RF circuit analysis, by nature, is an AC analysis that you typically run at high frequencies ranging from tens of Megahertz to tens of Gigahertz. At such high frequencies, the dimensions of your circuit may become comparable in order of magnitude to the wavelength, when wave retardation effects start to appear. In other words, your circuit starts to act like a distributed structure rather than a lumped circuit where signals propagate instantaneously. In the analysis of a low frequency circuit, two nodes that are connected to each other through a wire are assumed to have equal potentials or identical voltages. In RF circuits, however, the connecting wires act as [[Transmission Lines|transmission lines]], whose lengths play an important role in determining the voltages and currents at different points of the circuit.
The RF devices of RF. Spice are characterized and modeled based on their frequency-domain scattering (S) [[parameters]]. The S-[[parameters]] are tabulated as a function of frequency and interpolated in between the frequency samples. RF.Spice performs an AC analysis of these RF devices by converting their S-[[parameters]] to Y-[[parameters]] and using them in conjunction with SPICEâs nodal admittance matrix formalism. The high frequency AC analysis is carried out by the same analog and mixed-mode SPICE simulation engine. As a result, you can mix the RF devices in your circuits with all the other analog and mixed-mode devices of [[B2.Spice A/D]].
At the heart of RF.Spice lie the concepts of RF [[Transmission Lines|transmission lines]] and [[Multiport Networks|multiport networks]]. All the RF devices of RF. Spice can be divided into two groups: devices based on transmission line models, and devices based on multiple networks. As you will see in the later sections of this manual, RF.Spice's transmission line models are based on SPICE's standard LTRA model. [[Multiport Networks]] are characterized and modeled based on their frequency-domain scattering (S) [[parameters]].
The S-[[parameters]] are tabulated as a function of frequency and interpolated in between the frequency samples. RF.Spice performs an AC analysis of these RF devices by converting their S-[[parameters]] to Y-[[parameters]] and using them in conjunction with SPICEâs nodal admittance matrix formalism.
 The S-parameter-based RF devices of RF.Spice are primarily intended for use in two types of [[tests]]:
* AC Frequency Sweep Test
{{Note | S-parameter-based RF devices do not work with âLive Simulationâ or Transient Test as their models normally contain S-[[parameters]] at high frequencies only.}}
At the heart of RF.Spice lie 's simulation engines are the concepts same as the Berkeley SPICE and XSPICE engines of [[Multiport Networks|multiport networksB2.Spice A/D]] . The high frequency AC analysis is carried out by the same analog and mixed-mode SPICE simulation engine. As a result, you can mix the RF devices in your circuits with all the other analog and mixed-mode devices of [[Transmission Lines|transmission linesB2.Spice A/D]]. You can also mix transmission-line-type RF devices with digital parts and perform mixed-mode time domain simulations. Â From a simulation point of view, an RF circuit is made up of a collection of [[Multiport Networks|multiport networks]] that are interconnected via RF [[Transmission Lines|transmission lines]]. If the input of your circuit is connected to a source and its output is connected to a load, then you can compute all the voltages and currents at all the external or internal ports of the circuit (i.e. at the various circuit nodes). Or you can calculate the port characteristics of the overall network by designating input and output ports to your RF circuit.