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44 The PCB Design Magazine • July 2015 to fabrication. From this, one can approximate the required decoupling and adjustments can then be made to compensate. In Figure 8, you can see that the lack of planar capacitance creates high AC impedance (0.38 ohms) at 275 MHz. In this case, more de- coupling is required around 400 MHz. It may just be a matter of exchanging the 10nF capac- itors for 1nF in order to bring the "V" curve, created by the capacitors, up in frequency. The plane area could also be adjusted to move the curve up or down in frequency in order to posi- tion the resonant frequency of the planes spot on the 400 MHz fundamental—this is a trial- and-error process that should be done before layout. Over the years of analyzing customer boards, I have seen many variations of the four and six layer configurations in order to try and achieve the most cost-effective design. But realistically, adding a couple more layers is not expensive and provides more real estate for routing which will in turn yield a high-quality product with less crosstalk, and better EMC. Next month, I will look at higher layer-count boards, particu- larly 8-layer and 10-layer stackups that are pre- ferred for high-speed applications. STACkUP PLANNING, PART 2 continues Points to Remember • A four-layer board has the advantage of using planes for the distribution of power and ground (GND) to the ICs on both sides of the board and the planes also act as the return current path for signals. • Four-layer configuration boards should be avoided (unless completely shielded) as the microstrip traces tend to radiate. • Since all stackups are symmetrical, then it is best to work on just the top half of the stackup to begin with—this halves the construction time. • Six-layer boards have a huge advantage, over four layers, in that embedding high-speed signals between the planes can reduce elec- tromagnetic radiation by up to 10dB. • One minor disadvantage of four and six layer configurations is that there is no sig- nificant planar capacitance as the planes are separate by beefing up the center core/ prepreg. • A prelayout PDN analysis can import the stackup and evaluate the capacitance of the planes prior to fabrication. Adjust- ments can then be made to the decou- pling to compensate. beyond design Figure 7: Six-layer stackup using isola 370hR 2Ghz material.