Issue link: https://iconnect007.uberflip.com/i/539281
42 The PCB Design Magazine • July 2015 dB reduction of emissions. This is enough, in this case, to get the board past the FCC Class B, electromagnetic compliancy. There are four constraints to keep in mind: 1. Keep the mark-to-space ratio of the wave- form equal, as this eliminates all the even harmonics. 2. Route high-speed signals out from the center of the board, where possible, as any radiation will be in the opposite direction and will tend to cancel. 3. Route high-speed signals between the planes, fanout out close to the driver (200 mils) dropping to an inner plane and route back up to the load again with a short fanout. 4. Use the same reference plane for the re- turn signals as this reduces the loop area and hence radiation. Figure 7 illustrates a six-layer stackup using Isola 370HR 2GHz material which is a common- ly used product. This stackup configuration pro- vides many advantages: 1. 1080 glass style prepreg is used for the microstrip outer layers. With just 2.8 mil thickness, this material provides close coupling between the outer signal and ground (GND). 2. The GND plane is used for the common reference—return path—for layers 1 & 3 and VCC for layers 4 & 6. 3. The signal loop areas are small and there- fore produce less differential mode radia- tion. 4. The center is beefed-up by combining five sheets of 7628 material. This provides a total of 40 mils separation between the inner signal layers which reduces any broadside coupling that may occur. Also, routing these layers orthogonally will help reduce coupling. 5. EMI is reduced by routing the high-speed signal on layers 3 & 4 between the planes. One minor disadvantage of this (and the pre- vious four-layer) configuration is that there is not significant planar capacitance as the planes are separated by beefing-up the center core/pre- preg. Therefore, the decoupling must be care- fully selected to overcome this limitation. This is where a PDN Planner comes into play—a prelayout PDN analysis can import the stackup and evaluate the capacitance of the planes prior STACkUP PLANNING, PART 2 continues beyond design Figure 6: Comparison of radiated emissions from outer and inner layer routing.