PDF | This article presents the results of an experiment conducted on two distinct groups of students attending our ESP classes in the academic year PDF | The dictionary plays an important role in helping writers solve lexical problems in the L2 writing process. However, this is only when L2. Newnes Dictionary of Electronics This page intentionally left blank. Newnes Dictionary of Electronics Fourth Edition s. w. & l l O S, CEng, BSc, MIEE R. s.
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Modem dictionary of electronics / Rudolf F. Grafth ed., edition of this dictionary contained about 10, definitions of then current terms. And now, a scant. The Illustrated Dictionary of Electronics—8th Edition has been revised, clarified, and up- These symbols are used in illustrations throughout this dictionary. Electrical Engineering Dictionary. Ed. Phillip A. .. may result in a change in the electronic struc- ture, or in used in modern electric and electronic equip- ment.
Alternating supplies have the advantage that they can be transformed to high voltage for long-distance distribution: A domestic cordless telephone has radio links to a dedicated base station which is connected to the telephone system. Beam tetrodes usually have aligned grids and the resulting high-density electron stream prevents secondary emission from the anode from reaching the screen grid so preventing the tetrode kink which would impair efficiency. 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. See current amplifier, power amplifier, voltage amplifier. This is included in a bridge circuit which measures the power by the change in resistance resulting from the rise in temperature caused by the power absorbed.
The structure is illustrated in Figure B. Typical working voltages are shown in Figure B. In the npn transistor b the current crossing the base-emitter junction is carried by electrons from the emitter and holes from the base as explained under junction, but the impurity concentration in the emitter region is considerably greater than that of the base.
Thus the electrons greatly outnumber the holes. On entering the base region a few electrons leave it via the external base connection to give a small external base current.
Most of the electrons cross the very thin base region and are swept into the collector region, attracted by its positive bias, to give a considerable external collector current— despite the fact that the collector-base junction is reverse-biased. The collector output current can be as much as times the base input current. This amplification is the essence of transistor action. In the pnp transistor of Figure B. In both types of transistor electrons and holes play a part in the amplifying action, this justifying the description bipolar.
Because of their low input resistance and high output resistance bipolar transistors are best regarded as current amplifiers. J 7 Graphical symbols for a pnp and b npn bipolar transistors The graphical symbols for pnp and npn bipolar transistors are given in Figure B. When the circuit is placed in one of the states, it remains in it indefinitely unless compelled to leave it by the application of an external signal.
When thus triggered the circuit enters the other possible state and here too it remains unless a further triggering signal is received and this returns the circuit to its original state. Two external signals are thus necessary to restore the circuit to its original state and this illustrates one of the applications of bistable circuits—as pulse counters or frequency dividers. Because of their ability to remain in a given state, bistable circuits are extensively employed in digital switching applications, e.
Basically a bistable circuit may consist of a multivibrator with two direct inter-transistor couplings but there are many possible types. See graphics, painting software. It is usually expressed in bits kilobits or megabits per second. Black compression reduces the visibility of detail in the low-light regions of reproduced pictures. In a properly-adjusted system the black-level voltage at the input to a picture tube gives cut-off of the electron beam as indicated in Figure BA8W.
It may be included to limit the rise in voltage when the load is disconnected. It may be used to discharge capacitors when the power supply is switched off or it may be included to improve the regulation of the supply.
This may occur either fortuitously, for example, when the grid resistor is open-circuited or it may occur deliberately, e. C-coupled amplifier see Figure C. The circuit thus produces regular bursts of current and is used as a pulse generator in, for example, the sawtooth generators in TV receivers. The circuit diagram of a transistor blocking oscillator is given in Figure B. When the tuned circuit oscillates Ci is charged by base current to a high negative voltage which cuts the transistor off.
The transistor thus takes regular bursts of collector current and generates negative-going pulses across R2 at a frequency determined by the time constant RiCI. Such diagrams can be used to determine the stability of the device and to show what degree of negative feedback can safely be applied.
Bode equaliser An equaliser used to correct the frequency response of TV sound circuits in which the amount of equalisation can be adjusted, without change in the shape of the equaliser characteristic, by operation of a single control. The detecting device is a small resistive element, the resistance of which is strongly dependent on temperature.
This is included in a bridge circuit which measures the power by the change in resistance resulting from the rise in temperature caused by the power absorbed. The temperaturesensitive element is usually a thermistor or a filament in an evacuated bulb. It is named after its inventor George Boole and is used in the design of digital circuits.
This can be added to the normal supply to provide a high-voltage boost supply for the line output stage and other stages in the receiver. Clearly this location must be held in ROM. In a computer having a floppy-disk-based operating system, the ROM-based instructions may display a message to the user to insert the floppy disk containing the operating system and then to execute a batch file on that disk which configures the system.
A simple example is shown in Figure B. The output of the amplifier is developed across Re in the emitter circuit and the input is applied between base and emitter so that the potential of terminal b of the input moves in sympathy with the output signal. When an active device is used as a switch the bottomed condition is known as the on state.
