Issue link: https://iconnect007.uberflip.com/i/1359517
36 DESIGN007 MAGAZINE I APRIL 2021 the signal conductor and the ground conduc- tor that connects to the power source for the PCB. e more conductors and dielectrics that are between the signal in question and its ground, the more impact on signal integrity and EMC. ere are just not enough nodes in the models. It would require characterizing every change in the transmission line geom- etry, for not only the signal trace in question, but the other nearby traces and the power supplies for the ICs involved. Once a trace crosses a split in the ground plane, the space involved in the signal transmission expands to include the divergent path of the continuous ground conductor. is increases the time it takes to establish the signal voltage, requiring more wave cycles to achieve the signal levels. Each wave cycle increases crosstalk and pos- sible radiated emissions, which occur at every wavefront. Every other signal crossing the split will also have an expanded space to fill. ese signals will then share the space and affect the voltage for each signal. If a power supply is not properly designed, signal integrity is compro- mised for all the signals using that supply. If a signal is compromised, it affects not only that signal, but other signals in the adjacent areas of the PCB. Having a proper understanding of PC board design and how to totally contain EM fields will minimize or negate the need for simula- tion. We have to start with a different under- standing of how electronic circuits function. Containing EM fields requires three things: two conductors separated by a space. If one of these components is missing, the EM field will behave in well-defined ways. If there is only one conductor, such as a sphere, with no hole in it, any field contained inside the sphere can- not escape. If there is only one conductor and a space, the field will not be contained. If there is no space between two different conductors, such as between power vias next to each other, then there is no field in that space. Next, it is important to understand that you only get three components to use in managing the fields: conductors, spaces, and switches. ICs are just giant switch arrays. Finally, there are only three things you can do with EM fields: Store them, move them, or convert them into kinetic energy. is is not rocket science. Using this perspective is crucial in achieving signal integrity and EMC goals. It is all about the space. Empirical analysis of the PCB design can be a good first step in achieving signal integ- rity and EMC compliance. e first thing you Figure 2: Remember: If you have one conductor, such as a sphere, with no hole in it (2a), any EM field contained inside the sphere cannot escape. If there is only one conductor and a space (2b), the field will not be contained. (Images courtesy of Ralph Morrison, consultant)