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Where To Find This Example

AWR Version 15

Understanding AWR .emz Files

AWR Version 14

Understanding AWR .emz Files

Design Notes

Waveform Measurements on Voltage Eye Diagrams

This project demonstrates the types of measurements that can be made on voltage eye diagrams. Measurements are made to plot the amplitude, height, and the width of the eye, including the jitter in the crossing time. Open the tuner and change the MLIN Width parameter and watch all of these values change.

Overview

In this project, we show the types of measurements that can be made on a voltage eye diagram. All of these measurements are in the measurement browser under Nonlinear > Waveform. A pseudo random bit stream (PRBS) is generated, and the voltage at the load is displayed in the graph, "Vtime of Bit Stream". The resulting eye diagram is also displayed in the graph, "Veye of Bit Stream". The amplitude and height of the eye are then determined and listed in the table entitled, "Amplitude and Height". A second table, "Eye Width", shows the results of measuring the width of the eye. The jitter of the edge is shown in the table, "Jitter".

The PRBS is generated using a PORT_PRBS element, as shown in the schematic, "Bit Stream". The voltage is then measured at Port2, after the signal has gone through two transmission lines and discontinuities. Harmonic Balance is used as the simulation type throughout this project, although the measurements work equally well when using HSpice.

Amplitude and Height Measurements

The Eye_Amplitude and Eye_Height measurements are shown in the table "Amplitude and Height". The measurements require the same information as the eye measurement used in the graph, "Veye of Bit Stream". Eye_Amplitude and Eye_Height both measure the difference between logical "1" and logical "0" for the eye. The difference is that the height measurement also factors in the uncertainty of the 1 and 0 voltage levels - by subtracting three standard deviations of the thickness of the lower and upper voltage envelopes. Therefore, for noisy data, the eye's height will be noticeably less than its amplitude.

Width and Jitter Measurements

The width of the eye is shown in tabular form in "Eye Width" by using the measurement Eye_Width. The settings forthe measurement are the same as for the eye measurement shown in "Veye", with the addition of a Threshold parameter. Eye_Width is a measure of the horizontal opening of an eye diagram. The eye width is measured by first determining the logical 0 and 1 levels. The threshold parameter is then used to set the vertical window in which to look for the intersection of the traces of opposite signs. A threshold of 0 would result in a full height vertical window,whereas a threshold of 0.5 would result in a vertical window of 0 height (and therefore would be useless). The threshold setting makes a significant difference in the final answer when the data are noisy.

Jitter is the measure of the time variances of the rising and falling edges of an eye diagram as these edges affect the crossing points of the eye. The jitter measurement is shown in the Table, "Jitter". The settings are the same as the width measurement, with the addition of a field called Method. This field can be either set to Peak-to-Peak (the default) or RMS. Peak-to-Peak uses the minimum and maximum line crossing values, whereas RMS uses RMS values.

Schematic - Bit_Stream

Graph - Amplitude and Height

Frequency (GHz) Eye_Amplitude(PORT_2,2,0,0,0,1000,20) (V) Bit_Stream.$F_SPEC Eye_Height(PORT_2,2,0,0,0,1000,20) (V) Bit_Stream.$F_SPEC
0.015625 0.47889 0.429919

Graph - Veye of Bit Stream

Graph - Vtime of Bit Stream