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April 2015 • The PCB Design Magazine 33 duces again with the higher harmonics. But I have seen cases where even the 11 th harmonic— 4.4GHz in this instance—can create problems. So what does this EM radiation have to do with the PDN analysis? If the AC impedance is high at the fundamental frequency or at any of the odd harmonics, then the board will radiate. In Figure 4, I have superimposed the EM radia- tion on the PDN plot. Look at where both the radiation and plane resonance peak. If these co- incide, then you will have excessive radiation at that particular frequency. In this case, the fun- damental 400MHz has a very low impedance so it will not be an issue. But, the 7 th harmonic is high and the 5 th is borderline. Fortunately, the amplitude diminishes as the frequency de- creases. I have pointed out a possible issue on the 5 th and 7 th harmonics in Figure 4, so how do we fix it? Decoupling capacitors are only effective below 1GHz, so no matter how many are add- ed, to the PDN, they will not reduce the 2 and 2.8GHz peaks. However, above 1GHz, there are a number of ways to reduce the AC impedance: 1. On-die capacitance. Capacitors are placed on the IC itself by the manufacturer and gen- erally cannot be changed. However, in some cases, the capacitors are on the top of the IC. It may be possible to piggyback parallel capacitors to increase their effect. 2. Reduce the loop inductance of the de- coupling capacitors. This can be achieved by moving the decaps to the top side of the board so that the fanout vias have less distance to travel to the power and ground planes in the substrate. The loop inductance can also be re- duced by using multiple vias per land and spac- ing them close to each other to reduce the loop area. But this reduced inductance has minimal effect above 1GHz. 3. Select a material with lower dielectric constant. This will push the plane resonance to a higher frequency. beyond design LEARNING THE CURvE continues Figure 3: eM radiation from a DDR2 data signal @ 400Mhz.