u-blox ANTARIS 4 User Manual

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1.6.3 Shielding
If employing the countermeasures listed in Section 1.6.2 cannot solve EMI problems, the solution may be
shielding of the noise source. In the real world, shields arenot perfect. The shielding effectiveness you can
expectfroma solidmetal shieldis somewherein theorderof30-40dB.Ifa  thinPCB
copperlayer isusedasa
shield,thesevaluescanbeevenlower.Perforationoftheshieldwillalsoloweritseffectiveness.
Beawareofthenegativeeffectsthatholesintheshieldcanhaveonshieldingeffectiveness.Lengthyslotsmight
eventurnashieldintoaradiatingslotantenna.
Therefore,apropershieldhastobetightlyclosedandverywell
connectedtothecircuitboard.
1.6.3.1 Feed through Capacitors
Thebasicconceptofshieldingisthatametalboxwillterminateallelectricalfieldsonitssurface.Inpracticewe
havetheproblemthatweneedtoroutesomesignalsfrominside
tooutsideofthisbox.
Signal Layers
Supply Ground Layer (noisy)
Shield (free of supply currents)
Signal line leaving box
Feed through Capacitor
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Figure 19: Ideal shielding
The proposed setup for such a system is shown in Figure 19. A feed through capacitor removes all high
frequencycontent fromtheoutgoing signal line.It’s important tonote that anyconductor projectingthrough
theshieldingboxissubjecttopickingupnoiseinsideandre-radiatingitoutside,regardlessof
theactualsignalit
is intended to carry. Therefore, also  DC lines (e.g. the power supply) should be filtered with feed through
capacitors. When selecting feed through capacitors, it’s important to choose components with appropriate
frequency behavior.As with the ordinary capacitors, small value types will show better attenuationat high
frequencies (see Figure 20). For the GPS frequency band the 470pF capacitor is the optimum choice of the
MurataNFM21Cseries.
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
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u-blox ANTARIS 4 Specifications

General

Brand	u-blox
Model	ANTARIS 4
Category	GPS
Language	English