A
A battery (US) A battery used to provide the low-tension supply for electron-tube heaters or filaments.
abend Short for abnormal end. In computing fatal error conditions that require immediate termination of the user’s session.
aberration In cathode ray tubes, distortion of the image caused by failure of the electron beam to focus all points in the image accurately on the screen.
absolute permeability Of a material, the ratio of the magnetic flux density to the magnetising force which gives rise to it.
absolute permittivity Of an insulator, the ratio of the electric flux density to the electric field strength which gives rise to it.
absolute power level The magnitude of the power level of a signal at a point in a transmission system, usually expressed in decibels, relative to a power of 1 mW, known as zero power level. For example, a signal power level of 1 μW is known as −30dB.
absolute value device A device of which the output signal has an amplitude equal to that of the input signal but always has the same polarity irrespective of the polarity of the input signal.
absolute voltage level The magnitude of the voltage level of a signal at a point in a transmission system, usually expressed in decibels, relative to a voltage of 0.775 V, known as zero voltage level. 0.775 V RMS is the voltage corresponding to a power of 1 mW in 600 Ω. A reference voltage level of 1 V is sometimes used in expressing the voltage level of the output of a microphone.
absorber circuit In a transmitter, a circuit, usually employing an electron tube, which absorbs power from the transmitter when a break occurs in an oscillatory circuit so preventing the formation of an arc across the break which could cause damage.
absorption frequency meter (also known as wavemeter) An instrument for measuring the frequency (or wavelength) of an RF signal, in which a calibrated resonant circuit is coupled to the RF source, maximum energy being absorbed from it when the circuit is resonant at the frequency of the signal.
absorption modulation A method of producing amplitude modulation in which the amplitude of the carrier is varied according to the power absorbed from it in a resistance the value of which is made to vary in accordance with the instantaneous amplitude of the modulating wave.
A-B test A subjective comparison of the quality of two audio or video signals by switching alternately between the two. Two items of equipment, e.g. hi-fi amplifiers, may be compared in this way. A three-head tape recorder may be tested by switching alternately between the source and playback output while recording a suitable high-quality source.
accelerated life test A form of life test so designed that the time taken to determine the probable life of a component or device is appreciably shorter than the probable life. For example, in an accelerated life test on a switch it may be operated far more frequently than would occur under normal working conditions.
accelerating voltage In an electron tube the voltage applied between the accelerator and the cathode.
accelerator or accelerating electrode An electrode in an electron tube used to increase the velocity of electrons emitted from the cathode. It is usually in the form of a cylinder or a plate with an aperture so as not to intercept electrons but to direct them towards the anode or target electrode.
accentuation In audio-frequency engineering the emphasis of a particular band of frequencies, for example emphasis of the lower end of the audio spectrum, alternatively known as bass boost.
acceptance test A test carried out on a device or equipment to demonstrate that it meets all the requirements of the buyer.
acceptor circuit A resonant circuit designed to present a low impedance at a particular frequency. When connected across a high-impedance circuit it acts as a notch filter which attenuates signals at the chosen frequency. The commonest form of acceptor circuit is a series LC combination. See rejector circuit, series resonance.
acceptor impurity In semiconductor technology, a trivalent element such as boron, atoms of which can replace the tetravalent atoms in the lattice of a silicon or germanium crystal so making holes available as charge carriers. See p-type semiconductor.
acceptor level In the energy-level diagram of a p-type semiconductor an intermediate level slightly above the valence band which is empty at absolute zero temperature and to which electrons can be thermally transferred at other temperatures.
access time Of a store the time taken to obtain desired information. More specifically the interval between the instant at which the data is called for and the instant at which it is available at a specified location.
accumulator (1) A battery composed of secondary cells. The most widely used forms of accumulator are the lead-acid battery employed in cars, and the nickel-iron battery. (2) In a computer or electronic calculator, a small-capacity store (or memory) in which the results of an arithmetical calculation are kept until required for a further stage of the calculation.
acoustical impedance The complex ratio of the alternating sound pressure applied to an acoustic system to the resulting alternating volume velocity imparted to it. By analogy with electrical impedance, acoustical impedance is made up of acoustical resistance and acoustical reactance, and the latter can arise from inertance (acoustical analogue of inductance) and compliance (acoustical analogue of capacitance). Volume velocity is the product of linear velocity and the cross-sectional area of the system.
acoustic coupler A device enabling a normal telephone handset to be used for data transmission. The handset is placed in a cradle forming part of the terminal equipment and is thus acoustically coupled to a microphone and loudspeaker within the terminal. The digital signals are converted into audio signals for transmission over the telephone circuit and at the receiving end are reconverted to digital form. The system is suitable only for low-speed transmission.
acoustic delay line A delay line which makes use of the time of propagation of sound waves in a solid or liquid medium.
