标签: eye mask

a. Eye diagram      Eye diagram is an oscilloscope display. It’s generated when the digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizongtal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails.      Eye diagram is an important method to evaluate the quality of a lane, it can show out the impact of ISI and noise. Fig 1. eye diagram  b. Eye mask      Some oscilloscopes provide a feature to compare the measured data to an “eye mask.” An eye mask is a specifica-tion for allowed eye opening.      The advantage of an eye mask is that it tests for amplitude as well as timing compliance. The general physical media transmitter pulse shape characteristics are specified in the form of a mask of the transmitter eye diagram at any of the compliance measurement points. These characteristics include rise time, fall time, pulse overshoot, pulse undershoot, and ringing, all of which prevent excessive degradation of the receiver sensitivity. For the purpose of an assessment of the transmit signal, it is important to not only consider the eye opening, but also the overshoot and undershoot limitations. The parameters specifying the mask of the transmitter diagram (eye mask) can be found in the clause of the applicable physical layer specification whether it be copper or fiber physical media. The eye mask, through its use of a specified time range in which the transmit signal can change state from the logic low to logic high levels, is also specifying a measure of the allowed jitter.         It is strongly advised not to use the eye mask to verify that total jitter is within specification because of the nature of the test. a. 眼图 眼图是示波器测试得到的图形。使示波器水平扫描周期与接收码元的周期同步，由于示波器的余晖作用，示波器扫描所得的每个码元波形叠加在一起，这时示波器上看到的图形像人的眼睛，故称为眼图。 眼图是评估链路质量的重要依据，可以反应链路受码间干扰和噪声影响的程度。 b.眼图模板 很多通用链路都规定了它们的眼图的模板，它可以看作是眼图要达到的标准。 有些示波器具有将测试的得到的眼图同 ” 眼图模板 ” 比较的功能。眼图模块是对眼张开度的定义。眼图模板的好处是它对时序兼容性和幅度都作了定义。发送眼图模板定义了通用介质驱动器脉冲形状的特征。这些特征包括上升时间、下降时间、上下过冲、振铃，这些都对接收器的灵敏度提出更高的要求。若是以评估发送信号为目的，不能只考虑眼图张开度，上下过冲的限制也很重要。铜线或光纤的物理介质眼图模板参数都可以在 802.3 协议的物理层中找到定义。眼图不仅定义了上升时间范围，它也定义了允许的抖动。因为测不准的自然规律，强烈建议不要用眼图模板来证明总的抖动在允许范围内。