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1 Introduction

As the size of the system decreases, the operating frequency increases, and the functions of the system become complicated, which requires multiple different embedded functional modules to work simultaneously. Only the individual modules have good EMC and low EMI to ensure the realization of the entire system function. This requires that the system itself not only needs to have good performance to shield external interference, but also requires that it cannot generate serious EMI to the outside when working with other systems. In addition, switching power supplies are increasingly used in high-speed digital system design, and multiple power supplies are often required in a system. Not only is the power system susceptible to interference, but the noise generated by the power supply can cause serious EMC problems for the entire system. Therefore, in high-speed PCB design, how to better filter power supply noise is the key to ensure good power integrity. This paper analyzes the filter characteristics of the capacitor, the influence of the filter performance of the parasitic inductance and capacitance of the capacitor, and the current loop phenomenon in the PCB. Then some summary is made on how to select the bypass capacitor. This paper also analyzes the generation mechanism of power supply noise and ground bounce noise and analyzes and compares the various ways of placing bypass capacitors in the PCB.

2 Capacitor insertion loss characteristics, frequency response characteristics and capacitance filtering characteristics

2.1 Insertion loss characteristics of ideal capacitor

The ability of an EMI power filter to reject interference noise is usually measured by the Insertion Loss feature. The insertion loss is defined as the ratio of the noise power P1 transmitted from the noise source to the load and the noise power P2 transmitted by the noise source to the load when no filter is connected, expressed in dB (decibel). Figure 1 shows the insertion loss characteristics of an ideal capacitor. It can be seen that the slope of the insertion loss curve corresponding to a 1μF capacitor is close to 20dB/10.

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