal-to-noise. Synonymous: noise figure.

Noise figure See: noise factor.

Non-linear Any circuit or system which produces an output which is not directly proportional to its input at all times is said to be non­ linear.

Nonvolatile memory Type of memory in which data is maintained even when power is disconnected.

NOR gate Logic circuit whose output is high if all the inputs are low. If one or more inputs are high, the output is low.

NOT gate Logic circuit whose output is always the inverse of the input. Synonymous: inverter.

NPN transistor A bipolar transistor formed by three layers of semiconductor material- the outside two layers being of N-type material, the middle layer of P-type material.

NTSC Abbreviation for National Television System Committee.

An American committee, responsible for television standards. The

initials NTSC are often comically described as never the same colour

-referring to the constant colour changes inherent in the system.

N-type semiconductor Semiconductor material containing a higher concentration of negative charge carriers i.e. electrons, than positive charge carriers i.e. holes.

Numerical control Type of automatic control system in which a number generated by the controlling device is compared with a number generated by another device. The difference between the two numbers is detected by the controlling device and used to generate a control signal.

Nyquist diagram Graph of a system's performance, which may be used to determine the system's stability under untested criteria.

OCR Abbreviation for optical character reader.

Octave A difference or interval between two sounds, whereby one sound is twice the frequency of the other.

Off-line A computer peripheral which is unconnected to the computer is said to be off-line.

Ohm The unit of resistance, reactance and impedance. One ohm is the resistance between two points when a constant current of one amp flows as the result of an applied voltage of volt between the points.

Ohmic A material which follows Ohm's law is said to be ohmic. Ohmmeter An instrument which measure resistance.

Ohm metre The unit of electric resistivity. Symbol: 0 m.

Ohm's law Law which defines the linear relationship between the voltage applied across a material, the current produced through the material, and the resistance of the material. Ohm's law can be written:

V=IR

One shot Synonym for a monostable multivibrator.

On-line A computer peripheral which is connected to and

receiving or transmitting data from or to a computer, is said to be on­ line.

Opamp Abbreviation for operational amplifier.

Open circuit Term applying to a circuit or system whose output is not connected to any following circuit or system. The output is therefore unloaded. Measurements of electrical parameters at this time are said to be under open circuit or no lead conditions.

Operating point The point on a semiconductor device's characteristic curve, representing electrical parameters when defined conditions are applied to the device.

Operational amplifier An amplifier, generally in integrated circuit form, which is usable with only a few components and power supply connections.

Optical character reader A computer input peripheral or applications software which is capable of converting symbols printed on paper into digital signals.

Oracle The Independent Broadcasting Authority's version of broadcast teletext.

OR gate Logic circuit whose output is high if one or more of its inputs are low.

Oscillation A periodic variation of an electrical parameter. Oscillator A circuit or system which produces oscillation. Oscilloscope Test equipment which is able to produce a visual

display of one or more oscillations of voltage. Generally the device

used to display the voltages is a cathode ray tube and such oscilloscopes are often referred to as cathode ray oscilloscopes (shortened to CRO).

Output I The part of a circuit or system which produces an output signal. 2 The signal produced by a circuit or system.

Output impedance The impedance of the output of a circuit or system.

Overall efficiency The ratio of the power absorbed by a circuit or system to the power supplied by a source.

Overdamping Damping applied to a period oscillation which prevents the oscillation from completing one cycle before stopping.

Overdriven Term, generally applied to a linear system such as an amplifier, which refers to the state when the size of input signal is such that the system's output is non-linearly related. In the case of an overdriven amplifier the output sounds harsh and is known as distorted.

Oxidation A process in the manufacture of semiconductor devices when the semiconductor base material undergoes a reaction with oxygen to form a semiconductor oxide.

PA Abbreviation for public address system.

PABX Abbreviation for private automatic branch exchange.

Packing density The number of transistors or gates in unit area on an integrated circuit chip.

Pair Two similar conductors, insulated from each other but

running in parallel, forming a transmission line. Generally, the pair is in the form of wire e.g. twisted wire pair, coaxial cable.

PAL Abbreviation for phase alternation by line.

PAM abbreviation for pulse amplitude modulation.

Parallel Components are said to be in parallel if current from a single source divides and flows through them then later reunites.