See operating point. These cause changes in brightness or vertical positioning in reproduced pictures lasting longer than a field period. See ringing. For example ac, ab and bd are three branches of the network of Figure B. See node. Input 1 output Figure B22 Part of a network: A breadboard enables the performance of a new equipment to be checked. The network is shown in Figure B. The combination is extensively used for full-wave rectification. The rectified output has an open-circuit voltage across the reservoir capacitor approximately equal to the peak altemating input to the bridge circuit.
For some applications, e. The objective measured brightness is more properly called luminance.
The brightness control of an oscilloscope or TV receiver determines the grid bias applied to the cathode ray tube or picture tube and hence the light output from the screen.
In practice the switches are bipolar or field-effect transistors and by closing them appropriately by clocked or timed pulses, charge can be transferred from one capacitor to another and so move along the line of capacitors.
It is common practice to use emitter followers as buffer stages. Such networks are used in telephony to permit accurate matching to a line which is incorrectly terminated at its receiving end.
At one time this would have involved banks of tape readers, but today is more likely to be an array of hard disk drives. Subscribers may also leave messages on the bulletin board for other subscribers to read. It is indeed the electronic equivalent of an office notice board. See Applegate diagram, drift space, velocity modulation.
The theory is that, if the equipment survives the bum-in test, it will survive the less arduous conditions it is likely to encounter in normal service.
A bus is required to carry a 40 number of different types of information, e. Some success has been achieved in designing standard buses for inter-board connections within a computer system, and for interconnections between various manufacturers' equipment. An example is the capacitor which is connected across the resistor in a cathode bias circuit which is included to provide a low-reactance path for all signal-frequency components in the cathode current.
The resistor then carries only the DC component and a steady voltage is therefore set up across the cathode resistor.
See automatic cathode bias, byte A sequence of consecutive binary digits, shorter than a word, which is treated as a unit in computers and data'processing equipment. It is generally used for a specific purpose such as synchronising or addressing. It commonly contains 8 bits and is the smallest unit of data that can be put into or removed from an 8-bit store.
See binary word, address. Caches are used in disk read and write operations, in storing character bitmaps and, above all, in central processing unit operations. It replaces the cine camera formerly used for these purposes and has the advantages over them that no film processing is required so that instant playback is available and that the medium is reusable. The various types of tube that are used in black-andwhite or colour TV can be classified according to the type of target used i.
The earliest tubes such as the iconoscope and image iconoscope had photo-emissive targets and high-velocity scanning beams. The optical image and the scanning beam were projected on the same face of the target and the beam was arranged to scan the target obliquely so that the electron gun did not impede the light path.
The chief disadvantages of these tubes low sensitivity and the presence of strong shading signals in the tube output stemmed from secondary electrons released from the target as a result of bombardment by the high-velocity scanning beam.
To avoid these difficulties the beam velocity was reduced to a value at which no secondary emission occurred and in this way the orthicon and image-orthicon tubes were developed. The low-velocity scanning beam lands on the target and drives its potential negative until it reaches electrongun cathode potential. When the target potential has been thus stabilised, the scanning beam is slowed to zero velocity as it approaches the target. One of the chief problems with low-velocity tubes is that of focusing and deflecting such a beam.
This was solved by use of orthogonal scanning and a long magnetic lens: A combined transparent signal plate and target electrode was developed which enabled the optical image to be projected on one face whilst the rear face was scanned by the beam. Orthicon tubes were first used in TV in They were free of shading signals but lacked sensitivity and the target stabilisation could be lost on receipt of a large light input.
This is a highly-sensitive tube and was extensively used in black-and-white TV services for many years. It is, however, too bulky for use in colour cameras where three or four tubes are necessary. Instead, therefore, attention was directed towards photoconductive tubes vidicons which could be made very simple and compact.
In vidicon tubes, the target consists of a transparent signal plate on which is deposited a layer of photo-conductive material. The scanned face of the target is stabilised at electron-gun cathode potential as in any low-velocity tube and the signal plate is given a small positive bias. Current thus flows longitudinally through the target thickness and the magnitude of this current at any point on the target depends on the target resistance which in turn depends on the illumination of the target at that point.
Early vidicon tubes suffered from lag, i. Most modern colour television cameras incorporate three or four vidicon tubes. J A capacitance potentiometer using two capacitors 43 tapping points from which a desired value of voltage can be taken. In the simplest example there are only two capacitors as shown in Figure C.
The coupling capacitor may be a component connected in series with the circuits as in Figure C. In Figure C. The capacitance is proportional to the area of the plates and to the dielectric constant of the insulator and is inversely proportional to the thickness of the dielectric. There are many different types of capacitor varying in capacitance between a few pF and thousands of pF and varying in voltage rating from a few volts to several kilovolts.
