The WaveRunner Xi oscilloscope family offers several alternative statistical analysis tools based upon the analysis of persistence displays. These include persistence histograms and persistence trace mean, standard deviation, and range functions. All of these techniques are included in the extended statistical analysis option WRXi-STAT.

While persistence displays, such as eye diagrams, offer fast qualitative views of signals, they are often hard to quantify. Persistence histogram functions, on the other hand, allow the user to specify a narrow vertical or horizontal cross-section, or “cut”, and show the distribution of data points in that range. The functions are derived from the pixel maps of existing persistence displays and do not require re-acquisition of the data.

An example of a horizontal persistence histogram display is shown in Figure 1. The top trace (Channel 2) is the acquired eye diagram. Multiple signals have been acquired and stored using intensity-graded analog persistence. This display is typical of most eye diagrams and shows horizontal closure of the eye due to timing variations and vertical closure do to noise contamination.

Figure 1:

An example of a persistence histogram being used to analyze timing jitter on an eye diagram

Trace F1 is a horizontal persistence histogram function showing the distribution of the edge transitions in time. The associated dialog box shows the setup, including the location and width of the horizontal cut used to specify the area to be analyzed. Note that the region being analyzed is shown on the waveform between the horizontal dashed lines on the eye diagram. The width of the slice and its vertical position are user settable as the Slice Width and Slice Center controls in the persistence histogram setup shown in Figure 1.

The analysis function uses the location and number of pixels within the selected region to calculate the histogram. The function is automatically scaled to match the horizontal time scale of the eye diagram. Once the data is extracted from the persistence display it can be analyzed using cursors or the existing statistical parameters available in the WRXiSTAT option.

The second type of persistence histogram is shown on Figure 2. This figure shows a vertical persistence histogram used to study the same eye diagram.

Figure 2:

Using the vertical persistence histogram to study the vertical closure of the eye. Histogram parameters are used to compute the vertical eye opening (parameter P6)

A vertical analysis region is controlled using the setup dialog box to specify the width and location (center) of the analysis region. The area being analyzed is out-lined by two vertical bars on the eye diagram. The resulting histogram is displayed horizontally below the eye diagram. This histogram function is further analyzed using statistical analysis parameters. The histogram mean, standard deviation, and range are read out for the rightmost histogram. The vertical eye opening is computed by subtracting the minimum value of the right hand histogram (the top of the eye) from the maximum values of the left hand or base histogram. This calculation, performed using parameter math is shown in parameter P6.

While the same measurements can be made with conventional, parameter based histograms the persistence histogram is a more traditional analysis technique and much easier to set up. Persistence histograms, derived directly from the eye diagram, are more easily interpreted.