Parallel circuit A circuit containing two or more components connected in parallel.

Parallel plate capacitor A capacitor formed from two parallel conductive plates, between which is the dielectric.

Parallel resonant circuit A circuit containing a capacitance in parallel with an inductance, which exhibits resonance.

Parameter A criterion of an electronic component circuit, or system. Typical parameters are voltage, current, resistance, capacitance and so on.

Parametric amplifier I A microwave frequency amplifier, whose reactance is varied in a regular manner. 2 An audio frequency amplifier which can amplify or attenuate specific frequency signals, while passing other signals unaltered.

Pascal A high level programming language.

Passive Any component which does not introduce gain is known as a passive device.

P-channel The conducting channel of a field effect transistor of P­ type material. The term is also used to refer to the transistor i.e. P­ channel field effect transistor.

PCM Abbreviation for pulse code modulation.

PD Abbreviation for potential difference.

Peak-to-peak amplitude The difference between extreme values of a periodic oscillation.

Peak value The extreme value of a periodic oscillation. Period The time to complete a single cycle of an oscillation.

Symbol: T.

Periodic Term used to describe any variable which exhibits a regularly occurring form.

Peripheral devices Devices which connect to a computer. Permanent memory Non-volatile memory i.e. memory, the

contents of which remain intact without a supply of power.

PFM Abbreviation for pulse frequency modulation.

Phase The amount by which a periodic variable has progressed from a reference point. Phase can be measured as an angle or in radians. Two periodic variables with the same frequency and waveform which reach corresponding stages simultaneously are said to be in phase. If this does not occur, they are said to be out of phase.

Phase alternation by line A colour television system, variations of which have been adopted throughout Europe, in which the colour signal (known as the chrominance signal) is resolved into two components and transmitted separately. The phase difference of these two components is reversed on alternate lines, a procedure which helps to reduce errors due to received phase variations. Abbreviation: PAL.

Phase difference Difference in phase between two sine waves of the same frequency.

Phase modulation Modulation in which the phase of a carrier wave is varied by an amount proportional to the amplitude of the message signal. Abbreviation: PM.

Phase shift keying Alternative name for simple phase modulation of a digital signal. Abbreviation: PSK.

Photocell A transducer which converts light to some parameter of electricity.

Photodiode A semiconductor diode device, which conducts electric current by an amount proportional to the quantity of light falling on it.

Photoresist Photosensitive material which changes in molecular ways upon exposure to light. Photoresists are used in the manufac­ ture of semiconductors, integrated circuits and printed circuit boards.

Pick-up A transducer which converts recorded signals into electric signals.

Pico- Prefix denoting a multiple of 10·12. Symbol: p.

Picture element The smallest portion of a graphic or pictorial display system which can be resolved by the system. Often shortened to pixel.

Piezoelectric crystal A crystal which displays the piezoelectric effect.

Piezoelectric effect An effect observed in certain materials when a voltage is generated across the faces of the material as a mechanical stress is applied.

PIN diode A diode which contains a layer of intrinsic i.e. pure, semiconductor between the P and N layers.

PM Abbreviation for phase modulation.

PN junction The junction between two layers of semiconductors of P-type and N-type origin.

PNP transistor A bipolar transistor formed by three layers of semiconductor material- the outside two layers being of P-type material, the middle layer of N-type material.

Point contact device One in which the pn junction is formed at the contact between a metal 'cats-whisker' and the semiconductor material. Point contact diodes have advantages in some applications.

Polarised Term used to describe any component or device which must be inserted into a circuit a particular way round.

Positive feedback Type of control procedure in which part of the output signal of a circuit is fed back to the input terminal in such a way that the circuit regenerates the signal, resulting in greater amounts of signal fed back, resulting in further regenerates. Generally, the result of positive feedback is to form an oscillation.

Pot Abbreviation for potentiometer.

Potential Abbreviation for potential difference.

Potential difference The voltage across two points.

Potential divider A circuit consisting of a number of series components. Tapping at one of the junctions between components allows a fraction of the total applied voltage to be obtained. Synonymous: voltage divider.

Potentiometer A form of variable resistor with three contacts. A voltage is applied across the outer two (across the total resistance) and the third contact (the wiper) may be varied along the length of the resistance forming a variable voltage divider. Abbreviation: pot.

