Design and Implementation of a Weak Signal Detecting System Based On LabWindows.docx
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Design and Implementation of a Weak Signal Detecting System Based On LabWindows.docx
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DesignandImplementationofaWeakSignalDetectingSystemBasedOnLabWindows
DesignandImplementationofaWeakSignalDetectingSystemBasedOnLabWindows
Abstract
Detectingweakperiodicsignalsiswidelyusedinradar,communication,sonar,earthquakeandindustrialmeasurement.Generally,theweaksignaldetectionneedsspecialequipments,suchasphasesensitivedetectoranddemodulationmodule,andrequirestheusertooperateandprocessthereceivingdata.Thearticledesignsandimplementsaweaksignaldetectingsystem,whichcandetecthighfrequencynear-fieldsignalautomaticallyusinguniversaltestequipmentsandwithoutusers’intervention.Furthermoreitseffectisdemonstratedbydetectingaperiodicweaksignalradiatedfromcertainelectronicequipment.Thedetectingsystemdesignedinthisarticlecandetectasignalofonlyseveralmicrovoltsinarangeoftwentycentimeters.
Keywords:
weaksignaldetection,highfrequencynear-fieldsignal,LabWindows.
I.INTRODUCTION
Weaksignaldetectionisanintegratedandsophisticatedtechnology.Itdevelopsrapidlyandgetsincreasinglyattentionfor;itbreakstheshacklesofthetraditionalconceptionthatsomeweaksignalcannotmeasure[1].
Weaksignaldetectiontechnologyisamethodwhichadoptstheelectronics、physics、informationtheoryandcomputersciencetoanalyzethecausesofnoiseandlaws,toresearchthecharacteristicsandrelativitiesofsignalsandtodetectsignals,whichareburiedinthenoise.Theaimofweaksignaldetectionistoacquirethesignalfromthestrongnoise,ortouseadvancedtechniquesandmethodstoimprovethesignal-to-noiseratio(SNR)ofthedetectingsystem.
Quicklydetectingweakperiodicsignalsiswidelyusedinradar,communication,sonar,earthquakeandindustrialmeasurement.Thecapabilityofconventionaldetectionmethods,suchasphasesensitivedetector(PSD),islimitedwhendetectinghighfrequencynear-fieldweaksignals[2].Becauseofthenear-fieldsignalwiththesquareorcubeofthedistanceattenuation,commonlyweusespecialantennaandtestequipment,suchasspectrumanalyzertomeasuredirectly,orusetheparticularreceiveranddemodulation
moduletodemodulateandanalyzethesignal.However,eitherofthesemethodsrequeststhetesterstorealizethecharacteristicofthedetectedsignalsaswellastooperatethetestequipmentexpertly.
TheLabViewiswidelyusedinvirtualinstrumentdesignanddevelopment.Butinthespecializedtestingfieldforexampleweaksignaldetectiontherehavebeennocomprehensiveapplications.
Thispaperdesignsandimplementsahighfrequencynear-fieldweaksignaldetectingsystembasedonuniversaltestequipmentsandLabWindowscontrolsoftware.Forasimpleset-up,thisdetectingsystemcandetectthehighfrequencynear-fieldsignalautomaticallyandthewholetestingprocessdispenseswithusers’intervention.Thissystemadoptsdistributeddetectingmodeincludingthreeparts.Thefront-endisacustomantennawhichisportableandeasytoinstall.Themiddle-endisaswitcharraywhichcanselectandlocatethedetectingposition,andtheback-endcontrolsthetestingequipmentsuchasspectrumanalyzerthroughaGPIB-BUSbyacomputer.ThedetectingsoftwaredevelopedundertheLabWindows(anintegrateddevelopmentenvironmentofNICompany’s)obtainsthetestingdatafromthecommontestequipmentdirectlysothatwecancarryoutsomesignalprocessingarithmeticthoughthecomputer.ThewholesystemalsoimplementsremotecontrolviaaPDAplatform,andcouldexpanddetectionrangewithnetworkinginmulti-systems.Withtheactualtesting,thisdetectingsystemcouldeffectivelydetectandlocatesometargetelectronicequipmentsthatradiateweaksignals.
Thearticleanalysesahighfrequencynear-fieldweaksignal’scharacteristicsatfirst;thengivesthestructureoftheweaksignaldetectingsystemanddesignofeachmodule;atlastusestheLabviewdevelopmentenvironmenttoimplementthesystem’sback-endanddetectsaweaksignalradiatedfromcertainelectronicequipment.
II.HIGHFREQUENCYNEAR-FIELDWEAKSIGNALDETECTION
Beforedetectingthehighfrequencynear-fieldweaksignal,weshouldknowthesignal’scharacteristic,suchastheradiationcharacteristicofthesignalandtheradiationsourceattributeofthesignalandsoon..
A.Theradiationcharacteristicofhighfrequencynear-fieldweaksignal
AnyelectronicequipmentwillbeworkingwithelectromagnetismradiationandtheintensityofradiationisrelativetothesignalfrequencyandthePCBrouting.
Anexperimentsshowthatcertainequipmentfordetectingandlocatingexistselectromagnetismradiationwhenworks,especiallythecrystal’semissionasFigure1shows.Thesignal’scommoncharacteristicsare:
•Aperiodicsignal,thefrequencyisabout20MHz;
•Thesignalintensityisweakandisaffectedbythe
spaceelectromagneticinterference.
