RF circuits are typically characterized as multiport networks (usually one-port or two-port). In many practical cases, rather than computing the input or output voltages or currents, you might be more interested in the port characteristics of your RF circuit. For one-port circuits, you would designate an input port (Port 1) and would like to calculate its return loss or input impedance. For two-port circuits, you would specify an input port (Port 1) and an output port (Port 2) and would be interested in finding its insertion loss or gain. The most commonly used set of parameters for RF circuit characterization are the scattering (S) parameters. The "Network Analysis Test" is one of the AC-type tests of [[RF.Spice A/D]], which is of particular importance to [[RF.Spice A/D]]. Network analysis computes four sets of parameters: S, Z, Y and H. Of these, S-parameters and the "Smith Chart" are of primary interest, although Z-parameters are also frequently sought.
To run a network analysis of your RF circuit, open the Test Panel of the Toolbox. Check the checkbox labelled "Network Analysis" and then open the corresponding Settings Dialog. The top part of this dialog has three separate tabs: Connections, Sweep and Output, as shown below. In the Connections tab, you set the input port of the circuit as well as the output port, if it is a two-port network. The ports are defined by specifying their positive and negative pins. You also have to specify the port reference impedance (Z0). The default value of Z0 is 50 Ohms. The Sweep tab of the dialog is identical to the sweep section of AC Sweep Test Settings dialog. Here you set the start and stop frequencies and the step size. In the Output tab, you specify which port characteristics to compute at the end of the network analysis. You can choose only one of the four parameter sets: S, Z, Y or H. All [[parameters]] can be plotted on cartesian graphs with three data formats: Amp Only, Amp/Phase or Real/Imag. The magnitude data can be plotted on either linear or dB scales. S-parameters are the only option that can generate either a Smith chart or a polar graph.
<table><tr><td> [[File:Net1As another example, consider the RF circuit shown in the figure below, which was earlier examined in the discussion of AC frequency sweep test.png|thumb|left|250px|Network Analysis Settings: Connections TabThis circuit can be treated as a one-port network with its input port defined between nodes 2 and 0, i.]]</td><td> [[File:Net2e.png|thumb|left|250px|Network Analysis Settings: Sweep Tabbetween the source's internal resistor and the input T-line segment.]]</td><td> [[File:Net3As a one-port, the circuit has a single s11 and a single z11 parameter.png|thumb|left|250px|Network Analysis Settings: Output TabThe first figure below shows the Smith chart for the return loss (s11) over the frequency range 1-10 GHz at larger steps of 100MHz.]]</td></tr></table>The second figure below shows Cartesian plots of the real and imaginary parts of z11 over the same frequency range both with finer steps of 10MHz.
<table>
</tr>
</table>
Â
As an example, consider the RF circuit shown in the opposite figure, which was earlier examined in the discussion of AC frequency sweep test. This circuit can be treated as a one-port network with its input port defined between nodes 2 and 0, i.e. between the source's internal resistor and the input T-line segment. As a one-port, the circuit has a single s11 and a single z11 parameter. The first figure below shows the Smith chart for the return loss (s11) over the frequency range 1-10 GHz at larger steps of 100MHz. The second figure below shows Cartesian plots of the real and imaginary parts of z11 over the same frequency range both with finer steps of 10MHz.
<table>