Changes

*Scattering [[Parameters]]*Impedance [[Parameters]]

=== What You Will Learn === In this tutorial you will run a network analysis test of the simple transmission line circuit you built in the previous tutorial lessons. You will examine the Z- and S-parameters of your circuit and study the Smith chart.

[[File:RFTUT3_1.png|thumb|500pxleft|550px|The quarter-wave impedance transformer circuit tuned for f₀ = 2GHz.]]

As a first step, you will run a "Network Analysis" test of your RF circuit. Network analysis calculates the S/Z/Y [[parameters]] of your circuit based on the port(s) you define for your RF circuit. In this case, you will define a one-port network with an input port established at the input of the T-Line between Node 2 and ground. In the Toolbox, select the Test Panel and check the "Network Analysis" checkbox. Open the Test's Settings dialog. It has three tabs at the top: Connections, Sweep and Output. In the first tab, '''Connections''', you define the port(s) of your circuit. In this case, Port 1 is defined between Node 2 and the ground. Accept the default value of 50Ω for the "Reference Impedance". In the second tab, '''Sweep''', set the start and stop frequencies to 1GHz and 5GHz, respectively. Select a linear scale interval and set the step size to 10MHz. This will provide a smooth graph of the port characteristics. In the third tab of the dialog, '''Output''', go to the "Parameter Set" section and choose the '''S''' radio button to compute the scattering [[parameters]]. Since your circuit is a one-port, you will have the S11-parameter only. From the top "Graph Type" options, choose '''Cartesian (Mag/Phase)'''. Check the checkboxes labeled '''Decibels''' and '''Degrees''' for magnitude and phase, respectively.

[[File:RFTUT3_2.png|thumb|left|230px|The "Connections" tab of Network Analysis Test Panel.]]

[[File:RFTUT3_3.png|thumb|left|230px|The "Sweep" tab of Network Analysis Test Panel.]]

[[File:RFTUT3_4.png|thumb|left|230px|The "Output" tab of Network Analysis Test Panel.]]

[[File:RFTUT3_5.png|thumb|750pxleft|720px|Cartesian graph of the magnitude and phase of the S11-parameter.]]

[[File:RFTUT3_6.png|thumb|230px|The "Connections" tab of Network Analysis Test Panel.]]Next, go back to the '''Output''' tab of the Network Analysis Test Panel and choose the '''Z''' radio button in the "Parameter Set" section. Make sure you remove the check mark from the "Decibels" check box. From the top "Graph Type" options, choose '''Cartesian (Real/Imag)''', because you are more interested in the frequency response of the real and imaginary parts of the Z11-parameter, which are indeed the input impedance of your circuit. Run another network analysis and view the output graph as shown below. Note that at 2GHz, the real part of the impedance is 50Ω and its imaginary part vanishes as you would expect for perfect impedance match.

[[File:RFTUT3_6.png|thumb|left|230px|The "Connections" tab of Network Analysis Test Panel.]]</td></tr></table> Run another network analysis and view the output graph as shown below. Note that at 2GHz, the real part of the impedance is 50&Omega; and its imaginary part vanishes as you would expect for perfect impedance match.  <table><tr><td>[[File:RFTUT3_8.png|thumb|750pxleft|720px|Cartesian graph of the real and imaginary parts of the Z11-parameter.]]

The Smith chart is a very useful graphical tool for RF engineers. [[RF.Spice A/D]] allows you to plot the S-[[parameters]] of your circuit on the Smith Chart. For this purpose, open the Network Analysis Test Panel of the Toolbox once again, and this time choose the '''S''' radio button in the "Parameter Set" section of the Output tab. For the "Graph Type", check the "Smith" checkbox. In order to better view the data points on the Smith chart, change the step size to 500MHz in the Sweep tab of the test panel. Run a network analysis of your RF circuit with the following [[parameters]]:

[[File:RFTUT3 12.png|thumb|750pxleft|720px|The S11-parameter plotted on a Smith Chart. The tracking bar shows the points corresponding to 2GHz.]]

You can read the values of the data points on the Smith chart using the "Tracking Crosshairs" feature. For this purpose, click the '''Track Selected Plot''' [[File:b2TrackCH_Tool.png]] button of the '''[[Graph toolbar|Graph Toolbar]]''' and then move the mouse onto the surface of the Smith Chart. A tracking bar appears on the graph that connects the current cursor position to the data points. On the Status Bar, you can view the frequency associated with each data point as well as the real and imaginary parts of the reflection coefficient at that point. The 2GHz point is located at the center of the Smith chart.

[[File:RFTUT3 13.png|thumb|360pxleft|480px|Smith chart showing the variation of the S11-parameter with frequency at a fixed line length of 37.5mm.]]

[[File:RFTUT3 14.png|thumb|360pxleft|480px|Smith chart showing the variation of the S11-parameter with line length at a fixed frequency of 2GHz.]]

[[File:RFTUT3_15.png|thumb|500pxleft|550px|The quarter-wave impedance transformer circuit with an inductive load.]]

At 2GHz, your termination load now has an impedance Z_L = 100 + j80 Ohms. Run a new network analysis of your circuit with the [[parameters]] specified below:

[[File:RFTUT3_16.png|thumb|750pxleft|720px|Computed s11 data for line segment length L = 75mm and Z_L = 100 + j80 &Omega; over the frequency range 1-5GHz.]]

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