Issue link: https://iconnect007.uberflip.com/i/1071356
46 DESIGN007 MAGAZINE I JANUARY 2019 too late. Alternatively, and more proactively, designers recognize that the problem should be addressed at the board level where the radiation emanates. Since all products must comply with strict EMC regulations, all critical high-speed signals should be simulated to determine the amount of expected radiation. The EMC standards for FCC Class B limit the radiation to a 54-dB average with a peak of 74 dB (at a 3-m distance) above 1 GHz. However, harmonics can still cause unforeseeable problems (Figure 6). Generally, these issues are caused by routing traces on the outer (microstrip) layers where radiation is much higher than that of the inner (stripline) layers. Routing critical signals between the planes can reduce the EMI by more than 10 dB. Electromagnetic radiation from differential pairs can occur as either differential or common mode. Differential mode is typically equal and opposite, and therefore, any radiating fields will To be prudent, it is best to design the PCB layout with electromagnetic compatibility in mind rather than to be faced with excessive emissions at the prototype stage, or worse still, just before production. Unfortunately, too many of the problems are uncovered at the testing stage. Shielding and absorbing materials are then often used to reduce the emissions. The preferred approach is to identify the problems at the board level and to rectify them there. Even then, if particular offending frequencies are identified, engineers are still faced with the nightmare of locating which net segments are the cause of these particular emissions. Once excessive emissions are detected, the goal is to quench these emissions. Crude techniques involve the design of sealed enclosures, chokes and ferrites to restrain the launching of common-modes along cables and a variety of other methods that are too little and Figure 6: Electromagnetic radiation from MDQS0 strobe.