Test & Measurement World, July/August 2012

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in one device can cause variations in the power delivered to an adjacent device, a condition known as "chip crosstalk." Intersignal crosstalk, appears as sporadic jolts on the victim whenever an aggressor makes a logic transition. This interfer- ence lasts for the duration of that transition, whereas chip crosstalk has a longer duration. Crosstalk is exacerbated by pre-emphasis and de-emphasis, which enable high-speed serial data receivers to reach a low BER (bit-error rate), even when signals are so degraded their eye diagrams are closed. At the transmitter, pre-emphasis en- hances the high-frequency content of signals, which fends off the low-pass nature of the transmission lane's frequency re- sponse. Unfortunately, that high-frequency content generates the most crosstalk. It's worse at the receiver. As Miller said, "Two of the three standard equalization techniques, CTLE (continuous-time linear equalization) and FFE (feed-forward equalization), am- plify crosstalk noise. The third technique, DFE (decision-feed- back equalization), is the only one that doesn't make it worse, but nothing we have now makes it better." Another complication arises in systems with parallel buses. Because the channels in parallel buses are frequency locked, they have fixed-phase relationships. With fixed-phase rela- tionships, the time position of intersignal crosstalk in the eye diagram is also fixed. If crosstalk jolts occur at the crossing point in an eye diagram, they have a smaller impact on the BER than if the crosstalk jolts are offset by half a bit period and occur in the center of the eye at the victim. Figure 1 shows an eye diagram with no crosstalk jolts. In Figure 2a, the crosstalk jolts appear at the crossings, and in Figure 2b, the jolts appear at the eye opening's widest point. In many systems that use multiple serial lanes, including 40/100 GbE (Gigabit Ethernet), each lane operates at the same nominal frequency, but the lanes aren't locked. Because each channel operates on a clock recovered from the data, the relative phases are free to float. Figure 3 shows how un- locked, or "asynchronous," crosstalk smears the traces and is deceptively similar to the effect of random noise and jitter. Oscilloscopes analyze crosstalk If you're in complete control of a serial-data link's individual lanes, you have the luxury of performing systematic, straight- forward crosstalk analyses. Assume that you have a 100-Gbps link made from ten 10-Gbps lanes. Start by analyzing lane 1. Turn off lanes 2 through 9 and check the waveform and eye diagram of lane 1. Then, turn on a lane adjacent to 1. Any degradation you see is caused by crosstalk. Next, turn on each lane, and you can gauge the trouble. This is a simple diagnostic recipe for finding lanes that are more troublesome than others. If the lanes are frequency locked, you should be able to isolate the problem. If they're not locked, the degradation will look like a mix of random jitter and noise. In the unlocked case, it's better to examine the waveform, bit by bit, compar- ing the aggressor waveform to the victim waveform. Unfortunately, these techniques won't help you estimate the eye closure corresponding to a given BER, which is what FIGURE 1. An eye diagram that has no crosstalk will have no impairments. Courtesy of Tektronix. a. b. FIGURE 2. These eye diagrams display the effects of different victim-aggressor phase relationships. a) The aggressor and victim are in phase, and the crosstalk impairment is at the crossing point. b) The aggressor and victim are half a bit out of phase, and the crosstalk impairment is at the center of the eye. FIGURE 3. When the aggressor and victim are asynchronous, crosstalk noise varies across the victim's eye diagram. Test & Measurement World | JULY/AUGUST 2012 | –17– Photos courtesy of Tektronix.

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