Ublox TIM-5H Hardware Integration Manual

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Ublox TIM-5H Hardware Integration Manual Manual pdf 52 page image

TIM-5H - Hardware Integration Manual AppendixGPS.G5-MS5-07015-A-1 u-blox proprietary Page 52your position is our focusC.3 ShieldingIf employing the countermeasures listed in Appendix C.2 cannot solve EMI problems, the solution may beshielding of the noise source. In the real world, shields are not perfect. The shielding effectiveness you canexpect from a solid metal shield is somewhere in the order of 30-40 dB. If a thin PCB copper layer is used as ashield, 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 mighteven turn a shield into a radiating slot antenna. Therefore, a proper shield has to be tightly closed and very wellconnected to the circuit board.C.3.1 Feed through CapacitorsThe basic concept of shielding is that a metal box will terminate all electrical fields on its surface. In practice wehave the problem that we need to route some signals from inside to outside of this box.Signal LayersSupply Ground Layer (noisy)Shield (free of supply currents)Signal line leaving boxFeed through CapacitorFigure 38: Ideal shieldingThe proposed setup for such a system is shown in Figure 38. A feed through capacitor removes all highfrequency content from the outgoing signal line. It’s important to note that any conductor projecting throughthe shielding box is subject to picking up noise inside and re-radiating it outside, regardless of the actual signal itis intended to carry. Therefore, also DC lines (e.g. the power supply) should be filtered with feed throughcapacitors. When selecting feed through capacitors, it’s important to choose components with appropriatefrequency behavior. As with the ordinary capacitors, small value types will show better attenuation at highfrequencies (see Figure 39). For the GPS/GALILEO frequency band the 470pF capacitor is the optimum choice ofthe Murata NFM21C series.