The plates are often of aluminium and the most commonly-used dielectrics are impregnated paper, mica, plastics, ceramics, air and vacuum. The graphical symbol for a capacitor is given in Figure C. See electrolytic capacitor. Figure C. Dynamic RAMs are of this type. It is measured in decibels, the figure representing the ratio of the closest signal strengths that can be resolved. Such a diagram is obtained by combining a circular diagram with a figure-of-eight diagram as shown in Figure C.
A cardioid polar diagram is useful in radio direction finding and is obtained by combining the omnidirectional response of a vertical antenna with the figure-of-eight reponse of a loop antenna. This type of response can be obtained from a microphone which combines the omnidirectional pressure mode of operation with the figure-of-eight pressure-gradient mode of operation.
See pressure-gradient microphone, pressure microphone. Reading was by electromagnetic feelers or photoelectric scanning. The caret is positioned between characters, unlike a conventional rectangular cursor which is positioned directly over fixed-pitch characters. More majority carriers move towards the junction than are needed to supply the external current: This is permissible because the information to be transmitted is carried in the sidebands.
At the receiving end the carrier must be re-introduced to make detection possible and a suitable signal is generated locally for this purpose. The term is also used to describe the carrier-frequency component of a modulated wave. See amplitude modulation, angle modulation.
The arrangement avoids instability by feedback via the collector-base capacitance in the common-emitter stage and can thus be used for stable VHF and UHF amplification. The two transistors are often connected in series across the supply as shown in Figure C.
The circuit arrangement can be used with electron tubes and with field'Cffect transistors: The container protects the media against light, grease and dust and also makes it easier to handle. Audio compact cassettes and video cassettes in various formats are probably the best known examples. See velocity modulation. In electron tubes the source of electrons for the principal electron stream. In a glow-discharge tube the cathode may be cold and electrons are then liberated by bombardment of the cathode by heavy positive ions.
In photocells electrons are liberated from the cathode under the stimulus of incident light. In most electron tubes, however, the cathode is heated to liberate electrons and it may take the form of a filament directly heated by the passage of an electric current through it or the cathode may be indirectly heated.
See cold cathode, glow discharge, indirectly-heated cathode. The advantage of the circuit lies in its high input resistance and low output resistance which make it suitable as a buffer stage.
There are corresponding emitter follower and source follower circuits. The records are generally obtained by photographing the screen of a cathode ray oscilloscope and the provision for attaching a camera to the cathode ray tube is the only distinction between an oscillograph and an oscilloscope. For examining periodic waveforms the electron beam is deflected horizontally i. By adjusting the time-base frequency to an exact submultiple of the fundamental frequency of the waveform a stationary pattern 47 can be produced on the screen and detailed observation of the shape of the waveform is possible.
To help in obtaining a stationary pattern there is provision for synchronising the time-base generator to the waveform. The oscilloscope is one of the most widely used tools in electronics. In addition to its obvious application in examining waveforms to detect harmonics or the presence of spurious signals it can be used in conjunction with a wobbulator for the alignment of receivers. Oscilloscopes are also widely used as indicators in radars. They are also extensively employed in the examination of any physical quantity e.
See double-beam oscilloscope y oscillograph.
Such tubes are extensively used in oscilloscopes, in radars, and as picture tubes in TV receivers. Essentially the tubes comprise an electron gun for generating the beam, a system either electrostatic or magnetic for focusing the beam on the screen, a system also either electrostatic or magnetic for deflecting the beam, and finally a screen coated with a phosphor which emits light when struck by the beam. K, thermionic cathode; G, control grid; Ai, focusing anode; A: A small electron tube incorporating a triode amplifier and a simple cathode ray tube in which the area of luminescence on the screen can be controlled by the signal applied to the triode control grid.
It was chiefly used as a tuning indicator in radio receivers the area of the display being controlled from the AGC line. The graphical symbol for the device is given in Figure C.
See anion. TTiis was used for AM detection in the early days of broadcasting in the s. The electron stream can be made to circulate around the cathode within the space between cathode and anode, and can thus become velocity modulated as a result of passing close to the mouths of the resonators.
The resultant bunching of the beam can, by suitable choice of electric and magnetic field strengths give rise to oscillation at the resonance frequency of the cavities. The resultant energy can be directed into a waveguide by means of a coupling loop in one of the resonators. In this way microwave oscillations of considerable power can be generated. See cavity resonator, velocity modulation. At microwave frequencies such resonators replace the lumped inductance and capacitance used in tuned circuits at lower frequencies.
C battery US A battery used to supply the grid bias voltage for electron tubes. C core A magnetic core for a choke or transformer consisting of two Cshaped sections butted together to form a closed magnetic circuit. The C sections are made by winding magnetic alloy tape into a spiral and then cutting the resultant ring into two. C cores have better magnetic properties than laminated cores because the magnetic grain is parallel to the length of the tape.
C cores are also easier to assemble with choke or transformer windings. Ceefax The teletext service of the BBC. The electrodes may be immersed in a electrolyte and generate the EMF as a result of chemical action see voltaic cell.
