===Defining Sources and Loads===
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 [[B2RF.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]] 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 [[B2RF.Spice A/D]], whose "Use" checkboxes in the AC section of their source property dialog are automatically checked.
[[File:RFAC1.png|thumb|400px| AC voltage and current sources with internal series or shunt impedances.]]
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.
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.SpiceA/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.
[[File:RFAC3.png|thumb|600px| A more realistic version of the previous RF circuit including connecting transmission line segments.]]