acoustic feedback In general, feedback of sound from a loudspeaker to an early stage in the preceding amplifier. In the most familiar example, the sound is picked up by a microphone in a public address system but the effect can also arise when the sound is picked up by a microphonic electron tube or even a transformer. If the feedback exceeds a certain degree, sustained oscillation known as ‘howlback’ or ‘howlround’ occurs.
acoustic filter A network of acoustic elements designed to pass sound waves within certain frequency bands with little attenuation but greatly attenuating sound waves at other frequencies. Acoustic reactances can take the form of pipes, slots and boxes and these may be connected in series or in shunt so making possible a wide variety of types of filter as in electrical technology.
acoustics In general, the study of the production, propagation and effects of sound waves in solid, liquid and gaseous media. The term is also used to describe the properties of an environment which determine the distribution and absorption of sounds generated within it, i.e. a concert hall may be said to have good acoustics if the sound quality is good within it.
acoustic store A store utilising the properties of an acoustic delay line.
activation The treatment applied to electron-tube cathodes during manufacture to maximise electron emission. See reactivation.
active area Of a rectifier that area of the rectifying junction which effectively conducts current in the forward direction.
active circuit element Same as active device.
active current That component of the current in an alternating-current circuit which is in phase with the applied alternating voltage. The power dissipated in the circuit is the product of the RMS values of this component and of the applied voltage. See reactive current.
active device or active element A component in an electrical or electronic circuit which is capable of amplifying signals. The chief forms of active device are the electron tube and the transistor. Both require a source of power for their operation. See amplification.
active power In an alternating-current circuit the arithmetical mean of the product of the instantaneous voltage and instantaneous current over one period.
active transducer A transducer which requires a source of power for its operation other than that provided by the input signal. An example is an electrostatic microphone which requires a high-voltage source to charge the capacitor to enable the microphone to operate.
adaptive control Control which varies automatically so as to give optimum results from the controlling process. Automatic gain control is an example of adaptive control in which the gain of an amplifier or receiver varies automatically to give a constant level of output signal.
adder In general any device or circuit of which the output represents the sum of the inputs. In particular a combination of logic elements which accepts as inputs the signals to be added and the carry from any previous stage of addition and gives two outputs, one representing the sum of the inputs and the other the carry for a subsequent stage of addition. An adder can be composed of two half adders.
address In a computer or data-processing equipment, information, usually in the form of a binary word, which identifies a particular location in a store.
address register In a digital computer a register used to store addresses.
admittance (Y) A measure of the ease with which an alternating current flows through a circuit. More specifically, admittance is the complex ratio of the current in an alternating-current circuit to the EMF which gives rise to it. It is the reciprocal of impedance and therefore has real and imaginary components as shown by the expression
where Y is the admittance, G is the conductance and B the susceptance. The numerical value of the admittance is given by
The admittance concept is useful in solving problems where components are connected in parallel because conductances are added, and susceptances added or subtracted according to their sign to give the net admittance. The unit of admittance is the mho. See conductance, susceptance.
afterglow Same as persistence.
after-image In a cathode ray tube a visual display which lasts after the stimulus causing it has ceased. See persistence.
air cooling The cooling of components and active devices dissipating considerable power by transfer of heat to the ambient air by radiation and/or convection. See conduction cooling, convection cooling, forced-air cooling.
air gap Of an inductor or transformer, a small gap in the magnetic circuit deliberately introduced to increase the reluctance of the circuit and so prevent magnetic saturation of the core by the direct component of the current in the winding.
airtime The actual time spent by a subscriber using a cellular radio system such as a mobile telephone. The airtime is normally the basis of the charges levied by the service provider.
algorithm A commonly used sequence of arithmetical or logical operations that is applied to data.
alias In computing, an assumed name for a file or an application or a user.
aliasing The chunky appearance of an oblique or curved edge as displayed on a VDU or in a bitmap (Figure A.1). See anti-aliasing.
aligned-grid tube A tetrode or pentode tube in which the wires forming the screen grid are arranged to be in the electron shadow of the wires forming the control grid. Such construction ensures that the screen grid intercepts very few electrons (in spite of its positive potential) and screen-grid current is thus minimised.
alignment In general the adjustment of all the preset controls of an equipment to give the required performance. In particular, the adjustment of the tuning circuits of a radio receiver to ensure that they resonate at the required frequencies as the tuning is varied over the received waveband(s). In a superheterodyne receiver alignment is necessary to ensure the maintenance of the correct difference in frequency between the oscillator and signal-frequency tuned circuits as the tuning is varied over the waveband(s). Ideally the difference frequency should equal the intermediate frequency at all tuning settings, but in practice this ideal can be achieved at only two or three tuning points in each waveband.
allowed band In an energy-level diagram the energy range which electrons may possess. The conduction band and the valence band are examples of allowed bands. See forbidden band.
