Simrad ES80 - Page 472

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406273/C
Theprincipleisshownintheillustration.Curve(A)has
RampingsettoFast,andthelevelisincreasedfrom0
Vtomaximumlevelusingfromminimumtwo(2)up
tomaximumten(10)cycles.Attheendofthepulse,
maximumtendowntominimumtwocyclesareused
toreducetheoutputlevel.
Thenumberofcyclesuseddependontheq-factor
(bandwidthrelativetocentrefrequency)forthe
connectedtransducer.Thenumberoframpingcycles
willbeupwardlimitedtothenumberofcyclesinhalf
apulselength.
Curve(B)hasRampingsettoSlow.Theoutputlevelisincreasedfrom0Vtomaximum
levelusingthersthalfthepulseduration.Thesecondhalfofthepulseisthenusedto
reducetheoutputlevel.
Advantages
•Fastrampingnormallymeansthatonlytherstfewcyclesofthetransmitsignalare
usedtorampupthesignalamplitudeandthelastfewcyclesareusedtorampdown
theamplitude.
Forshortpulses,therampingmightconvergetoSlowramping.
TheadvantagewithFastrampingisthatthetransmittedpulsecontainsmoreenergy
relativetothepulselength.Thisincreasesthesignal/noiseratio.Whenyouapply
"chirp",thetransmissionfrequencychangesfromalowfrequencyatthebeginningof
thepulsetoahighfrequencyattheendofthepulse.Thecombinationofchirpand
Fastrampingwillsignicantlyincreasethesignal/noiseratio.
•Slowrampingmeansthatthersthalfpartofthetransmitsignalisusedtorampup
theamplitudeandthelasthalfpartisusedtorampdowntheamplitude.
TheadvantagewithSlowrampingisthatthepulsehaveasmoothstartandstop.This
signicantlyreducesthetransientsintothematchedlterandthereforeyouget
reducedsidelobesintime(range)atacostofslightlyreducedrangeresolutionatthe
outputofthematchedlterforFMpulses.
Thesidelobesintime(range)willthusbelowerwithSlowrampingthanwithFast
ramping.Smallersidelobesintime(range)willreduce"crosstalk",asastrongecho
fromonerangewillnotcontaminateweakechoesfromotherranges.Increasedecho
discriminationisparticularlyusefulwithechoesofdifferentstrength.
Araisedcosinewindowisusedasrampingfunction.
RampingwhenusingFMtransmission
•Fastrampingwillresultinmaximalreceivedsignalpowerforachosenalonger
effectivepulsedurationandtransmitpower,andawidereffectivefrequency
bandwidththanSlowramping.
Fastrampingisnormallyusedtoincreasefrequencybandwidth,rangeresolution,
andsignal-to-noiseratio.
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