Alternatively one of the electrodes may be light-sensitive and produce the EMF as a result of illumination by light see photocell. As a mobile user leaves the area served by one base station, the computer determines which of the surrounding base stations now offers the best transmission path and switches the user over to that base station in a transparent process.
Centronics printer standard The most widely used method of transferring data from a computer to a printer. The system uses pin plugs and connectors with eight parallel conductors carrying data, making it faster than alternative serial transmission systems.
Thus ceramic materials are extensively used in electronics in magnetic cores as ferrite , as dielectrics and as piezo-electric elements. Ceramics cannot usually be machined and are cast into shapes suitable for their application. This cyclic sequence ensures that a word cannot recur until the cycle is repeated. The channel must be wide enough to accommodate all the significant components of the signal, e. When channels are closely packed in a waveband, the spacing is made greater than the channel width to give a guard band between adjacent channels.
Television channels are identified by numbers in the UK, e. The effective width of the channel and hence its electrical resistance is controlled by the voltage applied to the gate. The term is, however, usually reserved for the curves which show how the current at an electron-tube electrode depends on its voltage or the voltage at another electrode, e.
This is also the value of impedance which, if used to terminate the line, gives an input impedance of the same value. Two types of transmission line commonly used for RF transmission are the twin-wire and coaxial cable.
For twin-wire and coaxial cables the characteristic impedance can be calculated from the dimensions of the cross-section as indicated in Figure C. RF energy can thus be transmitted along it without reflection. Examples of charge carriers are the positively- or negatively-charged ions in an electrolyte and the electrons and holes in a semiconductor. Such a device can be used for data storage. If the gate electrodes are thin, parallel and equidistant conducting strips then, by applying suitablyphased clock pulses to the strips, any charges placed on the semiconductor surface under the first gate are transferred along the surface of the device as in a shift register.
The method of manufacture of such devices has much in common with that of IGFETs but is simpler in that no contacts or diffusions are required. The devices can be manufactured with very small dimensions so that great packing density is possible. They can thus be used as serial stores and can compete with magnetic disks and tape: See volatile store. Storage tubes used in VDUs also use similar principles. There are two main types: Chebyshev response A response curve in which the deviations from the ideal curve are made as small as possible and approximately equal to each other over the frequency band of interest.
The parameters of filters are often chosen to give a Chebyshev response. The chip carrier is an alternative to the DIP for a large number of contact pins and gives considerable saving in space because of its smaller contact spacing. It also gives a better thermal performance and higher speed of operation because the contacts are shorter than the pins of a DIP.
Chireix system An amplifying system used in AM transmitters in which two constant-voltage phase-modulated signals drive two saturated amplifiers feeding a common load. The power output is a function of the phase difference between the two signals and this is controlled by the modulating signal. The system was developed in France by H Chireix in the s. See ampliphase system. For example an RF choke may be used at the input of an audio-frequency amplifier to prevent RF signals entering the amplifier.
In such an application the choke is likely to be followed by a shunt capacitance and for successful results the reactance of the choke must be large compared with that of the capacitance so as to give great attenuation at RF. This can be achieved by electrical or mechanical means and is used to transform a direct-current signal into an alternating signal.
DC amplification is often achieved in this manner, the chopped signal being amplified in conventional capacitance-coupled amplifiers and then reconverted into DC form. The defect is usually eliminated by constructing lenses of two different materials, e. Such combinations are known as achromatic pairs. In the NTSC and the PAL systems the chrominance signal is generated by so modulating the chrominance subcarrier that its instantaneous phase relative to that of the colour burst represents hue and its amplitude represents saturation.
Zo is the characteristic impedance of the transmission line. Circuit 1 A path consisting of a conductor or a system of conductors through which an electric current can flow.
For example an amplifier may be said to have a circuit. Vertically- or horizontally-polarised radio waves can become circularly-polarised after refraction in the ionosphere. Circular polarisation is used at some FM broadcasting stations to ensure that the signal can be received equally well on vertical and horizontal antennas. Clamps are used in television transmission to ensure that the parts of the waveform which represent black in displayed pictures are maintained at a constant potential.
To enable a clamp to operate at the desired instants it is driven by clamping pulses which are synchronised with the waveform to be clamped. Clapp oscillator A Colpitis oscillator in which frequency stability is improved by using separate capacitors for tuning and for providing capacitive feedback.
A circuit is shown in Figure CIO where Ci is the tuning capacitor and Ci and C3 are high-value capacitors which provide the connections to the active device and swamp any variations in the input capacitance of the device. This mode of operation is used in most small-signal stages of analogue equipment such as amplifiers and receivers, a linear part of the input-output characteristic being chosen to minimise waveform distortion as shown in Figure C.
It is an inefficient mode of operation because the mean current of the active device is independent of the amplitude of the input signal. In AF amplification where signal amplitudes are constantly varying the practical efficiency is unlikely to exceed half this figure.
For electron tube amplifiers the suffix I is added after the letters AB to indicate that grid current does not flow at any period during the input-signal cycle and the suffix 2 is added if grid current is allowed to flow.
