The eye pattern for high speed system/circuit gives important information. An eye pattern is obtained when a high speed circuit/system outputs a long psudo-random-bit- sequence (PRBS). A sampling oscilloscope is used to observe the output such that the scope is triggered to sample on every fourth or eighth clock cycle, and every sample point is plotted on the screen. Since the signal is PRBS, the scope is triggered to "look" at every fourth cycle, and every sample point is plotted on the screen, the picture one obtains is a superposition of ones and zeros output. The horizontal width of the lines gives the jitter (phase noise) and the rise and fall times of the data pulses can be measured from the "crossings".
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