Emi Mitigation Through Stitching And Decoupling Capacitors; Keep-Out Zone Determined Around Io And Other Connectors; Simple Capacitor Model And An Example Of Capacitor Impedance - Intel Quark SoC X1000 Design Manual

Electromagnetic Interference—Intel
Figure 61.

Keep-out Zone Determined Around IO and Other Connectors

17.4.4

EMI Mitigation through Stitching and Decoupling Capacitors

EMI emission can be mitigated with capacitors, which create low-impedance paths for
signals or noises to pass through. These paths could minimize current loop areas or
change plane resonant frequencies.
It is important to understand the capacitor model and its relationship with impedance.
A simple capacitor model is a series of resistor (R), inductor(L), and capacitor(C).
Therefore, the impedance of a capacitor can be expressed by the following equation.
The impedance is a function of frequency. It is lowest when the frequency is at
 
1 2 LC
the impedance is dominated by the capacitor value. Above this frequency, the
impedance is dominated by the inductor value. At this frequency, the impedance is
determined by the resistor value. The self-resonant frequencies normally are below 100
MHz. As the signal and clock speeds are ever-increasing. The concerned emissions
could reach above 100 MHz. The inductor impact is getting more critical. Choosing a
capacitor with low inductance is important.
Figure 62.

Simple Capacitor Model and an Example of Capacitor Impedance

Two main EMI mitigation methods using capacitors are introduced in the followings.
They are stitching capacitor and decoupling capacitor.
June 2014
Order Number: 330258-002US
®
Quark™ SoC X1000
Z f  
 . This frequency is call the self-resonant frequency. Below this frequency,
1
 
- - - - - - - - - -
= R + j 2fL –
 
2fC
®
Intel Quark™ SoC X1000
PDG
115