The PCB Design Magazine

PCBD-July2017

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74 The PCB Design Magazine • July 2017 As digital systems evolve and demand for new technology pushes the envelope for smaller and faster systems, transmission line losses, pre- viously considered to be negligible, are becom- ing a primary design concern. Pragmatic effects such as frequency-dependent losses come into play at clock frequencies above 1 GHz and are of particular concern for fast rise time signals, with long trace lengths, such as multigigabit serial links. This frequency dependence causes rise time degradation and reduces the upper bandwidth of the signal resulting in reduced channel data transfer. In this month's column, I will look at the impact of transmission line losses on signal integrity. In an ideal world, where transmission line losses are independent of frequency, the en- tire signal waveform would uniformly decrease in amplitude, over distance, and the rise time would remain constant. This reduction in am- plitude could easily be compensated for by ap- plying gain (cranking up the volume) at the re- ceiver. However, in practice, as signals propagate along a lossy transmission line, the amplitude of the high-frequency components is reduced, in magnitude, whereas the low-frequency com- ponents are unaffected. This selective attenua- tion, of high-frequency components, is the root cause of intersymbol interference (ISI) and col- lapse of the signal eye as in Figure 1. by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Transmission Line Losses BEYOND DESIGN Figure 1: Lossy transmission line with collapsing eye (simulated in HyperLynx).

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