Changes

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 will be more interested in the port characteristics of your RF circuit. 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. For a one-port circuit, you designate an input port (Port 1) and calculate its return loss or input impedance. For a two-port circuit, you specify an input port (Port 1) and an output port (Port 2) and find 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 computes four sets of parameters: S, Z, Y and H. 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.

<table><tr><td> [[File:b2MAN_Fig237.png|thumb|left|420px|The RLC circuit of Tutorial Lesson 2.]]</td></tr></table> As an example, consider the simple RLC RF circuit of Tutorial Lesson 2, which is shown here in the opposite figure. Firstearlier for nodal analysis, 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 consisting 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.  <table><tr><td>[[File:b2MAN_Fig233.png|thumb|left|720px|Cartesian plot of the real and imaginary parts of the Z11 parameter.]]</td></tr><tr><td>[[File:b2MAN_Fig234.png|thumb|left|720px|Cartesian plot of the real and imaginary parts of the Y11 parameter.]]</td></tr><tr><td>[[File:b2MAN_Fig235.png|thumb|left|720px|Cartesian plot of the magnitude and phase of the S11 parameter.]]</td></tr><tr><td>[[File:b2MAN_Fig236.png|thumb|left|720px|The S11 parameter plotted on a Smith chart.]]</td></tr></table> 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.  <table><tr><td>[[File:b2MAN_Fig238.png|thumb|left|720px|Cartesian plots of the magnitude of the S11, S12, S21 and S22 parameters.]]</td><tr><td>[[File:b2MAN_Fig239.png|thumb|left|720px|The S11, S12, S21 and S22 parameters plotted on a Smith chart.]]</td></tr></table> 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.  As another example, consider the RF circuit shown in the figure below, which was earlier examined in the discussion of AC frequency sweep testconnecting transmission line segments. 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><tr><td>[[File:RFAC3.png|thumb|left|640px| An RF circuit consisting of a two-port network and connecting transmission line segments.]]</td></tr></table>