The output signal is, of course, a reproduction of only one half of each cycle of input signal but linear amplification is possible by using two class-B stages in push-pull operation and most of the output stages in transistor amplifiers and receivers are of this type. The chief source of inefficiency is the no-signal or quiescent current which must be great enough to minimise crossover distortion.
For electron tube amplifiers the suffix 1 is added after the letter B to indicate that grid current does not flow at any time during the cycle and the suffix 2 is added if grid current is allowed to flow. This mode of operation, illustrated in Figure CIS, cannot be used for linear amplification but is extensively used for RF amplification, e. For electron-tube amplifiers the suffix 1 is added after the letter C to indicate that grid current does not flow at any period during the input-signal cycle and the suffix 2 is added if grid current is allowed to flow.
Very high efficiency measured by the ratio of output power to the power taken from the supply is possible from a classD stage because the input is of constant amplitude so permitting maximum use to be made of the current and voltage swings available in the output circuit.
I3 h-Vg characteristic of an electron tube with input and output signals for class-C operation click A transient signal of very short duration. The term describes the sound of the signal when reproduced by an acoustic transducer.
In the example illustrated in Figure C. I4 Input and output waveforms illustrating clipping: Although this is a widely quoted criterion when comparing the performance of computers, it is not the only factor affecting the speed at which computers execute tasks. In some applications memory access time or hard disk access time may be the limiting factor. In broadcasting the term is used to describe a circuit used for rehearsal or recording of a programme which is not radiated at that time to listeners or viewers.
CMYK colour model See colour model. C-network A network containing a single shunt element and two series elements, one in each leg as shown in Figure C. It can be regarded as a balanced form of the L-network. J5 A C-network coaxial cable; coaxial line; concentric line A transmission line formed from two conductors, one a wire and the other a cylinder concentric with the wire, the space between them being filled with a dielectric.
Cables of this type are extensively used for carrying RF signals at frequencies above approximately 10 MHz. Because the fields are confined within the outer conductor there is negligible loss by radiation from a coaxial cable. See characteristic impedance. For example in the Morse Code information can be expressed in normal language which can be spelled out, each letter of the alphabet being represented by a unique combination of dots and dashes.
Such a code is suitable for signalling by flashing a light, sounding a buzzer or interrupting an RF carrier. For digital computers a binary coding system is generally used. It is represented by OC in Figure C. I6 and its value depends on the value of the magnetic flux density initially induced in the material.
I6 A B-H hysteresis curve: CO represents the coercive field strength or coercive force coherence The maintenance of a fixed frequency. For example a laser is regarded as a coherent oscillator.
In interrupted oscillations, coherence implies the maintenance of phase continuity so that, at the beginning of each packet of oscillations the phase has precisely the value it would have had if the interruption had not taken place. See logic gate. This definition strictly embraces photo-emissive cathodes but the term is usually reserved for the type of cathode used in some gas'filled tubes.
Electron emission from the cathode arises as a result of bombardment by heavy positive ions released by ionisation of the gas and can be improved by coating the cathode with materials of low work function.
The carriers are minority carriers in the base region but majority carriers in the collector region. The collector-base junction is reverse-biased in normal transistor operation and hence the majority carriers are swept away from the junction and towards the collector terminal. For example in a delta-array tube the triangle which includes one red, one green and one blue phosphor dot.
The most familiar colour code is that used on resistors. As shown in Figure C. I 7 four bands of colour are used. The first nearest the end of the resistor indicates the first figure of the resistance value, the next band gives the second figure and the third indicates the multiplier, i. The fourth band indicates the tolerance. The code used is as shown in Tables C.
In an older system also indicated in Figure C. A similar code is used for capacitors to indicate the capacitance in pF. At the receiver, after detection, the colour difference signals are added to the luminance signal to yield the red, green and blue colour signals required as inputs to the picture tube. Colour models are used in television and in computer graphics and desktop publishing. The three colour models most regularly used are as follows. The values of the individual primaries are sometimes expressed as percentages and sometimes as numbers between 0 and see true colour.
Thus white is ,, all primaries turned fully on and magenta is ,0, full red and full blue but no green. Hue represents the wavelength or perceived colour such as green or violet.
Saturation represents the concentration of the colour; if saturation is low the colour is pale, i. Value represents the overall level of the colour. Note that if value is 0, the colour is black irrespective of the hue or saturation figures and if value is 1 X and saturation is 0, the colour is white irrespective of the hue.
Cyan, magenta and yellow allow a wide range of colours to be simulated by overprinting. The inks are assumed to be printed on a white background and the colours are subtracted.
Thus cyan and yellow make green while magenta and yellow make the crimson sometimes called 'warm red'. Key is black and is used both to print truly black areas and to produce darker shades of other colours. Values are expressed either as pecentages or as numbers between 0 and At the transmitting end an image of the original scene is analysed into its red, green and blue components and information about these components is contained in the chrominance signal. Modem colour TV systems are compatible and the transmitted signal is basically that of a black-and-white system to which the chrominance signal is added to give information about the hue and saturation of the colours in the scene.