alloy diode A pn diode manufactured by fusing a pellet of a trivalent or pentavalent element to the face of a wafer of semiconductor material. The wafer is heated until the element dissolves some of the semiconductor. After cooling the dissolved semiconductor crystallises out but sufficient of the element remains to give the required pn structure.
alloy transistor A transistor manufactured by fusing pellets of trivalent or pentavalent elements to the opposite faces of a thin wafer of semiconductor material. The combination is heated until the elements dissolve some of the semiconductor. After cooling the dissoved semiconductor crystallises out but sufficient of the elements remains to give the pnp or npn structure required, illustrated in Figure A.2.
all-pass network A four-terminal network of which the loss is independent of frequency. A symmetrical lattice network such as that shown in Figure A.3 is an example of an all-pass network. The phase shift of the network varies considerably with frequency and such networks are often used as phase equalisers because they can reduce phase distortion without affecting frequency response.
alpha cut-off frequency Of a bipolar transistor, the frequency at which the current amplification factor of the common-base circuit has fallen to 0.71 (1/√2) of its low-frequency value, i.e. is 3 dB down.
alphanumeric code A code using combinations of letters, numerals and other symbols to represent data.
alphanumeric display The display of information by means of letters and numbers. Such displays, usually on the screen of a cathode ray tube, are employed in computers and in information systems such as teletext.
alpha particle A positively-charged particle emitted from an atomic nucleus and consisting of two protons and two neutrons. An alpha particle is identical with a helium nucleus.
alternate display In an oscilloscope a means of displaying two or more signals by selecting the signals in sequence.
alternating current (AC or a.c.) or alternating voltage A current (or voltage) which periodically reverses its direction, varying sinusoidally with time about a mean value of zero. Domestic electricity supplies are usually in the form of alternating current, the frequency being 50 or 60 Hz. Alternating supplies have the advantage that they can be transformed to high voltage for long-distance distribution: high voltage implies low current and therefore low losses.
alternative denial gate Same as NAND gate.
aluminised screen Of a cathode ray tube, a form of construction in which the light-emitting phosphor of the screen is backed by a very thin coating of aluminium. This has the following advantages: (a) it protects the screen from bombardment by heavy negative ions so minimising ion burn, (b) it stabilises the potential of the screen so avoiding accumulations of charge which could produce shading effects on displays, (c) it increases the brightness of the display by reflecting forwards any light transmitted backwards from the phosphor.
ambient noise The noise present in the absence of a signal.
American Standard Code for Information Interchange (ASCII) A standard widely used in computers and data processing equipment to represent alphanumeric characters, punctuation marks and a selection of machine operations (such as line feed) by numbers. Originally ASCII was a 7-bit code, allowing a total of 128 characters and control codes, an eighth bit being used sometimes for parity checking. Many computers now use the eighth bit as part of the code; the extended character set made possible by the additional 128 characters often includes, e.g. the accented letters for most European languages and several currency symbols.
ammeter An instrument for indicating or recording the magnitude of an electric current in ampères (A). For large currents the measurement may be more conveniently expressed in kiloampères (kA) and for small currents in milliampères (mA) or microampères (μA). Current meters must be of low resistance so that, when inserted into a circuit, they have negligible effect on the current being measured.
Ampere’s law Same as Biot-Savart law.
ampere-turn The practical unit of magnetomotive force. For a magnetising coil it is equal to the product of the number of turns of wire on the coil and the current (in amperes) flowing in it.
amplification The process by which an electronic device or equipment increases the amplitude of a signal. In linear amplification, the output signal is a magnified but undistorted copy of the input signal but there are circumstances, e.g. in class-C operation, where the output signal, although related to the input, does not have a similar waveform. Some amplifiers are designed to magnify the signal current, others the signal voltage and power amplifiers are required to give substantial current and voltage output. A wide variety of active devices and circuit arrangements are used for amplification in electronic equipment and information on these is given under appropriate headings in this dictionary.
amplification factor (μ) Of an electron tube the ratio of the change in anode voltage to the change in control-grid voltage required to maintain the anode current constant. It is usually stated for particular values of electrode voltages.
amplifier A device or assembly of devices through which the current flow is controlled by an input signal so as to give at the output a magnified signal bearing some relationship to the input.
In some applications there is in fact little relationship between input and output, the amplifier operating as a switch controlled by the input signal. In other applications strict proportionality is required between input and output to give linear amplification.
