LVDS Eye Diagram

[ Team LiB ]

LVDS Eye Diagram Jitter, Noise, and Signal Attenuation As the bit stream travels from the transmitter on one end of a link to the receiver on the other end, it is subject to the following disruptive influences: Deterministic (i.e., predictable) jitter induced by the Link transmission line. Data-dependent jitter induced by the dynamic data patterns on the Link. Noise induced into the signal pair. Signal attentuation due to the impedance effect of the transmission line. The Eye Test Refer to Figure 12-10 on page 472. In order to ensure that the differential receiver receives an in-specification signal, an eye test is performed. The following description of the eye diagram was provided by James Edwards from an article he authored for OE Magazine. The author of this book has added some additional comments [in brackets]. Figure 12-10. LVDS (Low-Voltage Differential Signal) Transmitter Eye Diagram "The most common time domain measurement for a transmission system is the eye diagram. The eye diagram is a plot of data points repetitively sampled from a pseudo-random bit sequence and displayed by an oscilloscope. The time window of observation is two data periods wide. For a [PCI Express link running at 2.5Gbits/s], the period is 400ps, and the time window is set to 800ps. The oscilloscope sweep is triggered by every data clock pulse. An eye diagram allows the user to observe system performance on a single plot. To observe every possible data combination, the oscilloscope must operate like a multiple-exposure camera. The digital oscilloscope's display persistence is set to infinite. With each clock trigger, a new waveform is measured and overlaid upon all previous measured waveforms. To enhance the interpretation of the composite image, digital oscilloscopes can assign different colors to convey information on the number of occurrences of the waveforms that occupy the same pixel on the display, a process known as color-grading. Modern digital sampling oscilloscopes include the ability to make a large number of automated measurements to fully characterize the various eye parameters." The oscilloscope is set for infinite-persistence and a pattern generator is set up to generate a pseudo-random data pattern. Optimal Eye The most ideal reading would paint an eye pattern such as that shown in the center of Figure 12-10 on page 472 (labelled "Optimal Eye Opening"). It should be noted, however, that as long as the pattern painted resides totally within the region noted as "Normal," the transmitter and Link are within tolerance. Note that in these eye diagrams, the differential voltage parameters and values shown are peak differential voltages as opposed to peak-to-peak voltages documented in the specification. This is done because peak differential voltages can be represented in an eye diagram whereas peak-to-peak differential voltages cannot be represented in an eye diagram. See Figure 12-13 on page 475 for an example oscilloscope screen capture of an optimal eye. Figure 12-13. Screen Capture of a Normal Eye (With no De-emphasis Shown) Jitter Widens or Narrows the Eye Sideways Refer to Figure 12-11 on page 473. Jitter will cause a clock pulse to occur either before or after the "Optimal Eye Opening" resulting in an eye opening wider or narrower horizontally than the optimal width. Once again, as long as the amount of jitter doesn't cause the window to widen beyond the normal zone, it is still within tolerance. The jitter specification JT (see Table 12-1 on page 477) is a maximum of 3 UIs. See Figure 12-14 on page 476 for an example oscilloscope screen capture of an Eye Diagram showing how out-of-spec jitter causes horizontal widening or narrowing of the eye. Figure 12-11. Transmitter Eye Diagram Jitter Indication Figure 12-14. Screen Capture of a Bad Eye Showing Effect of Jitter, Noise and Signal Attenuation (With no De-emphasis Shown) Noise and Signal Attenuation Heighten the Eye Refer to Figure 12-12 on page 474. Noise or signal attenuation will cause the signal's voltage level to overshoot or undershoot the "Optimal Eye Opening" zone. As long as the amount of undershoot or overshoot doesn't cause the window height to dip below or extend above the normal zone, it is still within tolerance. See Figure 12-14 on page 476 for an example oscilloscope screen capture of an eye diagram showing how significant noise or signal attenuation causes the vertical widening or narrowing of the eye. Figure 12-12. Transmitter Eye Diagram Noise/Attenuation Indication

[ Team LiB ]