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The Network Analysis Test characterizes a circuit with one or two ports. It is used to determine the circuit’s behavior as seen through its port(s) and generates data in the form of Z (impedance) [[parameters]], Y (admittance) [[parameters]], S (scattering) [[parameters]] , and H [[parameters]]. In the case of a two-port circuit, the four Z [[parameters]] are Z11 (the input impedance), Z22 (the output impedance), and Z12 and Z21 (the cross-impedances). Network analysis is often used for characterization of circuits that operate at very high frequencies. The components operating at these frequencies are often modeled with tables of S [[parameters]].

The setup page of Network Analysis has three tabs: Connections, Sweep and Output. All of the [[parameters]] in these three tabs must be set carefully to run a successful network analysis test. The "Connections" tab is used to define the input and output ports of your circuit. If only one port of a circuit is being analyzed, then define "Port 1" only. If you have a two-port circuit, then check the checkbox labeled "Port 2" to enable its port definition. The ports can be defined either by their node numbers (positive and negative/reference node indices) or as part names. For example, your input port might be a voltage source. The reference or port characteristic impedance "Z0" is set to 50 Ohms by default. You can change its value if necessary. The "Sweep" tab of setup panel is identical to that of AC Sweep Test. This is where you set up the [[parameters]] of your frequency sweep including start and stop frequencies, frequency scale and step. The "Output" tab is used to specify the output data type as well as the graph type. Currently five graph options are available:

As an example, consider the simple RLC circuit of Tutorial Lesson 2, which is shown here in the opposite figure. First, we treat this circuit as a one-port network, with its port established at the voltage source. The characteristic or reference impedance Z0 is set to its default value of 50 Ohms. The frequency sweep is set from 1MHz to 1GHz on a decade scale with 50 points per interval. The figures below show the Z/Y/S [[parameters]] of this one-port network plotted on both Cartesian graphs and Smith chart.

Next, we consider the same RLC circuit as a two-port network. The first port is left intact at the voltage source (between Node 1 and the ground), while Port 2 is set up across the 50-Ohm resistor between Node 2 and the ground. The figures below show the Cartesian plot of the magnitude of the S11, S12, S21 an S22 [[parameters]] as well as the Smith chart for the two-port network.

RF circuits are typically characterized as [[Multiport Networks|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 [[B2RF.Spice A/D]], which is of particular importance to [[RF.SpiceA/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.