u-blox ANTARIS 4 - Page 27

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Figure 20: MuRata’s NFM21C Feed Through Capacitors
Anyfeedthroughcapacitorwillonlyachieveitsspecifiedperformanceifithasapropergroundconnection.
Iftheuseofaspecialfeedthrough capacitorisnotfeasibleforaparticulardesign,a simplecapacitorbetween
thesignallineandshieldinggroundplacedveryclosetothefeedthroughof
thesignallinewillalsohelp.Ithas
beenfoundthata12pFSMDcapacitorworksquitewellattheGPSfrequencyrange.Largercapacitancevalues
willbelessefficient.
Oneshouldkeepinmindthatafeedthroughcapacitorisbasicallyahighfrequency“short”betweenthesignal
line and ground. If the ground point that the capacitor is connected to is not ideal, meaning the ground
connectionorplanehasafiniteresistance,noisewillbeinjectedintothegroundnet.Therefore,oneshouldtry
toplaceanyfeedtroughcapacitorfarawayfromthemostnoisesensitive
partsofthecircuit.Toemphasizethis
onceagain,oneshouldensureaverygoodgroundconnectionforthefeedthroughcapacitor.
Ifthereisnogoodgroundconnectionavailableat thepointofthe feedthrough,orinjectionofnoiseintothe
non-idealgroundnetmustbeavoidedtotally,
insertingacomponentwithahighresistanceathighfrequencies
might be a good alternative. Ferrite beads are the components of choice if a high DC resistance cannot be
accepted. Otherwise, for ordinary signal lines one could insert a 1K series resistor, which would then form a
low-passfiltertogether
withtheparasiticcapacitanceoftheconductortrace.
SeealsoMuRatawebpageforextensivediscussiononEMCcountermeasures.
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
  Page 27