Figure1.Electricequipmentweaksignalradiationresultclosedtothe
antenna
B.TheradiationsourceattributeofHighfrequencynear-fieldweaksignal
Takethe24MHzcrystalsignalwhichbelongtoVHFbandincommunicationasanexample,thewavelengthisabout12.5meters.Butcomparedwiththewavelength,thedistancebetweentheantennaandtheequipment,thesizeoftheantennaandthestructureoftheequipmentisaveryshortdistance.Inotherwordsthetransmissionandreceptionofthesignalismuchsmallerthanawavelength.Thearearelativetotheantennaisofnear-field.
Thetransmissionpowerofelectromagnetisminnear-fieldisnotthroughthewave’sincidentandrefractionbutcoupling[3].
Inthenear-fieldtheelectricfieldormagneticfieldsignaliswiththesquareorcubeofthedistanceattenuationandtheratioofelectricfieldandmagneticfieldisnot377ohm,butdistinguishedfromtheradiationsource:
thenear-fieldofelectricdipole’sradiationisahighfieldandthewaveimpedanceismorethan377ohm.Ontheotherhandthenear-fieldofmagneticdipole’sradiationisalowfieldandthewaveimpedanceislessthan377ohm.Thatistosayonlysettingupthesignalradiationemissionmodelcorrectlyandconfirmingthecharacteristicofthesignalcandesignaproperantennatoimprovethereceptionefficiency.
Accordingtothecircuitstructure,theantennaoftheleakageandradiationshouldbetheclockroutingsonthePCB.Theclocksourceandthetransmissionline’simpedancemismatchcausesthetinyemission.Thefigure2andfigure3showthesignalradiationintensityindifferentdistance.Throughcalculation,thesignaliswithcubethedistanceattenuation.Inotherwordstheradiationsourceisofelectricfield[4].
Figure2.signalemissionofatargetelectronicequipmentRcmfromtheantenna
Figure3.Signalemissionofatargetelectronicequipment2Rcmfromtheantenna
C.ThedifficultiesofHighfrequencynear-fieldweaksignaldetection
1)Weaknessofthedetectedsignal
Astheexperimentshows,thesystemmodelisaweaksignaldetectionunderstrongnoisebackground.Figure1showsthesignalamplitudeisonly-17dBuVinelectromagneticshieldroom.Attheeverydayenvironmentevensometinyelectromagneticinterventionwouldburythesignalinnoise.
2)Designofminireceptionantenna
Fortheportabilityandeasyinstallation,thereceptionantennashouldbeminiaturized.Sincethefrequencyofthecharacteristicsignalis24MHz,theantennasizemustbe3metersevenusing1/4wavelengthtoreceive.Thewaytogettingbalancebetweenthesizeoftheantennaandthereceptionefficiencyiscrucialofantennadesign.
III.WEAKSIGNALDETECTINGSYSTEMBASEDONLABWINDOWS
Throughtheanalysisabove,wecandepictthestructureoftheweaksignaldetectingsystemanddesigneachmoduleasfollow.
A.Systemstructure
TheweaksignaldetectingsystembasedonLabWindowsincludingareceptionantenna、aswitcharrayandaprocessingmoduleisshowninFigure4.
Figure4.Systemstructure
Thefront-endisanactiveantennawhichismainlyresponsibleforsignalreceptionandamplification.Inordertolocatethedetectedsignal,theantennamusthavegooddirectionaswellashighsensitivity.
Themiddle-endisaswitcharraywhichisresponsiblefordetectingpositionselectionsothatimplementthedistributeddetectingandlocation.
Theback-endisaprocessingmoduleconnectsuniversaltestequipmentsuchasspectrumanalyzerthroughGPIB-BUSbyacomputer,andcouldbecontrolledbytheLabWindowsplatform[5].
B.Moduledesign
1)Designofactiveantenna
Theweaksignalthatelectronicequipmentradiateisanelectricfieldsignal,sotheantennamustbedesignedforthischaracteristic.Theantennamodelisshowninfigure5.Themainideaistoenhancethecouplingabilityofelectriccapacitybyincreasingtheareaoftheantennatoimprovethesignalreceptionintensityandgeneratethesymmetryreceptioneffectofantennacentralaxis.
IntheFigure5,thegreenareaisthemicrowavedielectricboardandtheyellowareaiscopper.
Figure6showstheantennacalculationanalysismodelthatisthedistributionofthesurfaceelectricfieldandthesurfaceelectriccurrent.
Figure6.antennacalculationandanalysismodel
Figure7showstheantenna’sEandHreception
Figure7.Antennamodelreceptionsimulation
Theamplifiercircuitusesamax4017whichisalow-noiseproductofMAXIM[6].Thechip’sgainandnoiseisshownasFigure8andFigure9.Theamplifierisplacedinashieldedbox.
Figure8.Gainofmax4017
Figure9.Noiseofmax4017
2)Designofswitcharray
Theswitcharrayiscapabilityofcontrollingtheswitchchipbycomputertoselectasignaloutputfrom8inputs.Thechipmusthavehighswitchspeedandlowinsert
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