Power Rate at which energy is used up or work is done. The electrical unit of power is the watt. Abbreviation: W. Symbol: P.

Power ratio The unit of acoustical or electrical power measure­ ment in comparison with a standard level is the bel. In practical terms, power ratios are usually expressed in decibels (dB).

Power supply A source of electrical power for electronic circuits. Usually the power supply is integral to the equipment.

Abbreviation: PSU (for power supply unit).

Power transistor A transistor which operates at high values of power.

PPM Abbreviation for pulse position modulation.

Preamp Abbreviation for preamplifier.

Preamplifier Part of an amplifying system which amplifies small applied input signals, generally amplifying in terms of voltage amplitude only.

Preferred values Predetermined component values. Their use makes component manufacture relatively simple, as only a selected few values need be manufactured, not every possible value.

Prestel See videotex.

Primary cell A cell whose structure does not allow it to be recharged.

Printed circuit board Method of manufacturing electronic products in which all or most of the circuit is constructed on a thin board (the printed circuit board). Connections between components are formed with thin strips of copper. Abbreviation: PCB.

Program The complete set of instructions which can control the operation of a computer.

Programmable read only memory Computer memory which may be programmed i.e. have data written into it, once. After this it may only be read from. Abbreviation: PROM.

Programming language Any language which may be understood by computers and humans. Computers ultimately require instructions in machine code, so this is the simplest programming language. It can be understood by humans but not easily. Low level programming languages resemble machine code and are thus still difficult in terms of human use. High level programming languages resemble human languages and are thus easier for humans to use.

PROM Abbreviation for programmable read only memory.

P-type semiconductor Semiconductor material containing a higher concentration of positive charge carriers i.e. holes, than negative charge carriers i.e. electrons. In effect holes are simply a depletion of electrons, but nevertheless can be viewed as small objects which carry a charge through a semiconductor.

Public address system Sound reproduction system used to amplify sound and thus allow it to be relayed to many people over a large area. Abbreviation: PA.

Pulse A single variation in voltage or current from a zero value, to a maximum and back to zero.

Pulse amplitude modulation Pulse modulation system in which the amplitude of a pulse is modulated with respect to the amplitude of a message signal. Abbreviation: PAM.

Pulse code modulation Pulse modulation system in which pulses are produced corresponding to the message signal.

Abbreviation: PCM.

Pulse modulation Any modulation system in which a train of pulses is used as the carrier. Abbreviation: PM.

Pulse position modulation Pulse modulation system in which the position of each pulse is related to the message signal.

Abbreviation: PPM.

Pulse width modulation Pulse modulation system in which the width of each pulse is modulated with respect to the message signal. Abbreviation: PWM.

Push-pull Circuit operation in which two devices operate totally out of phase.

Q-factor Abbreviation for quality factor.

Quadraphonic Referring to a sound reproduction system with four separate sound channels.

Quadrature Two sine waves of the same frequency but 90° out of phase are referred to as being in quadrature.

Quality factor A variable which describes the selectivity of a circuit. It is typically used in conjunction with resonant circuits. The quality factor may be calculated from the expression:

where Q is the quality factor, B is the bandwidth of the circuit, and is the centre frequency of the circuit. Abbreviation: Q-factor.

Quantisation The production of a number of quantised i.e. discrete, values which may be used to describe a continuous waveform. The best example of the use of quantisation is in the process of pulse modulation, where the sampled values are used to define some aspect of a pulse train.

Quartz A type of crystal which exhibits the phenomenon of piezoelectricity.

Quartz-crystal oscillator An oscillator which relies on the principle that crystal will vibrate at a fixed natural frequency.

Quiescent current Current which flows through any component or part of a circuit under normal conditions, when no signal is applied.

Quiescent point Point on a semiconductor's characteristic curve representing the parameters of the device when in a quiescent state.

Radar An acronym of radio direction and ranging. A system capable of locating distant objects using reflected radio waves.

Radiation Any form of energy transmitted as electromagnetic waves, or as streams of particles.

Radio The use of electromagnetic radiation within the frequency range of about 3 kHz to 300 GHz to transmit information without connecting wires.

Radiowave Any electromagnetic radiation with a frequency within the radio frequency range of about 3 kHz to 300 GHz.

