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/* Network Analysis */
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[[File:b2MAN_Fig5.png|thumb|left|270px250px|RF.Spice A/D Test Panel.]]
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== AC Test Types ==
===AC Frequency Sweep Test===
{{Note | Before running an AC analysis, you have to designate one or more AC sources for your circuit and set their AC amplitudes (and phases). During an AC analysis, [[RF.Spice A/D]] shorts out all of the non-constant voltage sources that have a zero AC magnitude and opens all the non-constant current sources that have a zero AC magnitude. Make sure that the voltage and current sources used as inputs have non-zero AC voltage/current values.}}
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[[File:b2MAN_Fig51.png|thumb|left|250px|AC Sweep Test Settings.]]
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All the sources with non-zero AC values are treated as sinusoidal sources with the specified peak voltage/current amplitudes and phases and default zero offsets. You can view or edit the AC values of voltage and current sources by double clicking on the parts and selecting the "Small-Signal AC and Distortion" tab of their property dialog. Make sure to check the "Use" checkbox in the section titled "AC Properties for Small-Signal AC Properties only". Given the specified voltage and current source values, the DC operating point of your circuit is calculated, and all nonlinear elements are replaced with their small-signal circuit models. If the DC operating point of your circuit with the sources shorted out is not where you want it, you may have to place some constant (DC) voltage sources in the circuit to compensate. Alternatively, you can set a nonzero value for the "Small-Signal Offset" parameter of an AC source as shown in the figure below.
Just as in low frequency circuits, RF circuits must be excited using a voltage or current source. However, in order to ensure a successful RF circuit simulation, your source must be a single-frequency (sinusoidal) AC source. Remember that in [[RF.Spice A/D]] circuits, you must specifically designate a source to operate as an AC source for all tests of the AC type. The RF Menu of [[RF.Spice A/D]] provides two additional types sources: the AC voltage source with the keyboard shortcut "Alt+V" and the AC current source with the keyboard shortcut "Alt+I". These sources are identical to the regular voltage and current sources of [[RF.Spice A/D]], whose "Use" checkboxes in the AC section of their source property dialog are automatically checked.
{{Note | For AC-type RF circuit analysis, you can only use AC voltage or current sources with a single common frequency.}}
In most RF circuits, the sources are modeled to have an internal source impedance typically denoted by Z<sub>s</sub>. This source impedance is usually real-valued and typically has a value of 50 Ohms. To model the source impedance, you can simply use a resistor in series with the AC voltage source or a resistor in parallel with the AC current source as shown in the opposite figure. If you need a complex-valued source impedance, you can use a "Complex Impedance" and connected it either in series or in parallel with the AC voltage or current source, respectively.
<table><tr><td>[[File:RFAC2RFAC1.png|thumb|400pxleft| A simple RF circuit driven by a 480px| AC voltage source and current sources with a resistive loadinternal series or shunt impedances.]]</td></tr></table>
Similarly, most RF circuits have a load impedance typically denoted by Z<sub>L</sub>. This load impedance, too, is usually real-valued and typically has a value of 50 Ohms. To model the load impedance, you can simply use a resistor at the output port of your RF circuit. The opposite figure shows a simple RF circuit consisting of a two-port network N1 connected to an AC voltage source with a 50Ω internal resistance and terminated at a 100Ω resistive load. Note how the negative input and output pins of the two-port device have been grounded.
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[[File:RFAC2.png|thumb|left|480px| A simple RF circuit driven by a voltage source and with a resistive load.]]
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Your load impedance can also be a combination of resistors, capacitors or inductors to model capacitive or inductive loading. Note that in that case you will have a complex-valued load impedance that varies with the operational frequency. In some other cases, you may prefer a user-defined "Complex Impedance" as your load, which cannot be simply modeled as a combination of RLC elements. A resonant antenna load is a good example of this case. The port characteristic data for the antenna structure can be generated by an electromagnetic simulator like [[EM.Cube]] and then imported to [[RF.Spice A/D]]. If you have the input impedance values as a function of frequency, then you should define a complex impedance load. If you have the return loss (s11) data as a function of frequency (as is usually the case), then you can define a one-port as your load.
===Using Transmission Lines for Connecting Parts===
You can use Generic T-Line segments or physical transmission line types to connect the various RF parts and devices in your circuit. The opposite figure shows the same simple RF circuit of the previous figure, but containing two T-line segments, one connecting the source to Port 1 of two-port N1 and the other connecting Port 2 of N1 to the resistive load. Note how the negative input and output pins of both T-line segments have been grounded. Lossless transmission line segments cause a phase shift of the propagating signal, while lossy transmission lines also cause additional signal attenuation.
<table><tr><td>[[File:RFAC4RFAC3.png|thumb|200pxleft| Setting the parameters in 640px| A more realistic version of the AC Frequency Sweep Test Panelprevious RF circuit including connecting transmission line segments.]]</td></tr></table>
===Defining RF Circuit Observables===
{{Note | Typically the input and output voltage, currents and powers are of primary interest. These are measured at the input port (between the source impedance and input transmission line segment) and output port (between the output transmission line segment and the termination load).}}
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[[File:RFAC4.png|thumb|left|250px| Setting the parameters in the AC Frequency Sweep Test Panel.]]
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In the previous RF circuit, the input port has node index 2 and the output port has node index 5. Therefore, the voltage v(2) and v(5) are designated as the simulation output data. The figure below shows a plot of the computed input and output voltages over the frequency range 1-10 GHz. A frequency step size of 10MHz has been set for the frequency sweep.
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[[File:RFAC5.png|thumb|500pxleft|550px| Defining the input and output voltages for the previous RF circuit.]]
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[[File:RFAC6.png|thumb|640pxleft|720px| Graph of the computed input and output voltages of the previous RF circuit over the frequency range 1-10 GHz.]]
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== Network Analysis ==
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.
{{Note | Network analysis is an AC circuit analysis that is typically performed for high frequency circuits.}}
For amplitude graphs, you have the option to plot them in Decibels (dB) scale. For phase graphs, you have the option to express them in degrees. You also need to specify which parameter set to calculate as the output of network analysis. The four options are Z, Y, S or H.
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<td> [[File:b2MAN_Fig56.png|thumb|200pxleft|250px|Network Analysis Settings: First Page.]]
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<td> [[File:b2MAN_Fig57.png|thumb|200pxleft|250px|Network Analysis Settings: Second Page.]]
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<td> [[File:b2MAN_Fig58.png|thumb|200pxleft|250px|Network Analysis Settings: Third Page.]]
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As an example, consider the RF circuit shown in the opposite figure, which was earlier examined in the discussion for nodal analysis, consisting of AC frequency sweep testa two-port network and connecting 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.
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<td> [[File:Net4.png|thumb|640pxleft|720px|Smith Chart.]]
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<td> [[File:Net5.png|thumb|640pxleft|720px|Cartesian graph of Z-parameters.]]
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