The chrominance information is conveyed by a colour subcarrier which, with its sidebands, is interleaved with the sidebands of the black-and-white luminance signal. A number of different subcarrier modulation systems are in current use.
Colpitts oscillator A sinusoidal oscillator in which the frequencydetermining element is a parallel LC circuit connected between input and output terminals of an active device, positive feedback being obtained by connecting a tapping on the capacitive branch of the LC circuit to the common terminal cathode, emitter or source of the active device. The 60 Figure C. In VHF oscillators the inter-electrode capacitances of the active device can be used as the series-connected capacitors providing the positive feedback.
A command normally consists of a keyword which may be followed by one or more arguments which specify how the command is to be executed. The command is interpreted and executed by the operating system or by an interactive language such as BASIC or by an application program.
Command and control equipment may integrate radio and telephone communications with computer-controlled displays to show, e. This arrangement gives a very low input resistance and a very high output 61 output input output input Figure C. Current gain is less than unity but voltage gain can be considerable. The output signal is in phase with the input signal.
The basic form of the circuit is shown in Figure C. This circuit arrangement has a high input resistance and a low output resistance. Voltage gain is approximately unity but current gain can be considerable. Because of the unity voltage gain signals applied to the base emerge with negligible loss from the emitter. For this reason the circuit is often known as an emitter follower.
The base form of the circuit is shown in Figure C. It is used where equipment requires a high input resistance or low output resistance and as a buffer stage. This form of connection gives a low input resistance and a high output resistance although the ratio is not so high as for the common-base circuit. Current gain and voltage gain can be considerable. The output signal is in antiphase with the input signal. This is the most widely used of the three basic transistor circuits in analogue equipment probably because it is capable of greater output power than the others.
An ideal differential amplifier responds only to the difference between the input signals so that there should be zero output for equal input signals. The ratio is best measured by use of alternating input signals to avoid difficulties caused by dc offset voltage. If the two inputs are of equal amplitude and are accurately in antiphase, the common-mode signal amplitude is zero. If, on the other hand, the two inputs are in phase, then their common amplitude is also that of the common-mode signal.
A perfect differential amplifier does not respond to common-mode signals but see common-mode rejection ratio. Such receivers usually have several wavebands, facilities for controlling the bandwidth of the IF amplifier, a BFO, an efficient a,gx.
The bumps are of the order of 1 pm in diameter and the face of the disc accommodates about tracks. The disc is replayed by a lowpower laser beam which is reflected back along its incident path in the absence of a bump but is scattered if it strikes a bump. Servos are used to enable the laser beam to follow the tracks accurately, to keep the beam in focus on the disc surface and to maintain constant linear speed of the disk as it passes the beam a master oscillator being used to provide a reference standard.
As a consequence of the constant track speed, the rotational speed of the disc decreases from about rpm to rpm as the laser beam moves from its innermost to its outermost position. Compact discs are used to record mono and stereo sound and the single recorded side gives a maximum playing time of about 75 minutes of stereo sound. A system of PCM is used to record the two sound signals and other signals associated with the servo systems.
Sound reproduction from compact discs is greatly superior to that from conventional long-playing disks in freedom from distortion, signal-to-noise ratio, dynamic range and stereo separation.
A further advantage of CDs over other recording media such as longplaying records and tape is that there is no physical contact between the rotating disk surface and the reading head.
There is therefore no wear on the disk. Consequently, unless it meets with some catastrophic accident, a CD should sound the same after thousands of playings as when it was new. Compact discs are also widely used for the mass distribution of computer software. Known as CD-ROMs, these discs have a capacity of over megabytes, much larger than that of floppy disks, facilitating the distribution of very large collections of software.
For major software houses it is considerably cheaper to mass produce their software on CD63 ROM than on floppy disks and this is sometimes reflected in software prices. Other uses for compact disc include the storage of photographs see PhotO'CD and video see video disk. The process is used to improve the signal-to-noise ratio of the system and to be effective, the compression must take place before the point at which noise is introduced and expansion after that point.
For example in a radio system the compressor is at the transmitter and the expander at the receiver, noise being introduced in the radio link. See compression, expansion. The logic inverter stage illustrated in Figure C. A positive signal applied to the base of an npn transistor increases the collector current but the same signal applied to the base of a pnp transistor decreases the collector current. Thus if the same signal is applied to the bases of a complementary pair of transistors, the transistors operate in push'pull without need for a phase-splitting circuit.
The alternative is a reduced instruction set computer RISC , which has a limited repertoire of commands, but is faster.
It is thus the reciprocal of stiffness. Compliance is analogous to capacitance in electrical oscillating circuits. A spring can store mechanical energy when it is compressed just as a capacitor can store electrical energy when it is charged. Compression permits the transmission of higher mean volume levels thus improving signaUto-noise ratio. Various compression algorithms are used, according to the type of data in the file.