See current amplifier, power amplifier, voltage amplifier.
ampliphase system A modification of the Chireix system in which the efficiency is improved by amplitude modulation of the drive signals.
amplitron A microwave amplifying electron tube used for high-power pulsed operation, e.g. in radar transmitters. The basic form of construction of the tube is shown in Figure A.4. The input and output ports are connected to a slow-wave structure surrounding a cylindrical cathode. There is a strong magnetic field parallel to the axis of the cathode.
amplitude In general the magnitude of a signal. In particular, of a sinusoidally-varying quantity, the peak value, i.e. the maximum departure from the average value. For example, if a voltage is given by V = Vo sin ωt, the amplitude is Vo. The value of a sinusoidally-varying quantity at any instant is termed the instantaneous amplitude.
amplitude discriminator In general, a circuit the output of which is a function of the amplitude of two input signals. In particular, the term is used to define a device which gives an output when the two input signals have equal amplitude.
amplitude distortion In a circuit, component or amplifier, distortion arising from variation of gain or attenuation with the amplitude of the input signal. It arises from non-linearity of the input-output characteristic and is produced by hard limiters when the input signal amplitude exceeds the extent of the linear characteristic. See amplitude limiter, characteristic curve.
amplitude gate Device or circuit capable of slicing.
amplitude limiter A four-terminal device which maintains a constant amplitude of output signal for amplitudes of input signal exceeding a certain value. There are two basic types of limiter. In one, which may be termed a hard limiter, a constant-amplitude output is ensured by chopping the peaks of large-amplitude signals, the characteristic having the form shown in Figure A.5 (a). The resulting waveform mutilation is unimportant, for example, in the hard driven limiter often included before the detector in an FM receiver. This ensures that the signal delivered to the detector is free of amplitude variations (which could cause noise) and contains only frequency variations. A second type of limiter is required in broadcasting and recording to ensure that transmitters or the recording medium are not overloaded by signals of too great an amplitude. Limiters for such applications must maintain linearity of the input-output characteristic and the limiting action is achieved by automatic reduction of the gain of the limiter for large-amplitude input signals so as to maintain the output signal at the limiting level. The characteristics for this type of limiter are shown at Figure A.5 (b): no distortion occurs except at the instants when the gain is changed. See characteristic curve.
Figure A.5 (a) Non-linear characteristic of a hard limiter and (b) characteristics of a non-distorting limiter for two values of gain
amplitude modulation (AM) A method of modulation in which the amplitude of the carrier wave is made to vary in accordance with the instantaneous value of the modulating signal. In radio transmission, considerable use is made of amplitude modulation of a carrier wave and Figure A.6 illustrates the effect of modulating a radio-frequency wave by a low-frequency sinusoidal signal. Such a modulation system is used, for example, in long-, medium-, and short-wave broadcasting and for the vision signal in television broadcasting. In these practical applications the difference between the carrier-wave frequency and the modulating-wave frequency is much greater than suggested in this diagram. It is also possible to amplitude modulate a pulse carrier. See double-sideband transmission, pulse amplitude modulation, single-sideband transmission, suppressed-carrier transmission, vestigial-sideband transmission.
analogue comparator A device used to check the output of an analogue-to-digital converter.
analogue computer A computer that operates on analogue signals, i.e. signals of which the significant property can have any value. These values are represented by corresponding values of voltage or current within the computer circuits which then carry out the required mathematical operations on the signals.
Analogue computers employ operational amplifiers with negative feedback for such operations as multiplication, addition, integration and differentiation. Potential dividers are used for division. Function generators are also needed, e.g. to give trigonometrical ratios. The final result from the computer is displayed on a cathode ray tube or on a chart.
Because of the large degree of negative feedback used with operational amplifiers, their outputs are reasonably accurate but there is inevitably some error (e.g. due to drift) and hence there is a limit to the accuracy of the results from the computer. This is normally of little significance because the inputs fed into the computer are normally from measurements which are themselves subject to error. Where precise accuracy is essential a digital computer is used.
analogue device A device which operates on analogue signals, e.g. an analogue computer or an audio amplifier.
analogue signal A signal, the significant property of which can have any value. The significant property may be the amplitude, phase or frequency of an electrical signal, the angular position of a shaft or the pressure of a fluid. An audio signal is an example of an analogue signal. Signals are often termed analogue where it is necessary to contrast them with digital signals. See analogue computer, digital signal.
analogue switch A switch with characteristics which make it suitable for use with analogue signals. Such signals can have a wide range of voltage and current, and switches for use with them must have values of resistance in the ‘on’ and ‘off states which are independent of the applied voltage and current. For solid-state analogue switches field-effect transistors are often used.
analogue-to-digital conversion (ADC) The process of converting an analogue signal to digital form. See digital signal.
analogue-to-frequency converter A device which accepts an analogue signal as input and gives an output with a frequency proportional to the input signal.
ancillary store Same as backing store.
Anderson bridge A modified form of Maxwell bridge used for measuring inductance in terms of capacitance and resistance. As shown in Figure A.7 the bridge has an additional resistor R5. The balance conditions (which are independent of frequency) are:
and the Anderson bridge has the advantage that the two balance conditions are independent.
AND gate A logic gate which gives a logic-1 output when, and only when, all the input signals are at logic 1. The graphical symbol for an AND gate is shown in Figure A.8. See logic level.
