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34 DESIGN007 MAGAZINE I APRIL 2021 Feature Article by Dan Beeker NXP SEMICONDUCTORS We are living in an age where the demands on electronic product designs are constantly evolving. e IC technology and operating speeds continue to pose significant challenges for teams as they work to develop their prod- ucts. e increased transistor switching speeds and less forgiving compliance standards make signal integrity and electro-magnetic compli- ance more difficult to achieve. e status quo seems to have become, "We expect to fail EMC testing." What can be done to increase the likelihood of compliance, and proper function? In some cases, the engineering community looks to simulation as a method of evaluating the PCB design. ere are certainly a number of power- ful tools available, but they are usually expen- sive and difficult to use properly. ese tools are only of value if the transmission lines are not broken. e simulated results oen differ from the measured behaviors, forcing model tweaks and lost time. In most cases, you have an incomplete model and inaccurate measure- ments, which even when reconciled, do not reflect the real behaviors of the design. It is nearly impossible to get good measurements for this purpose. e probes will affect the sig- nal, as does the location in the transmission line where the measurement is taken. Many teams just do not have the bandwidth or expertise to achieve success using this process. e fundamental issue is that most simula- tion tools are not capable of evaluating bro- ken transmission lines. (I say "most" because I am not intimately familiar with all of them.) A broken transmission line is a signal or power conductor that is not one dielectric away from ground. If a signal or power conductor is not directly adjacent to a continuous dielectric bounded by a continuous ground conductor, the EM fields do not stay where the design requires. e field will fill the space between Alternatives to Simulation Figure 1: If the return plane is split, the field must fill the space between the signal trace and the ground copper. (Image courtesy of Rick Hartley, consultant)

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