Horology: time measuring systems or devices – Combined with disparate device
Reexamination Certificate
2002-04-25
2004-07-27
Miska, Vit W. (Department: 2841)
Horology: time measuring systems or devices
Combined with disparate device
C345S208000, C345S440000, C702S073000, C702S079000, C702S080000
Reexamination Certificate
active
06768703
ABSTRACT:
BACKGROUND OF THE INVENTION
Eye diagrams are a conventional format for representing parametric information about signals, and especially digital signals. Various prior art eye diagram testers are known, but we shall call the technique described in “METHOD AND APPARATUS FOR PERFORMING EYE DIAGRAM MEASUREMENTS”, as well as the preferred method to be disclosed herein (and any corresponding circuit apparatus), an Eye Diagram Analyzer, or EDA for short.
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 individual measurement samples taken upon separate instances of a signal occurring on a channel of interest, and which were then stored in a memory. Each measurement sample contributes to a displayed dot. The eye shape appears 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. A further difference with the continuous analog technique is that rare or infrequently occurring events, once sampled, do not appear faint in the display or disappear with the persistence of the CRT's phosphor. This latter difference is often quite an advantage, since it is often the case that such otherwise “hard to see” features of the trace are very much of interest.
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 SUT signal) centered about the reference, with sometimes perhaps several additional eyes (cycles) before and after.
Different (X, Y) regions within a (sample) space containing 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 Eye Diagram Analyzer measures is the number of times, out of 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. Points along the visible eye diagram trace describe something about those (time, voltage) combinations that were observed to actually occur in the data signal under test. The value of a (time, voltage) combination is represented by its location, but the color or intensity of the measured result is determined in a way that assists in appreciating the meaning of the measured data. 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 let the analyzer 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., X
11
) for a computer operating system. (A one-to-one correspondence between display pixels and measurement points is not required. 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 for an image when the correspondence between the display's pixel locations and the measurements that are the original image description is not one-to-one.)
A modern eye diagram trace itself is thus not a single time domain waveform (think: ‘single valued function’), but is instead equivalent to 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 (cycle) of the data 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 various rules for determining the intensity and color of the displayed pixels of the eye diagram trace (e.g., very long, perhaps “infinite”, persistence) the eye diagram presents readily seen evidence of infrequently occurring failures or other hard to spot conditions.
The EDA of the incorporated “METHOD AND APPARATUS FOR PERFORMING EYE DIAGRAM MEASUREMENTS” operates by applying the clock signal from the SUT to a comparator circuit whose output is then delayed by a fixed amount, say about a half cycle, or some integral multiple thereof. The delayed clock comparison is then the reference mentioned above, and it used by determining when individually threshold-compared and then delayed data signals (the SUT data channels) are sampled.
This sampling of the individually threshold-compared and then delayed data signals is actually performed twice in rapid succession, a very brief (but selectable) amount of time apart. If these two successive samples are different, then the input signal transitioned through the voltage region of interest, and we call this a hit. This is the manner of sampling that accomplishes the taking of the (time, voltage) pairs that are the basic data of the eye diagram measurement, and it is an alternative to digitizing with a conventional Analog-to-Digital Converter (ADC). We use it because it works at frequencies that are impractical for ADCs.
Different sampling voltages are obtained by varying the comparison thresholds for the data signals. Different times are obtained by varying the amount of delay in the data channel path, while leaving the clock data signal path essentially fixed. Skew between data channels is removed by introducing corresponding increases or decreases in the individual delays of the data channels.
An advantage of this technique is that, once skew is calibrated out, it allows the delay and threshold comparison for each data channel to vary as needed to complete the measurement. That is, the EDA will dwell on a measurement point for say, a specified number of clocks, or say, until some other condition is met, before moving on to the next measurement point. It sometimes happens that the rate of progress among the channels is not all the same. This particular technique allows each channel to be measured at a different measurement point, simultaneously with the other channels at their measurement points, if that is appropriate. That is, each data channel can have its own
Eskeldson David D.
Nygaard, Jr. Richard A
Agilent Technologie,s Inc.
Miller Edward L.
Miska Vit W.
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