RAM Abbreviation for random access memory.

Random access memory Type of computer memory which may be accessed randomly i.e. directly (non-sequentially).

Raster Term describing the pattern of lines on a television-type display screen, which occurs at all times.

Ratings Specification sheets for transistors cover many facets of the device's operation but most parameters are needed only by the designer. The ratings which need to be known for replacement purposes are VCE(max)' the maximum collector to emitter voltage; /c, the collector current; hre• the gain; and J;, the cut-off frequency. The output power also needs to be observed.

RC Abbreviation for resistor-capacitor.

RC network Abbreviation for resistor-capacitor network. Any circuit or network which consists primarily of a resistor and a capacitor.

Reactance The part of the total impedance of a circuit which is due to capacitance in inductance, and not to resistance. Reactance causes the current and voltage to become out of phase (in a circuit of pure resistance, current and voltage are in phase). Symbol: X.

Unit: ohm.

Reactive load A load with reactance, which thus causes the applied current and voltage to be out of phase.

Reactor A component with reactance i.e. a capacitor or inductor.

Read To retrieve information previously stored in a computer-type memory device.

Read only memory A computer-type memory device, from which information can only be read from, and not stored into. The information held in a read only memory is generally stored at the manufacture stage and is specific to the operation of the computer. Abbreviation ROM.

Read-write head Device used to record and retrieve information to and from a magnetic memory.

Real time operation Use of a computer during the actual time a process is occurring, to monitor and control the process.

Receiver The part of a communication system which receives encoded information from a transmitter, and decodes it to the form required.

Record Any permanent or semi-permanent storage of electrical information.

Rectifier Any device which passes current in only one direction. A rectifier is thus an AC-to- DC converter.

Redundancy I The use of extra components in a circuit or system to ensure that breakdown of one component does not affect operation of the circuit or system. Redundancy is a method of increasing reliability. 2 Inclusion of extra information in a transmitted signal which may be eliminated without loss of essential information.

Refractive index The ratio of the velocity of light in free space to that in the material.

Refresh The restoration of information stored so that the information is not lost, typically in a dynamic memory device, or in devices with a destructive read operation.

Regeneration Synonym for positive feedback.

Register One of the temporary storage locations within the central processor of a computer, used to store the results of operations and calculations performed.

Regulator A circuit or device which maintains a constant output voltage or current, regardless of input voltage or output current requirements.

Rejection band The band of frequencies which are not passed through a filter.

Relative permittivity The ratio of the difference between the permittivity of a capacitor dielectric and the permittivity of free space.

Relaxation oscillator An oscillator which relies for its operation on an increasing and decreasing current or voltage within each period of oscillation.

Relay An electrical component in which an applied voltage or current electromagnetically operates a switching mechanism. The contacts of the switch can be isolated from the electromagnet providing a means whereby separate circuits may be interfaced without the need for electrical contact. Modem relays, although providing the same function, are often of a solid state form.

Reliability The ability of a component, circuit, or system to perform its functions for a given period of time.

Reluctance The magnetic equivalent of resistance.

Repeater A device or circuit which amplifies, regenerates, or restores to its original condition a signal in a telecommunications system, which has deteriorated due to transmission over a distance.

Resistance The ability of a material to resist the flow of electric current and to convert electrical energy into heat. A material's resistance is given by the ratio of applied voltage across it to the current flow through it caused by this voltage. Symbol: R.

Unit: ohm.

Resistivity The ability of a material to have a resistance dependent on the material's cross-sectional area and its length. The resistivity

of a material is given by:

where p is the resistivity, R is the resistance, A the cross-sectional area, and L the length.

Resistor An electronic component which possesses resistance. A pure resistor possesses only resistance, no capacitance or inductance, but all practical resistors possess some small amount of capacitance or inductance. Usually these are sufficiently small to be negligible.

Resonance Phenomenon arising when a circuit or system is excited by an applied signal, so that a small input signal produces a relatively large output signal, at the system's resonant frequency.

Resonant frequency The frequency at which a resonant circuit naturally resonates. Symbol: m.

Reverberation The persistence of sound inside an enclosure, due to multiple reflections from the inside surfaces of the enclosure.