Real-time compression and decompression processing is sometimes built into filing systems, greatly enhancing their effective capacity. See compression. Computers are sometimes called data-processing equipment. The information input must be in a form which the computer can accept and there are two principal types of computer, the analogue and the digital. See analogue computery digital computer. As an example of the interactive design possible by this means, three-dimensional representations of car bodies, building frameworks or printed-wiring-board layouts can be created and modified by the use of lightpens and mice.
The problem to be solved by the new program is defined using CASE tools which may employ a graphical user interface to indicate relationships between repository members and outside events. CASE is a much faster and more powerful technique than conventional programming for certain kinds of application.
Ultimately the central processing unit only understands a limited range of instructions—machine code or machine language—which it reads in binary form.
The earliest method of programming involved writing machine code directly. This was a cumbersome task since it required an intimate knowledge of the processor and provided no assistance in error finding. Today some applications and subroutines, especially those that involve intensive arithmetic, are still programmed in machine code because of its speed of execution.
Instead of writing the program directly in machine code, however, a programming language called an assembler is used. This uses as source code a sequence of mnemonics which represent the processor instructions; these are easier to understand than the binary numeric codes themselves.
The source code is written in a word processor or text editor and when finished is converted to executable machine code in a process called assembly. Because assembler source code inevitably is closely related to the machine code to which it is converted, an assembler is regarded as a low-level computer language.
For many applications computer programs are written in high-level languages such as C and BASIC in which the source code bears no direct relationship to the machine code that is ultimately executed. High-level languages use keywords to represent common operations such as displaying output on the VDU or searching text for a certain character sequence. They also allow the programmer to define subroutines or procedures for frequently used sequences of operations and these can be called as required from any point in the program.
In most high-level languages the completed source code must be converted to executable machine code in a process called compilation before it can be executed. Some high-level languages, however, do not require compilation. Instead the source code itself is executed in real time by a program called an interpreter. Some of the computer languages in common use are: One benefit is the comparative ease with which one set of source code can be differently compiled for use on computers having widely differing central processing units and operating systems.
English keywords and simple syntax make this one of the most widely-used languages, especially in microcomputers where it is generally interpreted rather than compiled. Algol in which the instructions are presented in the form of algorithms.
Forth used primarily for process control and scientific applications, is unusual in being a compiling language that is interactive. Forth consists of a dictionary of keywords and a compiler; applications are implemented by repeatedly defining new keywords in terms of the original or previouslydefined keywords until one keyword is defined which embraces the entire application. Because its use fosters an orderly and systematic approach to programming it is much used in education although originally developed for scientific applications.
Occam has been specially developed for use with transputers. The data may be fed in via a keyboard and may be received in alphanumeric or graphical form. For a purely-resistive AC circuit, the voltage and current are in phase and the conductance is the ratio of current to voltage. The heat sink may be a finned radiator or a prepared surface on a panel. More specifically conductivity is the reciprocal of resistivity. The conductivity of metals, for example, is better, i.
See constant-velocity recording. The term is generally applied to the Va-Vg characteristics of an electron tube for constant h which are used in assessing the performance of high-power triodes as output stages in transmitters.
Because positive i. The constancy of this product has given this type of filter its name. Both disadvantages can be overcome by the inclusion of m-derived sections or half-sections. See m-derivation. Attenuators and equalisers are usually designed as constantresistance networks so that they can be inserted in circuits without affecting resistance terminations.
In practice such a characteristic cannot be used over the whole of the audio spectrum because of the possibility of exceeding the groove spacing at low frequencies and giving inadequate signal'to-noise ratio at high frequencies. Moreover the chosen characteristic must take into account the fact that sound amplitudes are smaller at high frequencies than at low.
Practical recording characteristics tend therefore to be a compromise between constant amplitude and constant velocity. See constant-amplitude recording. The transmission and reception of such a wave establishes a link between transmitter and receiver but to enable information to be sent over the link the continuous wave carrier wave must be keyed on and off in accordance with a code e. Morse or must be modified in amplitude, phase or frequency, i.
See interrupted continuous wave, modulation. This range must be compressed before a sound progranmie can be recorded or transmitted. Methods of expansion are available to re-establish the original dynamic range.
It is determined by the amplitude of the video signal input to the picture tube and hence by the gain of the preceding amplifiers. Such characters represent instructions to perform some operation on the output device rather than printable characters. From a 69 keyboard these codes may be issued by using the Ctrl key in conjunction with alphabetic and other keys.
The control grid is usually situated very close to the cathode and is constructed in the form of a wire spiral the pitch of which can be adjusted during manufacture to give the required degree of control over the density of the electron stream. Thus the pitch of the grid determines the mutual conductance of the tube.
In a cathode ray tube the control grid sometimes known as the modulator is often in the form of a disk containing a small aperture through which the electron beam passes. An adequate clear space must be provided around the device for this method to be successful. The advantage is that the user may roam freely within the unit's range during a telephone conversation.