AND-NOT gate Same as NAND gate.
anechoic chamber A room used for testing loudspeakers and microphones in which all sound reflections have been eliminated by lining all the surfaces, including floor and ceiling, with sound-absorbing material.
angle modulation General term for modulation in which the phase angle of one signal is varied in accordance with the instantanteous value of another. Frequency modulation and phase modulation are two examples of angle modulation.
angle of current flow or angle of flow For an amplifier with a sinusoidal input signal the fraction of each cycle, expressed as an angle, during which current flows in the amplifier. For a class-A amplifier the angle of flow is 360° (2π radians), for a class-B amplifier it is 180° (π radians) and for a class-C amplifier it is less than 180°.
angle of lead See phase angle.
angular frequency (ω) The frequency of an alternating quantity expressed in radians per second. Because there are 2π radians in each cycle the angular frequency is equal to 2πf, where f is the frequency in cycles per second (Hertz). Thus
anion Negatively-charged ion formed in a gas by ionisation or in an electrolyte by dissociation and which moves towards the positively-charged electrode (anode) under the influence of the potential gradient. See cation.
anisotropic material A material which has different physical properties in different directions. Certain crystals are anisotropic in that their optical properties depend on the direction of incident light relative to the crystal axis. Certain magnetic materials, too, are anisotropic. See isotropic.
annunciator Any device, such as a lamp or a buzzer, used by equipment to attract the attention of its user.
anode Of an electron tube the positively-charged electrode at which the principal electron stream from the cathode leaves the tube as an external current. The anode is therefore the output electrode of the tube.
anode AC resistance See electrode AC resistance.
anode-bend detector An AM detector which relies for its action on the curvature of the Ia–Vg characteristic of an electron tube.
This curvature causes the tube to respond unequally to increase and decrease in the amplitude of the modulated input signal and thus there is a modulation-frequency component in the anode current. This is illustrated in Figure A.9. A pentode is generally used as an anode-bend detector and it is biased near anode-current cut off where characteristic curvature is most marked and detection, therefore, most efficient.
anode characteristic The graphical relationship between the anode voltage and anode current of an electron tube usually expressed for given values of control-grid and screen-grid voltage. The shapes of the characteristics for a triode and a pentode are given in Figure A.10.
anode (collector or drain) dissipation That part of the power supplied to the anode (collector or drain) circuit of an electron tube or transistor which is converted into heat at that electrode. The power thus dissipated is the difference between the DC power input to the electrode and the AC power delivered to the output load.
anode (collector or drain) efficiency The ratio, usually expressed as a percentage, of the AC power delivered to the output load of an active device to the DC power input to the anode (collector or drain) circuit. For a theoretically perfect active device with straight, parallel and equidistant characteristics the maximum efficiency for a sinusoidal signal is 50%. Pentodes and transistors can give efficiencies almost equal to this but the efficiency of triodes is much less because of the limited anode voltage swing available before the onset of grid current.
anode (collector or drain) load The impedance of the external circuit between the anode (collector or drain) and the cathode (emitter or source) of an active device. The current variations in the load, the voltage variations across it or the power dissipated in it constitute the output of the active device.
anode modulation Same as Heising modulation.
anode stopper See parasitic stopper.
antenna The system of conductors used at a radio installation to radiate or receive electromagnetic waves. To be effective as a radiator the antenna must have dimensions comparable with the wavelength and thus medium- and long-wave transmitters must have very large antennas, hundreds of feet in height or length. VHF and UHF transmitting antennas can be very much smaller but must usually be mounted at the top of a hill, a tower or a high building to give adequate service area. Receiving antennas can be small because the low pickup can be compensated by high gain in the receiver circuits. There are many different types of antennas and these are defined in the Dictionary of Audio, Radio and Video (Butterworths, 1980).
anti-aliasing The use of pixels in intermediate colours to smooth the jagged appearance (aliasing) of oblique or curved edges in bitmaps and VDU displays (see Figure A.11). Anti-aliased text is more readable than text displayed without anti-aliasing. The term is also applied to an analogous process used in converting digitised sound to analogue.
anti-cathode Target of electron tube, particularly an X-ray tube, on which the electron beam is focused and at which the X-rays are generated.
anti-hunting circuit A circuit incorporated in a feedback system to prevent self-oscillation. The circuit often consists of an RC combination designed to absorb energy at frequencies at which the system is likely to ring or oscillate.
antinode Of a standing wave, a point or plane at which a particular variable has a maximum value. Thus antinodes on a transmission line are points at which current or voltage is at a maximum. For a bowed string, antinodes are points at which the amplitude of vibration is at maximum.
antiresonance (US) Same as parallel resonance.
aperiodic circuit A circuit without natural resonance. The term is commonly applied to an LC circuit which is so heavily damped that the impedance is substantially constant over a wide band of frequencies centred on the resonance value. It is also applied to an LC circuit for which the resonance frequency is so remote from the frequency band in use that there is little change in the impedance over the working frequency range.
