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PCBD-Apr2015

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32 The PCB Design Magazine • April 2015 Also, the same PDN connections (planes) that are used to transport high-transient cur- rents are used to carry the return currents for critical signal transmission lines. If high-fre- quency switching noise exists on the planes, coupling may occur, resulting in ground bounce, bit failure or timing errors. Many fail- ures to pass electromagnetic compliancy (EMC) are due to excessive noise on the PDN coupling into external cables and radiating emissions. If you are not familiar with a PDN plot (AC impedance vs. frequency), it can be awfully daunting at first. Figure 2 shows the ICD PDN Planner with a typically 400MHz fundamental frequency and with an optimized capacitor se- lection. Please refer to my previous series PDN Planning and Capacitor Selection to understand the effects of bypass and decoupling capacitors on the PDN, as this column will focus on the plane resonance. The AC impedance (thick red curve) should be below the target impedance up to the maxi- mum bandwidth. For a 400MHz fundamental frequency, the maximum bandwidth is 2GHz to take in the 5 th harmonic. But I am frequently asked one question: Does it have to be low all the way up to 2GHz? In Figure 2, you will notice the ringing in the top right corner of the plot. This is the plane resonance. As the frequency approaches half wavelength, the planes (power and ground) act as an unterminated transmission line and start to resonate. This resonance is not a problem unless it falls on the fundamental frequency or one of the odd harmonics. A Fourier series ex- pansion of a square wave is made up of a sum of odd harmonics. If the waveform has an even mark-to-space ratio then the even harmonics cancel. Figure 3 illustrates the typical electromag- netic (EM) radiation spectrum analyzer plot for a 400MHz DDR2 data signal. The fundamen- tal frequency generally has little radiation, but then increases up to the 5 th harmonic and re- beyond design LEARNING THE CURvE continues Figure 2: PDN with optimized decoupling.

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