AT LEAST FOUR significant problems contribute to eye collapse and an increased bit error rate in high-speed serial links. Our previous two columns addressed how reflections and losses affect ISI, collapse of the eye, increased deterministic jitter and higher bit error rates. This month we look at how channel-to-channel crosstalk contributes to ISI and collapse of the eye. Crosstalk from one channel to another can range from as high as 20% in some cases to as low as 0.01% in other cases. And just how much is too much, of course, depends on the received signal strength and how much noise margin is needed at the receiver. In an extreme case, the received signal may be as low as -30 dB from the transmitted signal and still acceptable. If a signal-to-noise ratio of at least 10 dB is desired, then the crosstalk should be less than -40 dB. This is less than 1%.
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