aperture (1) Of an optical lens the opening in the diaphragm (entrance pupil) which controls the amount of light passing through the lens. The size of the aperture is usually quoted as a fraction of the focal length of the lens, e.g. an aperture of f/8 signifies that the diameter of the aperture is equal to one eighth of the focal length. The aperture control is used, for example, to adjust the amount of light entering a television camera to ensure that the tonal range of the scene to be televised is correctly located on the characteristic(s) of the camera tube(s). (2) In television the term is used to mean the size of the spot where the scanning beam meets the target in a camera tube, flying-spot telecine or picture tube. See aperture distortion. (3) The term has a third meaning when applied to antennas. See the Dictionary of Audio, Radio and Video (Butterworths, 1980).
aperture corrector An equaliser designed specifically to offset aperture distortion.
aperture distortion In television, distortion arising from the finite size of the scanning spot. Such distortion can arise at the transmitting end where the spot is effectively the cross-sectional area of the electron beam where it meets the target in camera tubes or in flying-spot telecines, or at the receiving end where the spot is the cross-sectional area of the scanning beam at the picture tube screen. Because of the finite spot size an instantaneous change in tonal value occurring along a scanning line is reproduced as a pulse with a finite rise or fall time. Thus the spot size determines the degree of detail which can be transmitted and reproduced. It is possible to compensate to some extent for the loss of definition arising from aperture distortion. See aperture, aperture corrector, rise time.
Applegate diagram A diagram used to illustrate the principle of electron bunching in velocity-modulated tubes. Figure A.12 is a typical diagram. It is a graph in which the distance travelled by electrons from the buncher gap is plotted vertically and time is plotted horizontally. OA represents the path of a typical electron and the slope of OA measures its velocity. In a velocity-modulated tube the electrons leaving the buncher gap are accelerated by a positive voltage on which is superposed the RF signal it is required to amplify. This imparts a sinusoidal variation to the electron velocities. Suppose OA represents the velocity when the RF voltage is passing through zero and that the RF voltage executes its negative half-cycle immediately afterwards as suggested in Figure A.12. Then electrons leaving the gap after those represented by OA have lower velocities as suggested by BC and DE until point P is reached after which the velocities increase. The diagram shows that at a particular distance d from the gap the lines meet at a point showing that the electrons gather in bunches at such a distance. By repeating the construction for further cycles of RF the diagram shows that bunches occur at this same spacing from the gap at intervals of 1/f, where f is the frequency of the RF signal. See bunching, klystron, travelling-wave tube, velocity modulation.
application A task performed on a computer, such as desktop publishing or databasing, or a suite of software that forms an integrated package to perform that task. For example, several programs may be included together with various kinds of data file used by the programs. Often the various files that make up an application are stored together in one directory.
application(s) software Software that provides an application, in contrast with operating systems, utilities and data.
Application Specific Integrated Circuit (ASIC) Sometimes known as a gate array, this is a generic integrated circuit that has been tailored for a specific application. A single ASIC might replace several conventional logic integrated circuits, allowing a reduction in both circuit board size and power consumption. The term is also sometimes applied to a microprocessor dedicated to a single purpose.
arc Conduction of an electric current through an ionised gas usually at high current density and accompanied by emission of light. See ionisation, spark.
arcback In a gas-filled tube, conduction between anode and cathode resulting in current flow in the reverse direction to normal.
arc discharge In a gas-filled tube, conduction between anode and cathode as a result of electronic emission from the cathode. See glow discharge.
arc drop The voltage between the anode and cathode of a gas-filled tube when it is conducting normally.
architecture The overall design of a computer or data-processing system with special reference to the performance and interaction of its components, its connection to peripherals and other systems and facilities for future extension.
archive Any file in a non-volatile storage medium. More specifically, one or more files that have been subjected to a compression process so that they occupy less space in the storage medium.
arc-over An arc between two conductors or between a conductor and earth.
arc through In a gas-filled tube, conduction between anode and cathode in the normal direction but at a time when the anode should be non-conductive.
argument In computing additional data which is entered immediately following a command to specify the way in which the command is to be executed. For example, in the MS-DOS command
the character string ‘A:*.*’ forms the argument, specifying that all files on drive A are to be copied to the currently selected drive and directory.
arithmetic logic unit (ALU) That part of a central processing unit that performs arithmetical and logical operations.
array (1) A series of similar devices arranged in a meaningful pattern. In computers gates and memory cells are usually arranged in a rectangular pattern of columns and rows, known as an array or matrix. Arrays of antennas are used at short-wave transmitters. (2) In computing a cluster of similar variables sharing the same name but distinguished by numeric suffixes enclosed in parentheses.
artificial antenna Same as dummy antenna.
artificial earth A system of conductors mounted a few feet above the ground under an antenna and used in place of an earth connection. Such a system is likely to be used where the earth conductivity is poor.
artificial intelligence Computers and computer-controlled machines so organised that they behave, to some extent, like human beings, but without the propensity for mistakes. Perhaps the closest approach has been in so-called expert systems.
