Total jitter (TJ) is defined by many high-speed, serial-data standards to ensure relatively low bit error rates (BERs) in the presence of timing misalignment. As the date rates are increased, this parameter becomes more critical. For a high-speed serial data link operating at rates beyond 2.5 Gb/s, the required error rate cannot exceed 10~(-12). Timing misalignment, or jitter, contributes more to the overall BER than any other effect in a typical high-speed serial link and is caused by limited-channel bandwidth, system noise, phase noise, crosstalk, and power supply leakage -- among other things (see sidebar, Taxonomy of Jitter). Total jitter, the sum of all the timing errors of the signal, can be measured with either an oscilloscope or bit error rate tester (BERT) by observing the timing error of the crossover point or the BER.
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