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PCBD-Oct2014

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26 The PCB Design Magazine • October 2014 as a monopole or loop antenna. Unfortunately, the high-frequency components of the funda- mental radiate more readily because their short- er wavelengths are comparable to trace lengths (particularly stubs), which act as antennas. Consequently, although the amplitude of the harmonic frequency components decrease as the frequency increase, the radiated frequency varies depending on the antennas/traces char- acteristics. Computer-based products tend to radiate on the odd harmonics. High emissions are generally detected at the 3 rd , 5 th and sometimes the 7 th harmonic of the fundamental clock fre- quency. If this also occurs where the AC im- pedance of the power distribution network is high, then the radiation is even higher. So at what speed should there be concern about wave propagation rather than just the current in conductors? beyond design SIGNAL INTEGRITY, PART 1 OF 3 continues Figure 2: radiated emissions from the 10ns edge rate (left) and 1ns (right). Figure 1: Edge rate changes over the past 25 years.

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