Network Analysis

In addition to wave propagation delays along the interconnections, the transmitted signals are distorted by reflection noises. Reflection noise is the excessive voltage caused at the driving and receiving ends of an interconnection during signal switching. It strongly depends on the characteristic impedance of the line and the impedance of the driving and receiving circuitry. Reflection noise is also produced when the interconnect cross-section undergoes changes along the signal propagation path.

Multiple nets can be selected for analysis, but each net is modeled separately without coupling it to other nets. When a differential net is selected the other differential pair, net is also automatically selected, and both differential pair nets are included in the same network analysis model.

Figure 1.

Creating a network analysis model starts with selecting a net from the net listing. The net name is initially assigned to the model name which can be changed by users. When multiple output or bi-directional pins are available for the net, one of them must be selected for active driver pin. The pulse period of input signal is automatically obtained from the operating frequency of the net. Clicking Input Signal allows you to view the input signal property and change them. After checking Define Pulse Data, you can also change the high/low state of each bit of the input signal.

Figure 2.

Clicking Device Models allows you to view the device models selected for the output and input pins. Users can change the device model selection when multiple models are available for the pin.

Figure 3.

Waveform Analysis

Waveform analysis is running transient circuit simulation on the network model for user-specified simulation time to obtain time-domain voltage waveforms at the input and output nodes. Signaling time refers how long the driver pin keeps transmitting signals. To see the waveforms after the driver pin becomes inactive, the simulation time should be longer than the signaling time plus the signal delay of the input signal.

The analysis is performed upon selecting Analyze menu. Upon completion of the analysis, waveforms at all input and output nodes are displayed. For each output pin, you can view:

• measurement voltage (Vmeas)
• input signal (*_in)
• enable signal (*-en)

For each input pin, you can view:

• static overshoot voltage high (Vmax)
• static overshoot voltage low (Vmin)
• logic threshold voltage high (Vinh)
• logic threshold voltage low (Vinl)

Selecting or unselecting an input node for waveform display makes the differential pair node be automatically selected or unselected. Many displays and measuring options are available in the waveform viewer. The analysis results can be listed in a table form by clicking the Result Data tab. They can be also shown in MS Excel. The waveform data can be saved in a file with the use of Save or Save As menu. Save menu saves the file in Signal_Integrity/Waveform directory under the PCB design job folder. The model name plus .spw is used for the file name. The saved waveform data can be read into the waveform viewer alone or together with other waveform data for a review or comparison.

Figure 4.

Eye Diagram Analysis

Eye Diagram analysis is running transient circuit simulation on the network model to obtain eye diagrams at the input nodes. An eye diagram or eye pattern is obtained from overlapping multiple waveforms at the receiver node of a net generated by randomly selected high/low states of the input signals for the selected net and adjacent nets. The coupling effects from the adjacent nets are reflected in the waveforms. Prior to running analysis, users can change the analysis control parameters such as the number of random pulses and bit pattern style.

Upon completion of the analysis, the eye diagrams at input nodes are displayed in the waveform viewer. Check Eye menu allows users to check if the eye size is large enough with the use of eye mask. The height and width values of the eye mask are initialized with the ones defined for the net, and users can change them. Among many available display and measurement options, Box can be effectively used for measuring the eye size. With X box checked users can make a rectangular box by placing two vertical lines. The eye mask height is used for the box height. On the other hand, with Y box checked users can make a rectangular box by placing two horizontal lines. The eye mask width is used for the box width. The eye diagram waveform data can be saved in a file with the use of Save or Save As menu. Save menu saves the file in the Signal_Integrity/Waveform directory under the PCB design job folder. The model name plus .spe is used for the file name. The saved eye diagram waveform data can be read into the waveform viewer alone or together with other eye diagram waveform data.

Figure 5.

Network Parameter Analysis

Upon completion of the analysis, the S-parameter magnitude values among ports are initially displayed in the network parameter viewer. You can change the port pair selections and change the value type to decibel, phase, real number, or imaginary number. You can choose the network parameter type for the display among S-parameter, Y-parameter, and Z-parameter. Many display and measurement options are available in the network parameter viewer. Result Data menu is used to list the network parameter values in a table format. The network parameter values can be saved in a Touchstone format file. Additionally, they can be viewed in MS Excel.

Figure 6.

TDR Analysis

TDR analysis is running transient circuit simulation on the network model to obtain reflected waveform to generate the impedance waveform (TDR) seen from the driver. TDR analysis only considers the wave reflected to the driver stage. Multiple reflection is not considered.

Figure 7.

You can select multiple network analysis models for analysis. The selected models are analyzed one by one, and the analysis results are displayed together. The network models with analysis results are saved in Signal_Integrity/NWK directory under the PCB design job folder by selecting Save menu. The model name plus .NWK is used for the file name. The saved models are listed in the left-hand side of the dialog. You can select the saved models anytime and copy the model to view the model, view the analysis results, edit the model, and rerun analysis.

Outputs