Composite eye diagrams

Details Composite eye diagrams Composite eye diagrams

2002-01-31

2004-10-26

Huff, Marc S. (Department: 2857)

Data processing: measuring, calibrating, or testing

Testing system

Of circuit

C702S066000, C324S076120

Reexamination Certificate

active

06810346

ABSTRACT:

BACKGROUND OF THE INVENTION
Eye diagrams are a conventional format for representing parametric information about signals, and especially digital signals. We shall refer to an item of test equipment or a measurement circuit arrangement that creates an eye diagram as an eye diagram tester, whether it is found in an oscilloscope, a BERT (Bit Error Rate Tester), a logic analyzer, or, as a separate item of test equipment. 'Scopes and BERTs each have their own inherent (and different) types of circuit architecture that they use to create eye diagrams, and thus belong to the class of Eye Diagram Testers. The preferred method (and by implication, any corresponding circuit apparatus) to be disclosed herein is that of the incorporated Application, is different than those used in 'scopes and BERTs, and is especially suitable for use within a logic analyzer, as well as an item of stand-alone test equipment. We shall call this different method (and any corresponding circuit apparatus) an Eye Diagram Analyzer, or EDA for short. By the definitions above, an EDA is a particular type of eye diagram tester.
A modern eye diagram for a digital signal is not so much a trace formed continuously in the time domain, as it is an “eye” shape composed of closely spaced points (displayed dots, or illuminated pixels) representing many (probably at least thousands, easily millions, and perhaps orders of magnitude more) individual measurement samples taken upon separate instances of a signal occurring on a channel of interest. Each measurement sample contributes to a displayed dot. To borrow an idea from the world of analog oscilloscopes, it is as though an infinite persistence continuous time domain trace (for the signal of interest) were cut apart into lengths corresponding to one, five or ten clock times, and then stacked on top of each other. In the sampled digital case however, portions of the eye shape only appear continuous because the collection of dots is rather dense, owing to the large number of times that the signal is sampled. Unlike a true continuous technique, however, there may be detached dots that are separated from the main body of the eye shape. In any event, the vertical axis is voltage, and the horizontal axis represents the differences in time (i.e., various offsets) between some reference event and the locations for the measurement samples. The reference event is generally an edge of a clock signal in the system under test, represents directly or through some fixed delay the expected point in time when the value of an applied data signal would be captured by some receiving circuit in an SUT (System Under Test), and is derived from an application of the SUT's clock to the Eye Diagram Analyzer. The time axis will generally have enough length to depict one complete eye-shape (cycle of a DUT signal) centered about the reference, with sometimes perhaps several additional eyes (cycles) before and after. In general, the number of cycles shown depends upon how the measurement is set up, and could be more than several.
While not forgetting that oscilloscopes and Bit Error Rate Testers each have their own measurement paradigms, it will be useful for us briefly consider how the particular technique of interest to us operates. Different (X, Y) regions of an eye diagram represent different combinations of time and voltage. Assume that the eye diagram is composed of a number of pixels, and temporarily assume that the resolution is such that each different (X, Y) pixel position can represent a different combination of time and voltage (and vice versa), which combinations of time and voltage we shall term “measurement points.” What the analyzer measures is the number of times, out a counted number of clock cycles, that the signal on the channel being monitored passed through a selected measurement point. Then another measurement point is selected, and the process repeated until there are enough measurement points for all the pixels needed for the display. The range over which the measurement points are varied is called a “sample space” and is defined during a measurement set-up operation. And in reality, we define the sample space and the resolution for neighboring measurement points first, start the measurement and then figure out later how to ascribe values to the pixels of the display. The “display” is, of course, an arbitrary graphic output device such as a printer or an X Window of some as yet unknown size in a window manager (e.g., X11) for a computer operating system. (The one-to-one correspondence between display pixels and measurement points we assumed at the start of this paragraph was just a convenient simplification for ease of explanation. It will be appreciated that it is conventional for display systems, such as X Windows, to “. . . figure out . . . how to ascribe values to the pixels . . . ” when the correspondence is not one-to-one.)
The substance of an eye diagram, then is that it represents various combinations of circumstances that occurred in the data signal being characterized by the Eye Diagram Analyzer. However, the eye diagram trace itself is not a single time domain waveform (think: ‘single valued function’), but is instead an accumulation of many such instances; it can present multiple voltage (Y axis) values for a given time value (X axis). So, for example, the upper left-hand region of an eye might represent the combination of an adequate logical one at an adequately early time relative to the SUT's clock signal, and an eye diagram whose trace passes robustly through that region indicates to us that a signal of interest is generally achieving a proper onset of voltage at a proper time. Furthermore, we note that there are also other regions, say, near the center of an eye, that are not ordinarily transited by the trace, and which if that were indeed to happen, would presumably be an indication of trouble. Thickening of the traces is indicative of jitter, a rounding of a corner is indicative of slow transitions, and so on. An eye diagram by itself cannot reveal in the time domain which isolated instance of the signal caused such an exception, as other types of measurements might, but it does provide timely and valid information about signal integrity within a system as it operates. In particular, by incorporating very long (perhaps “infinite”) persistence the eye diagram presents readily seen evidence of occasional or infrequently occurring failures.
An eye diagram, then, is information about signal behavior at various time-voltage (X, Y) combinations. A simple system would be to indicate that the signal was “there” or that it wasn't. That is, respectively put either an illuminated pixel or a non-illuminated pixel at the various (X, Y) locations for the different instances of “there.” This is about what an analog oscilloscope would do if it were used to create an eye diagram for some signal. However, we would notice that some parts of the trace were brighter than others, and understand that this is a (somewhat useful) artifact caused by finite persistence on the one hand (old stuff goes away) and relative rates of occurrence on the other. That is, the display ends up having an intensity component at each pixel location. This is fine as far as it goes, but we would rather not rely upon the persistence of phosphor for this effect, since the most interesting indications are apt to be also the faintest. Since we are not using an analog 'scope, anyway, and have an instrument (an EDA) with memory (akin to a digital oscilloscope, timing analyzer or logic analyzer), we can gather data and decide after the fact what pixel value is to go with each (X, Y) pixel location. Those pixel values can be variations in color, intensity, or both, according to whatever scheme is in use (and there are several). The general idea is that the operator configures the EDA to render the display in a way that makes the condition he is most interested is quite visible, and also such that the eye diagram as a whole is generally easy to interpret. Thus, the reader is reminde

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