materi ppt asddst ( noise )

8/18/2019 Materi Ppt asdDst ( Noise )

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NOISE

Noise may be defned, in electrical terms, as any unwantedintroduction o energy

tending to interfere with the proper reception and reproduction oftransmitted signals.• Many disturbances of an electrical nature produce noise inreceivers, modifying the signal in an unwanted manner. In radio receivers, noise mayproduce hiss in the loudspeaker output.

• In television receivers snow, or confetti !colored snow"becomes superimposed on# the pictute.

• In pulse communications systems, noise may produce unwanted pulses or perhaps cancel out the wanted ones.It may cause

serious mathematical errors.

• Noise can limit the range of systems, for a given transmitted power. It a$ects the sensitivity of receivers, by placing a limit on


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&. E'(E)N*+ NOISE

(he various forms of noise created outside the receiver come underthe heading of

eternal noise and include atmospheric and etraterrestrial noise andindustrial noise.Atmospheric Noise• (he ma-ority of these radio waves come from natural sources ofdisturbance. (hey represent atmospheric noise, generally called static. Static iscaused by

lightning discharges in thunderstorms and other natural electricdisturbances

occurring in the atmosphere.• *tmospheric noise consists of spurious radio signals withcomponents distributed

over a wide range of freuencies.• *tmospheric noise becomes less severe at freuencies above about/0 M12 because of two separate factors. 3irst, the higher freuencies arelimited to line4of

sight propagation, i.e., less than 50 kilometers or so. Second, the nature of the mechanism generating this noise is such


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&. E'(E)N*+ NOISEExtraterrestrial Noise1. Solar noise

•. (he sun radiates so many things our way that we should not betoo surprised to %nd that noise is noticeable among them, again thereare two types.•. 7nder normal uiet conditions, there is a constant noise

radiation from the sun, simply because it is a large body at a veryhigh temperature !over 8OOO9:on the surface". It therefore radiatesover a very broad freuency spectrum which includes the freuencieswe use for communications.•. 1owever, the sun is a constantly changing star whichundergoes cycles of peak activity from which electrical disturbances

erupt, such ascorona ;ares and sunspots.•. (he solar cycle disturbances repeat themselves approimatelyevery II years.

2. Cosmic noise•. Since distant stars are also suns and have hi h tem eratures


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&. E'(E)N*+ NOISEExtraterrestrial NoiseSummary

Space noise is observable at freuencies in the range from about 5M12 tosomewhat above &.


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&. E'(E)N*+ NOISEIndustrial Noise• @etween the freuencies of I to 800 M12 !in urban, suburban and

other industrialareas" the intensity of noise made by humans easily outstrips thatcreated by any other source, internal or eternal to the receiver,7nder this heading, sources such as automobile and aircraft ignition,electric motors and switching euipment, leakage fromhigh4voltage

lines and a multitude of other heavy electric machines are allincluded.

• 3luorescent lights are another powerful source of such noise andtherefore should not be used where sensitive receiver reception ortesting is being conducted. (he noise is produced by the arc

discharge present in all these operations, and under thesecircumstances it is not surprising that this noise should be mostintense in industrial and densely populated areas.

• (he nature of industrial noise is so variable that it is diAcult to

analy2e it onan basis other than the statistical. It does however obe the


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>. IN(E)N*+ NOISE7nder the heading of internal noise, we discuss noise created by anyof the active or

passive devices found in receivers. Such noise is generally random,impossible to treaton an individual voltage basis, but easy to observe and describestatistically. !andom noise power is proportional to the bandwidthover which it is measured.


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>. IN(E)N*+ NOISEThermal-Agitation Noise (he noise generated in a resistance or the resistive component is

random and is referred to as thermal, agitation, white or "ohnsonnoise. #t is due to the rapid and..random motion of the molecules!atoms and electrons" inside the component itself. Inthermodynamics, kinetic theory shows that the temperature of aparticle is a

way of epressing its internal kinetic energy. (hus the temperatureof a body is thestatistical root mean suare !rms" value of the velocity of motion ofthe particles in thebody. It becomes apparent that the noise generated by a resistor ispropoftional to its absolute temperature, in addition to being

proportional to the bandwidth over which the noise is to bemeasured. (herefore


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>. IN(E)N*+ NOISEThermal-Agitation Noise7sing Euation !>4&", the euivalent circuit of a resistor as a noise

generatormay be drawn as in 3igure >4&, and from this the resistor#s euivalentnoise voltage $nmay be calculated. *ssume that !% is noiseless and is receiving themaimum noise

power generated by !& under these conditions o maimum powertranser, !% must beeual to !. 'hen


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>. IN(E)N*+ NOISEThermal-Agitation Noise


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>. IN(E)N*+ NOISEShot Noise (hermal agitation is by no means the only source of noise in

receivers. (he mostimportant of all the other sources is the shot eect, which leads toshot noise in allamplifying devices and virtually all active devices. #t is caused byrandom variations in

the arrival o electrons (or holes) at the output electrode o anampliying device anappears as a randomly varying noise current superimposed on theoutput. Bhenampli%ed, it is supposed to sound as though a shower of lead shotwere falling on a metal sheet. 1ence the name shot noise.

3or a diode, the formula is eactly


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>. IN(E)N*+ NOISETransit- Time NoiseIf the time taken by an electron to travel from the emitter to the

collector of a transistor becomes signi%cant to the period of the signalbeing ampli%ed, i.e., at freuencies in the upper 613 range andbeyond, the so4called transit-time eect takes place, and the noiseinput admittance of the transistor increases.

Miscellaneous Noise• lic!er *t low audio freuencies, a poorly understood form of noisecalled icker or modulation noise is ound in transistors. #t is proportional to emittercurrent and *unction temperature, but since it is inversely proportional to requency, it may be completely ignored above about

?00 12. It is no longer very serious.• "esistance (hermal noise, sometimes called resistance noise, italso present in transistors. It is due to the base, emitter, and collectorinternal resistances, and in most circumstances the base resistancemakes the largest contribution.

• Noise in mixers Miers !nonlinear amplifying circuits" are muchnoisier than am li%


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/. NOISE :*+:7+*(IONAddition o# Noise due to Se$eral Sources


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materi ppt asddst ( noise )

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