Confronted with ever faster digital signals that have become essential to meet the demand for speedier processing and data transfer, engineers are discovering that signal integrity problems often aren't discovered until the latter stages of a development cycle. This means they must pay more attention all the way through the development cycle to factors that affect a signal's characteristics - improperly terminated transmission lines that can result in reflections and ringing, noise induced in power and ground by switching transients, and intersymbol interference, for example.
A new tool called Eye Scan can help. This enables a logic analyzer to scan all incoming signals for activity in a particular time and amplitude range. The time range is centered on the system clock and the amplitude can cover the entire voltage range of the signal. In eye scan mode, the Agilent 16760A Logic Analyzer examines regions of time and voltage for signal transitions that occur. The horizontal axis represents time and the vertical axis represents voltage, just like an oscilloscope.
The results are displayed as a graph image that is similar to an oscilloscope eye diagram. As can be seen in figure 1, the display colors correspond to the amount of signal activity detected.
Figure 1. An Eye Scan Display.
The scan proceeds first along the time axis, as shown in the figure 2. When the user-specified range of time has been scanned from Tmin to Tmax, the threshold voltage is incremented, beginning at VTH(min), and the entire time range is scanned again at a new threshold. This is repeated until all time and voltage regions have been scanned. The user can adjust the scan range and resolution for both time and voltage.
Figure 2. Time and voltage axes are scanned for signal transitions.
There are a number of viewing options: All channels on a single bus or multiple buses can be overlaid on the display. Individual channels can be highlighted in the composite display. Signals can be examined individually. In addition, results can be viewed for each individual run - or results from multiple runs can be all superimposed on the screen.
By selecting the option 'Accumulate results from run to run', the user can view the results of multiple runs superimposed on the display.
A Typical Application
Here is a typical Eye Scan application: Let's say a user wants to vary the power supply voltage in the system under development in several increments and then view the worst-case eye opening for all the power supply voltage settings in a single view. First the power supply voltage would be set to a starting level and then eye scan is run. Then the power supply voltage would be incremented and eye scan would be run, once more. After running eye scan for all the power supply settings, the user could view the worst-case eyes for the entire test.
One big advantage of eye scan is that by designing connectors into the board, all the signals can be connected at once.
To make eye measurements using oscilloscopes, before the advent of Eye Scan, only four connections could be made at a time, so measuring hundreds of nodes required moving probes hundreds of times. So engineering teams could spend months making scope measurements on prototypes for the purpose of margin validation. But by attaching the logic analyzer to the board via the test connector, connections can be switched virtually instantaneously.
However, the biggest time saving and the biggest boost of confidence comes from the greater measurement throughput of eye scan compared to an oscilloscope. An oscilloscope does not trigger and paint a waveform on every clock cycle. The digital circuit sees and responds to every clock cycle. So at best an eye diagram, whether on an oscilloscope or on eye scan, is a statistical picture of what the circuit sees. The scope may not trigger when something interesting happens. Therefore to have a high level of confidence that your circuit meets its time and voltage margins, the engineer would have to let the scope run long enough to see all the rare errors.
Eye scan, on the other hand, does respond to every successive clock event, for as many clocks cycles as the user chooses (up to 60 million clock cycles) at each time and voltage coordinate. But it is only looking at one time and voltage coordinate at any one time.
The bottom line is, either a scope or eye scan can miss an event. The difference is that eye scan makes thousands more observations of the signals per unit of "wall clock" time than any oscilloscope. So the probability of finding the occasional error in a given span of "wall clock" time is orders of magnitude higher with eye scan.
The test parameters and their impact on total test time are spelled out in Table 1. (Note that the test times appear across the bottom row.)
Table1. Benchmark Times.
In column one, there are 30,000 regions to be examined. The voltage is scanned from -600 mV to +600 mV, in 10-mV steps, for a total of 120 steps. Whereas the time is scanned from -1.25 ns to +1.25 ns, in 10-ps steps for a total of 250 steps.
120 voltage steps x 250 time steps = 30,000 regions
In the remaining examples in Table 1, the number of regions is reduced to one-fourth of that number - 7500 regions - by doubling the magnitude of the voltage from 10 mV to 20 mV and by enlarging the time increments from 10 ps to 20 ps. Notice the difference between column two and column one, where everything is the same except for time and voltage resolution. The time has been reduced by a factor of 6.7, just by reducing the resolution slightly on each axis. The reduction is greater than a linear factor of four, which one might expect, because some of the measurement time is governed by processing time in the computer.
Comparing columns three and two in the table highlights the difference in measurement time as a function of the number of clock cycles examined at each time/voltage point. The time does not increase proportionally with the number of clock cycles. With ten times as many clock cycles, the elapsed measurement time increases only by a factor of 3.33.
Once eye scan has given the user a comprehensive overview of signal integrity, the user can then localize trouble situations. One may elect to examine these cases in more detail with an oscilloscope. Or, in some cases, the information from Eye Scan may be a sufficient diagnostic to localize the cause and thereby solve the problem. For example, skew between two signals can be visualized and measured very quickly using just Eye Scan.
An example where the oscilloscope and eye scan can work together effectively might be where a connector is suspected of introducing a signal integrity problem. The symptom can be viewed by observing the signals at the receiver with Eye Scan. An oscilloscope can then be used to examine the signals before and after the connector to verify that the connector is introducing the problem.
Shown in figure 3a is a display of bimodal jitter on an 800 MT/s PRBS (800-megatransfer-per second, pseudo-random-binary sequence) signal on an oscilloscope The same signal is displayed on a logic analyzer with Eye Scan in figure 3b. The oscilloscope used in the examples is an Agilent Infiniium 54846A oscilloscope, which has 2.25-GHz bandwidth and an 8-GSa/s sampling rate.
Note the difference in the appearance of the "shoulders" of the distribution in the histogram between the oscilloscope and eye scan. Eye scan uses a log vertical scale, while the oscilloscope uses a linear vertical scale for the histogram. This tends to emphasize the "tails" of the distribution. This makes it easier to visualize how much activity is present in the "tails" of the distribution.
Figure 3a. Bimodal jitter on a scope.
Figure 3b. Bimodal jitter on a logic analyzer with Eye Scan.
In summary, Eye Scan enables the user to measure signal integrity behavior on tens, or even hundreds, of signal nodes much more quickly than an oscilloscope. One can acquire comprehensive signal-integrity information on all the buses in a design, under a wide variety of operating conditions, in minimum time. The end result is greater confidence that the design meets the engineer's signal integrity goals. Once eye scan has identified a problem, one can then turn to an oscilloscope to investigate the nature or causes in more detail.
Agilent Technologies Application Note "Designing High-Speed Digital Systems for Logic Analyzer Probing," publication number 5988-2989EN (www.agilent.com/find/probeguide)