Reverberation time The time required from the cessation of a sound, for the intensity to fall by 60 dB (that is, one millionth of the original value). The unit of reverberation time is the second.

Reverse bias Voltage applied to a PN junction, such that the P­ type layer of semiconductor is negative with respect to the N-type layer. Synonymous: reverse voltage.

Reverse voltage Synonymous: reverse bias.

Rewrite Synonymous: refresh.

RF Abbreviation for radio frequency.

Rheostat A variable resistor used specifically to alter the current flow in a circuit.

Right hand rule See: Fleming" s right-hand rule.

Ringing The delay which a system exhibits in returning to its quiescent state after a sharp pulse input, due to inherent resonance within the system. Generally a period of oscillation occurs, gradually dying away. Damping the system reduces this period.

Ripple A small AC signal superimposed on a DC voltage or current, typically found on the 0utput of a DC power supply, where the frequency of the ripple is mains frequency i.e. 50 Hz or sometimes twice this frequency.

Rise time The time taken for a pulse's leading edge to rise from

10% to 90% of its final value.

RMS Abbreviation for root mean square.

ROM Abbreviation for read only memory.

Root mean square Term used to describe the effective value of an AC waveform. It is the square root of the mean value of the squares of the instantaneous values of the waveform. In the specific case of a sinewave the root mean square value is equal to the peak value divided by -./2. Abbreviation: RMS.

RS flip-flop See: bistable.

Rumble Unwanted noise heard in a hi-fi system, caused by mechanical vibrations in the record playing deck, of low frequency.

Sampling The extraction of portions of an electrical analog signal, used to produce a series of discrete values.

Satellite Artificial body in orbit around the earth for purposes of communications, either one-way from the satellite to the earth, or two-way from earth to satellite and back.

Saturation When the output current of an electronic device is constant and independent of input.

Sawtooth oscillator A relaxation oscillator which produces a sawtooth shaped waveform.

Scanning Process of controlling the electron beam horizontally across and vertically down the face of a cathode ray tube device.

Schematic Circuit diagram.

Schmitt trigger Bistable circuit in which the binary output is determined by the magnitude of the input signal in such a way that the circuit exhibits hysteresis -the output changes when the input exceeds a predetermined level, and changes back when the input falls below a lower predetermined level.

Scramble Process of rendering a communications signal unintelligible at the receiver unless a descrambling circuit is used.

Screen I Surface of a cathode ray. 2 Shield to prevent electro­ magnetic interference.

SCS Acronym for silicon controlled switch.

SECAM Acronym for sequential couleur a memoire; a line­ sequential colour television standard.

Secondary cell Rechargeable cell.

Secondary emission Emission of electrons from a material as the result of a bombardment by high-velocity electrons or positive ions.

Secondary voltage Voltage developed across the secondary windings of a transformer.

Selectivity Ability of a radio receiver to discriminate against carrier frequencies different to that selected.

Semiconductor device Device whose operation is based on the use of semiconductor material. In addition to transistors and diodes there is a wide range of components which make use of semiconductor effects.

Semiconductor material Material whose conductive properties depend on the addition of minute quantities of impurity atoms. Unlike normal conductors, semiconductors increase in conductivity with an increase in temperature.

Sensitivity I The change in output of a device per unit change in input. 2 Ability of a radio receiver to respond to weak input signals.

Sensor Transducer.

Serial transmission Communication method in which characters are transmitted in tum along a single line.

Series Components in series have one current flowing through each.

Shift register Digital store of information, in which the informa­ tion is displaced one place in either direction on application of a shift pulse.

Short circuit Unwanted electrical connection between two points in a circuit.

Short wave Radiowave in the wavelength range from 10 to 100 metres.

Shunt Parallel connection.

Siemens The SI unit of electrical conductance. Symbol: S. Signal Variable electrical parameter.

Signal generator Device which can generate a controlled signal.

Signal-to-noise ratio The ratio of the value of signal at a point in a system, compared with the value of noise at the same point. Usually expressed in decibels.

Silicon Semiconductor element, most widely used element to form semiconductor devices.

Silicon controlled rectifier Abbreviation: SCR. Thyristor. Simplex Communications channel operating in one direction only. Sinusoidal Waveform identical in shape to a sine function.

