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Time Domain Reflectometry (TDR) Swept Variables Example
This project demonstrates a TDR* simulation that uses swept variables. A Harmonic Balance Pulse source is used to excite a microstrip line terminated with a complex impedance (100 -j*100). The swept variable is used to sweep the length of the microstrip line from 1000 to 4000 mils at a step of 1000 mils.
The goal of this project is to demonstrate various ways AWR can display results from swept variable simulations. A detailed discussion on the swept variable framework is at the end of these notes.
This schematic has a very simple TDR setup to measure the input signal, signal at the load, and the reflected signal separately. A circulator, directly after the pulse source, acts as a reflectometer since any reflected signal is directed to port 3. The project frequency is set to 500 MHz. The length of transmission line is swept at values of 1000, 2000, 3000, and 4000 mils. Notice that port 2 has a complex impedance to generate a reflected signal. The line is setup to be a 50 ohm line (25 mils wide on a 25 mil, Er=10 dielectric). The output port impedance is setup so that the real and imaginary values can be tuned. Notice that if zreal = 50 and zimag = 0, there will be zero reflected signal. "
Pulse on MLIN" Graph
This graph is setup to have three sets of axes, one for each state of the pulse on the line. The measurement dialog is setup to show the voltage versus time at the different locations. The swept variable combo box for each measurement on this graph is set to Display all traces. You will see on the graph you can see the pulses delayed by different amounts due to the different lengths of microstrip line. Each axis has 4 different traces, one for each swept variable value.
Notice the parameter markers near the graph. These allow the user to determine which trace is the result of which swept parameter settings.
Instead of displaying all traces for the transmitted (port 2) and the reflected (port 3) signals, change these measurement settings to Select with Tuner instead of Plot all traces. When you do this, you will notice only one of the swept variable traces on the graph. Then open up the tuner, and change the tuner value. Notice a couple things. The tuner can only go to discrete values, the values of the swept variable. Notice the values on the graphs change as well as the parameter markers.
Swept Variable Framework
It is very simple to setup a swept variable.
1. Use an equation in the schematic to define a variable to be swept, and its default value.
2. Assign this variable to the appropriate model parameter(s).
3. Add a swept variable block, SWPVAR, to the schematic from the Circuit Elements > Simulation Control node in the Elem tab of the project browser.
The ID parameter of the SWPVAR block is used in the measurement dialog box for setting up measurements. The VarName parameter is the name of the variable defined in step 1. The Values parameter is a vector of numbers; the variable named in the VarName block will be swept through these values.
The measurement setup determines how to display the data when swept parameter blocks are used in a schematic. If you specify a schematic in the Data Source Name block with swept parameters, one new combo box will be added for each swept variable block in that schematic.
For each swept variable you have the following choices:
Use for x-axis: Put this swept block values on the graph x-axis of the graph.
Plot all traces: Puts one trace on the graph for each swept value.
Select with tuner: Puts one trace on the graph and then allows the user to use the tuner to change which swept value result gets displayed.
Disable sweep: Allows the user to turn off the sweep for that variable. The default value from the equation is used in this case.
[Swept parameter values]: Allows the user to display results using one specific swept variable value.
Simulation frequency is treated as a separate swept variable, with the exception that you cannot disable this sweep. Additionally, you can choose between multiple sweep plans using the right-pointing angle bracket (>) next to this combo box. You can choose between the following settings for frequency:
1. Project frequencies, set in the Project Options. Called FPRJ.
2. Schematic frequencies, set in the local properties for each schematic or EM structure. Called FDOC.
3. Swept frequency control block, Circuit Elements > Simulation Control > SWPFRQ. You can add SWPFRQ blocks to any schematic to add more unique frequency sweeps. The ID of the block(s) will appear in the list when choosing the frequency sweep from the > menu.
Note: when you add SWPVAR blocks to a schematic, all of the possible combinations of all of the swept variables will be simulated. The measurement selections choose how to display the data, not which data is simulated. The only exception is when one of the swept variables is disabled in all of the measurements on the schematic. It is easier and less obscure to simply disable the SWPVAR block in the schematic. During simulations, you can watch the status of the swept variables in the status box, if the simulation is slow enough to watch these values update.
* These simulations are performed using the Harmonic Balance simulator, not the linear TDR measurements. The TDR_Analysis example demonstrates the linear measurements.
Schematic - MLIN_TDR
Graph - Pulse on MLIN