artificial language A language based on a set of rules established prior to the use of the language.
artificial line A network of inductors and capacitors designed to simulate the electrical properties of a transmission line over a desired frequency range. See capacitance, inductance.
artificial load A substitute for the normal load of an equipment which has the correct impedance and can safely dissipate the power generated in it. The artificial load for a transmitter simulates the impedance of the transmission line and antenna but does not radiate.
aspect ratio The ratio of the width to the height of a television picture. This is now standardised at 4:3 to agree with cinema-film practice, but see high definition television.
assembler, assembly language See computer language
astable circuit A circuit with two possible states, both unstable, so that it alternates between the two without need of external triggering signals. Astable circuits can, however, readily be synchronised at the frequency of any regular signal applied to them. An example of an astable circuit is a multivibrator in which both inter-transistor couplings are via capacitors as shown in Figure A.13. The two possible states of this circuit are: TR1 on and TR2 off; TR1 off and TR2 on.
astigmatism A defect in an optical or electron lens which causes focusing in different axial planes to occur at different points along the lens axis. As a result of astigmatism, a point object gives rise to an image in the form of a vertical line at one point on the axis and in the form of a horizontal line at another point as shown in Figure A.14. Normally the best compromise is between these two points where the image has the form of a circle, known as the circle of least confusion, which represents equal vertical and horizontal resolution.
asymmetrical circuit A circuit the two sides of which have different electrical properties with respect to a reference potential, usually earth. The term is often applied to circuits in which one side is earthed. See symmetrical operation.
asymmetrical deflection Electrostatic deflection in a cathode ray tube where the deflecting potentials applied to the plates are not symmetrical about a reference potential (usually the final-anode potential). In particular the term is applied to electrostatic deflection where one plate of a pair is effectively earthed and the deflecting potential is applied to the other.
asynchronous logic In a computer or data-processing equipment, logic in which operation is started by a signal generated at the completion of the previous operation. The term is used to distinguish this form of operation from that in which operations are controlled by an internal clock. See synchronous logic.
asynchronous transmission A method of data transmission in which characters are sent at irregular intervals, each being preceded by a start bit and followed by a stop bit. It is the most widely used method of data transmission between microcomputers.
atomic number Of an element, the total number of unit positive charges carried by the nucleus of an atom. For a neutral atom this is the number of protons in the nucleus and also the number of orbiting electrons. See atomic structure.
atomic structure The atom is regarded as a dense nucleus containing protons and neutrons and with a net positive charge, around which revolve a number of electrons neutralising the positive charge of the nucleus. The total number of protons is equal to the atomic number of the element. The innermost electron orbit can contain at most two electrons, the second 8 and the third 18: the number needed for completion is given by 2n2, where n is the number of the orbit. The number of electrons in the outermost orbit determines the properties of the element. For example, it is difficult to remove an electron from or to insert an electron into an outermost orbit which has a complete complement of electrons. Thus atoms with completed outermost orbits are chemically inert: such atoms are those of the gases helium, argon, krypton etc. If the outermost shell has only one or two electrons these can be removed with little effort, making elements with such atoms good electrical conductors. Typical of such elements are copper and silver. If the outermost orbit has four electrons, covalent bonds can be formed between neighbouring atoms in a crystal lattice and the element has semiconductor properties. The obvious examples are germanium and silicon.
attack time The time that elapses between the instant that a signal at the input of a component or circuit exceeds the threshold needed to activate the component or circuit and the instant that the component or circuit begins to respond to the input.
attenuation coefficient (or constant) See propagation coefficient.
attenuation/frequency distortion or attenuation distortion In a system, equipment or component, variation of the gain or attenuation with the frequency of the input signal. Attenuation distortion is illustrated by the frequency response curve and the most commonly-encountered forms are shown in Figure A.15. An important feature of attenuation distortion is that it can be corrected: for example, the effects of an equipment suffering from ‘top cut’ can be offset by use of an equaliser with a complementary ‘top lift’ response.
attenuator A network used to reduce the amplitude of a signal without distorting it. The reduction factor (attenuation) may be fixed or variable. A fixed attenuator is often called a pad. Attenuators may be inserted in lines or between equipments and are then designed not to introduce any impedance change, i.e. they are designed to have a particular value of iterative impedance. Alternatively attenuators can be designed on an image-impedance basis to match unequal impedances. Networks often used in attenuators are T-networks and π-networks and their balanced equivalents H-networks and O-networks. Figure A.16 gives an example of an O network with 10-dB attenuation and with an iterative impedance of 300 Ω.
audio frequency (AF) Any frequency of sound wave which is normally audible to the human ear. The lower limit is usually quoted as 15 Hz but it is difficult at such low frequencies to differentiate between the sensations of hearing and feeling. The upper limit is often given as 20 kHz but this limit is dependent on age and falls below 10 kHz for very old people. In electronics, audio signals which are intended for reproduction by loudspeakers are regarded as high quality if the frequency range from 30 Hz to 15 kHz is covered with minimal distortion.