Slew rate The rate at which the output of a circuit can be driven from one limit to the other.

Smoothing circuit Circuit designed to reduce ripple in a direct current or voltage.

Solid state circuit A circuit in which the current flows through solid material instead of through a gas or vacuum.

Super alpha pair See: Darlington pair.

Synchronous Clocked.

Telecommunications The transfer of information by any electromagnetic means.

Telemetry Measurement at a distance using electromagnetic

means.

Telephone Communication of speech and/or other sounds via electromagnetic means.

Television Communication of video and audio information by electromagnetic means.

Thermal runaway Semiconductor materials are very sensitive to heat - germanium much more so than silicon. Circuit design has to take account of this and many components have to be included to prevent increased current flow due to heat. Without such protection heat induced current will raise the temperature leading to a further increase in current and so on, a process known as thermal runaway which can destroy a semiconductor. See: heat sink.

Thermionic emission Electron emission from the surface of a body, due to the temperature of the body.

Thermistor A semiconductor whose resistance varies with temperature. Some have a negative temperature coefficient, that is resistance falls with an increase in temperature, others have a positive temperature coefficient. Typical applications are to provide compensation for the effects of heat on circuit operation.

Thevenin's theorem A theorem used to simplify the analysis of resistance networks.

Threshold of hearing The sound level or intensity which is just audible for an average listener. For a pure sinusoidal tone of 1,000

Hz it corresponds approximately to a root mean square pressure of

Thyristor Three junction, four layer semiconductor rectifier which conducts when either the voltage across it reaches a breakdown point or when triggered by a pulse at its gate electrode. Once triggered it remains conducting until the voltage across it becomes zero.

Transducer Any device that converts one parameter into another, where one of the parameters is an electrical signal.

Transformer Device which transforms electrical energy at its input to electrical output. Usually the voltages of the electrical energies differ.

Transistor Semiconductor device in which the current flowing between two electrodes may be modulated by the voltage or current applied to other electrodes.

Triac Bi-directional thyristor.

Tunnel diode A heavily doped semiconductor diode which exhibits a negative-resistance characteristic i.e. over part of its characteristic increased forward bias leads to a reduction in the current flowing.

Type numbers The numbers in a transistor designation rarely describe anything about its characteristics. In the 2N series adjacent type numbers are frequently widely differing devices. European and British transistors are frequently coded with the first letter A

(germanium) orB (silicon) followed by a second letter which indicates the type:

A Diode

C AF (low power) D AF(power)

E Tunnel diode

F HF (low power)

L HF(power)

UHF Ultra high frequency.

P PhotoType

S Switching (low power)

U Switching (power) Y Diode (power)

Z Zener diode

Ultrasonics Sound frequencies above the limits of human ears, generally classed as above 20kHz.

Ultraviolet radiation Electromagnetic radiation of wavelengths between visible light and X-rays.

Unijunction transistor Three terminal transistor comprising an n­ type silicon bar with a base contact at each end (base I and base 2) and a p-type emitter region. Current flow from one base to the other is controlled by the emitter current; when the emitter voltage reaches a certain level the emitter-base I junction virtually short circuits.

Valency The ability of atoms to unite with other atoms due to the electrons that exist in the outer orbit, or valency band, being able to form a shared orbit with other atoms.

Varicap diode Varactor. When reverse-biased, all PN junctions exhibit capacitance, as the depletion layer at the junction forms an insulator between the conductive regions. This property is used in the varicap, in purposes such as automatic tuning and AFC in radio receivers.

VHF Abbreviation for very high frequency.

Voltage drop Voltage between any two points of a circuit, due to the current flow between them.

Watt SI unit of power. Symbol: W.

Wave A periodic motion, through a medium (which may be space) in which the propagation from a point is a function of time and/or position.

White noise Noise with a wide, flat frequency response.

Word A stream of bits corresponding to a unit of information in a digital circuit.

Wow Low frequency (below 10 Hz) periodic variations in the pitch of the sound output of a sound reproduction system.

Write To enter information into a storage element.

Yagi aerial Directional aerial array -most television aerials are based on the Yagi aerial.

Zener diode Voltage regulating diode. A PN junction diode which has a defined reverse breakdown voltage. Once in the breakdown region large increases in current produce negligible variation in the voltage across it.