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7.Noiseandinterferenceincommunicationssystems

Sofarinthiscourse,wehaveinvestigatedtheuseofmodulationandfrequencydivision

multiplexingasmeansoftransmittingsignals.WehaveconsideredAM,PMandFM,discussed

theirpropertiesandtheirimplementation.Also,wehavelookedatthedesignofreceiversusing

superheterodyningprinciples,andwewenttogreatcaretoavoidinterferencefrombothimage

frequenciesandadjacentchannels.Butletsberealistic:wecantexpecttocompletelyeliminate

anyundesirablesignalsandpresentthedetector(demodulator),withanideallyfilteredsingle

channel!Whatweneedtoaskourselvesisthis:obviouslywewillgetsome,albeithopefullynot

alot,ofunwantedstuffappearingattheinputtothedetectoralongwiththewantedsignal,in

whichcase,howwillthedetectorrespond?I.e.howmuchunwantedstuffcouldtherebebefore

wecanlongersensiblydetectthedesiredsignal?Also,wouldFMbeless,ormore,sensitivethan

AM.DoalltypesofAMdetectorsrespondthesameway?Anotherquestioniswheredoesthis

undesirablestuffcomefrom?

Letsstartwiththelastpoint:

1)Clearly,wemayexpectsomedegreeofinterferenceformadjacentchannelsastheIFfilterof

atypicalreceiverisnotperfect.However,wemustalsoallowforinterferencefromnon-adjacent

channelsbecauseofimageeffectsinsuperhets.

2:)Wealsooughttoconsiderinterferenceformthingslikeelectricmachinery,computerboards

andanyotherelectricalitemthatcanpotentiallygenerateRF?Ironically,eventhedifferentparts

ofourownequipmentaresourcesofinterference(cross-talk);thesignalspassingthrough

parasiticinductancesandcapacitances

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