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Design007-Sept2019

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SEPTEMBER 2019 I DESIGN007 MAGAZINE 59 low-inductance, low-impedance path between all ICs on the PCB that need to communicate. To reduce the inductance, you must also mini- mize the loop area enclosed by the current flow. The most practical way to achieve this is to use power and ground planes in a multilayer stackup. In this two-part column, I looked at the alternatives to planes, why planes are used for high-speed design and the best combina- tion for your application. 6. DDR3/4 Fly-by Topology: Termination and Routing DDR3/4 fly-by topology is similar to daisy- chain or multi-drop topology but has very short stubs to each memory device in the chain to reduce the reflections. The advantage of fly-by topology is that it supports higher-fre- quency operation and improves signal integrity and timing on heavily loaded signals. If you are employing high-frequency DDR4, then the bandwidth of the channel needs to be as high as possible. However, with today's extremely fast edge rates, the sequencing of the stubs and the end termination and the associate load can make a measurable difference in signal quality. In this column, I explored how best to route DDR3/4 fly-by topology (Figure 2). For more background reading, check out my two-part series on "PCB Design Techniques for DDR, DDR2, and DDR3" [2] . 5. Microstrip Coplanar Waveguides The classic coplanar waveguide (CPW) is formed by a microstrip conductor strip sepa- rated from a pair of ground plane pours, all on the same layer affixed to a dielectric medium. In the ideal case, the thickness of the dielectric is infinite. But in practice, it is thick enough so that electromagnetic fields die out before they get out of the substrate. A variant of the copla- nar waveguide is formed when a ground ref- erence plane is provided on the opposite side of the dielectric. This is referred to as a con- ductor-backed or grounded CPW. CPWs have been used for many years in RF and micro- wave design as they reduce radiation loss at extremely high frequencies compared to tra- ditional microstrip. As edge rates continue to rise, they are coming back in vogue. In this col- umn, I described how conformal field theory could be used to model the electromagnetic effects of microstrip coplanar waveguides. 4. The Dark Side: Return of the Signal All PCB designers should be aware of the impact of crosstalk on signal integrity. As signal traces come into close proximity of an aggres- sor signal, part of that signal is unintentionally electromagnetically coupled into the victim trace as noise. I have mentioned before that current flow is a roundtrip; the current must return back to the source to complete the loop. Figure 2: Fly-by topology for clock, address, command, and control routing.

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