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Design Notes

HARMONIC BALANCE EYE DIAGRAM EXAMPLE

This example shows the effects of differential and single ended microstrip and stripline structures on high speed digital waveforms.

Overview

Eye diagrams and the associated time domain waveforms are shown for four different structures: single ended microstrip, single ended stripline, differential microstrip, and differential stripline.  The dimensions for all of these structures were tuned for the best possible match over the frequency range of 0.5-15GHz.

A Harmonic Balance pseudo random bit source is used.  This source allows for fast, frequency domain analysis of time varying signals by automatically configuring the source to supply the spectral representation of the desired waveform.  To run the circuit simulation, click on the Analyze button on the tool bar.

By varying the data rate of the digital data the different structures can be compared to see how they perform from a signal integrity perspective.  The data rate is already setup for tuning, click on the Tune button on the toolbar, and vary “ModFreq” variable.  Note that the plots will get distorted while tuning due to the change in the timing for the different data rates - press the Sweep button in the tune dialog box to refresh the axis ranges.

“Balanced_Microstrip” and “Balanced_Stripline” Schematics

These schematics use a pseudo random bit sequence harmonic balance source, PORT_PRBS, which can be found in the Elem tab under Circuit Elements > Ports > Signals.  The variable used by the source is “ModFreq” which is defined in Project tab under Global Definitions.

The attached graphs show the associated eye diagrams.  For comparisons, please see the "All Eye Diagrams" graph, described below.

“Unbalanced_Microstrip” and “Unbalanced_Stripline” Schematics

These schematics use the same pseudo random bit sequence source.

The attached graphs show the associated eye diagrams.  For comparisons, please see the "All Eye Diagrams" graph, described below.

Waveform Graphs

The "All Eye Diagrams" graph is used as a collection document; the eye diagrams for all four structures are copied in it for easy comparison.  Maximize this graph and use the Tuner to compare results at different data rates. While stripline is the more lossy medium, it does not distort the signal as much as microstrip, especially for high data rates.  Comparing unbalanced to balanced shows that differential lines preserve the signal much better than single ended lines.

There are two graphs showing the voltage vs. time waveforms that generate the resulting eye diagrams shown on the schematics.  These are called “Balanced Waveform” and “Unbalanced Waveform.”  Only part of the data (2 ns) is shown so that the bits can be discerned at all frequencies.  To change the x-axis range, right-click on the graph, select Properties, then click on the Axes tab.

Spectrum Graphs

There is 1 graph showing the power vs. frequency spectrum of the PRBS source used in each of the schematics.  This graph is called “PRBS Spectrum.”

Eye Measurement Tables

Tables with self-descriptive names show the following measurements:

• "Eye Height in Volts"

• "Eye Width in ns"

• "Overshoot in Volts"

Note the absence of overshoot in the stripline structures.  The frequency displayed in the table is the fundamental frequency of the 64-bit test sequence; i.e. the data rate (or ModFreq) divided by 64.  At the highest data rate of 40 GHz, the signals are so degraded in the microstrip structures that some tabulated measurements can not be made accurately.

Schematic - Balanced_Stripline

Schematic - All Eye Diagrams

Schematic - Unbalanced_Microstrip

Graph - Balanced Waveforms

Graph - Balanced Microstrip