auto-coincidence gate Same as exclusive-OR gate.
autodyne or auto-heterodyne A system of heterodyne reception in which the same active and passive components are used for oscillation and mixing (as in a self-oscillating mixer). See mixer, self-oscillating mixer.
automatic brightness control In a television receiver, a circuit which automatically adjusts the brightness of the display in accordance with the level of ambient light near the receiver. A photocell may be used to measure the ambient light, its output, after amplification, being used to control the grid bias of the picture tube.
automatic cathode bias Use of a resistor in the cathode circuit of an electron tube to provide grid bias. A typical circuit is given in Figure A.17. The cathode current in Rk sets up a voltage across this resistor which biases the cathode positively with respect to the negative HT supply. The control grid is connected to HT negative by the grid resistor Rg. Thus the grid is biased negatively with respect to the cathode as required.
The circuit can be used only when the mean cathode current is constant as in class-A operation and the value of cathode resistance required is, from Ohm’s law, given by Vg/Ik where Vg is the bias voltage and Ik the mean cathode current. Thus if the grid bias is −8 V and the mean cathode current 40 mA, the cathode resistance required is 200 Ω.
For a triode tube the mean cathode current is equal to the mean anode current but for more complex tubes such as pentodes, the cathode current includes also the screen-grid current.
In addition to providing grid bias a resistor in the cathode circuit also gives negative feedback which reduces the gain of the tube. If this reduction is undesirable the feedback can be minimised by by-passing the resistor by a capacitor as shown in Figure A.17. The capacitance should be such that the reactance at the lowest signal frequency is small compared with Rk.
automatic check In computers and data-processing equipment the use of hardware or software to verify the accuracy of processes carried out by the equipment.
automatic chrominance control (ACC) In a colour television receiver, an (AGC) circuit incorporated in the chrominance amplifier to ensure that the relative amplitudes of the chrominance and luminance signals are maintained. This is necessary to ensure correct colour rendering in spite of variations in signal level at the receiver input.
automatic contrast control See automatic gain control.
automatic frequency control (AFC) A circuit in a receiver which ensures that errors in the frequency of the oscillator or the tuning of a circuit are kept within certain limits. Before 1939 some amplitude modulation receivers had such a control (known as automatic tuning) and a discriminator was incorporated in the intermediate frequency amplifier to derive an error voltage which was applied to a reactance tube connected across the oscillator tuned circuit. In frequency modulated receivers the detector can be designed to provide the required error signal in addition to the audio frequency output. AFC is desirable in any receiver with preset (e.g. push-button) tuning to compensate for the inevitable slight drift in oscillator frequency.
automatic gain control (AGC) The maintenance of a substantially-constant input signal amplitude at the detector of a radio or television receiver (in spite of variations in amplitude of the input signal to the receiver) by automatic adjustment of the gain of the intermediate frequency and/or radio frequency stages. This adjustment is carried out by a control signal derived from the detector or from a post-detector stage. The purpose of AGC is to minimise the effects of signal fading on audio frequency output or picture contrast and to ensure that all received signals, no matter what their amplitude, give substantially the same output. When applied to radio receivers, AGC was originally known as automatic volume control and when applied to television receivers is sometimes termed automatic contrast control or automatic picture control. See forward automatic gain control, reverse automatic gain control, variable-mu tube.
automatic grid bias Use of a resistor in the grid (or cathode) circuit of an electron tube to produce grid-bias voltage as a result of grid (or cathode) current flow. Grid-current bias is commonly used in electron-tube oscillators and cathode-current bias in electron-tube amplifiers. See cathode bias.
automatic picture control Same as automatic gain control.
automatic tuning Same as automatic frequency control.
automatic volume control (AVC) Same as automatic gain control.
automation (1) The theory or technique of making a process automatic. (2) The control of processes by automatic means.
auto-transformer See transformer.
avalanche breakdown In a reverse-biased pn junction, a rapid increase in current which occurs at a particular reverse voltage as a result of cumulative multiplication of charge carriers. The effect is caused by the strong electric field across the junction which gives the few charge carriers so much energy that they liberate other hole-electron pairs by impact ionisation. The new carriers so created liberate further hole-electron pairs and thus the process becomes regenerative, leading to a very rapid increase in reverse current. See charge carriers, ionisation.
avalanche photo-diode (APD) A photo-diode in which the electrical output is considerably increased by avalanche breakdown. Such diodes are operated with a reverse bias close to the breakdown voltage so that liberated photo-electrons have sufficient energy to create hole-electron pairs by impact ionisation.
average detector A detector of which the output approximates to the mean value of the envelope of the input signal. See detection.
azimuth The alignment of a directional antenna or of a recording or playback head in a tape recorder.