The handset is powered by an internal rechargeable battery which is recharged when it is plugged into the base unit; this requires a mains supply. The discharge is often accompanied by a hissing noise and becomes more marked as the radius of curvature of the conductor is reduced. Sharp points, therefore, give pronounced corona discharge and to minimise the effect the surface of the conductor must be smooth. If the input pulses are at regular intervals the device is more properly known di frequency divider.
Usually a counter is a circuit which can take up a number of different states and which changes from one state to another in a particular sequence each time an input pulse is received. The circuits may be physically separated, power being C, a b Figure C. The common component can be in series as Ci in Figure C. It is equal to the ratio of the mutual impedance resistive, inductive or capacitive between the circuits to the geometric mean of the two impedances of like kind in the two circuits.
The maximum possible value of k is unity and values used in practice are often as low as 0. The coupling coefficient for each circuit is given in the text If two tuned circuits LiCi and L2C2 are coupled by mutual inductance M as shown in Figure C.
To avoid such coupling it may be necessary to employ electric or magnetic screening or decoupling. This occurs in the tetravalent atoms of germanium and silicon which therefore behave in some respects as though they had eight electrons in the outermost shell. This is sufficient to fill the shell which accounts for the very low electrical conductivity of pure germanium and silicon.
CPS emitron tube Same as orthicon. A common cause is that the processor begins executing an endless loop of instructions to which the user has no input.
His only recourse is to reset and restart the system. Any processed data that had not been stored at the time of the crash is usually lost irretrievably. Coupled circuits giving a two-peaked response, with the peaks near to the resonance frequency, are often employed as bandpass filters in RF and IF amplifiers in a receiver. As an example of a mechanical system the friction in the moving parts of a measuring instrument is commonly adjusted to give critical damping so that the pointer takes up its final reading very quickly and without overshoot which is the start of oscillation.
The value of the critical grid voltage depends on the anode voltage, becoming less negative as the anode voltage is reduced as shown in Figure C. In this way the tetrode kink can be eliminated so improving the ability of the tube to handle large output voltage swings. In AM reception this can be caused by non-linearity in RF stages. As a result the carrier of a wanted signal becomes modulated by the unwanted modulation of another signal.
It takes the form of a discontinuity at the datum level of a sinusoidal signal as shown in Figure C. See class-B operation. Figure C29 Crossover distortion 74 crossover frequency 1 In dividing networks the frequency at which equal power is delivered to each of the adjacent frequency channels assumed correctly terminated.
It is the frequency, in fact, at which the signal crosses over from one channel to the other. In two-unit loudspeakers the crossover frequency is usually between Hz and 2 kHz. For example if the initial target potential is below V[ the secondary emission ratio is less than unity and the number of electrons striking the target exceeds those lost from it.
Thus the target potential is driven negative until it stabilises at a potential near that of the electron-gun cathode. This is the type of target stabilisation used in all low-velocity camera tubes.
If, however, the initial target potential is between Vi and Vi and if the final anode potential of the electron gun is also between these two voltages then the secondary emission ratio is greater than one and the target releases more electrons than it gains from the electron beam. If the target potential is initially below that of the final anode all the secondary electrons released are collected by the final anode and the target potential rises until it is approximately equal to that of the final anode.
If the target potential is initially above that of the final anode the retarding field between target and anode returns the secondary electrons to the target and the target potential falls until again it is approximately equal to the final anode potential.
This is the type of stabilisation which occurs in high-velocity TV camera tubes and in cathode ray tubes. An example is the induction of unwanted speech signals into one pair of conductors from neighbouring pairs in a telephone cable. See superconductivity. These were used for detection in the early days of broadcasting and successful results depended on finding and retaining sensitive points on the crystal surfaces.
The resonant properties of the crystals are used when the filter is required to accept or reject a narrow band of frequencies. The AF signal is applied to a crystal bimorph and the resulting vibrations of the bimorph are mechanically coupled to the diaphragm. Such loudspeakers generally operate over a limited frequency range at the upper end of the AF spectrum and form one unit the tweeter of a multi-unit loudspeaker.
The graphical symbol for a crystal loudspeaker is given in Figure C31 a. Figure C31 Graphical symbols for a crystal loudspeaker and b crystal microphone crystal m i c r o p h o n e A microphone which relies for its action on the piezoelectric effect.
In one type the sound waves strike a diaphragm and the resulting mechanical vibrations are coupled to a crystal bimorph. T h e voltages developed across the faces of the bimorph constitute the microphone output.
In another type of crystal microphone there is n o diaphragm and the sound waves impinge directly on a sound cell which consists o f two crystal bimorphs mounted back-to-back. The graphical symbol for a crystal microphone is given in Figure C. Such oscillators have great stability and are used as frequency standards, in signal generators and as master oscillators in radio transmitters. By maintaining it at a constant temperature, a crystal oscillator can give very high frequency stability.
Such ovens have been used in broadcasting transmitting stations to achieve the required stability of carrier frequency. The movement of the stylus point caused by the groove modulation is coupled to a crystal bimorph and the voltage developed between its opposite faces constitutes the pickup output.
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