You can use [[RF.Spice A/D]] to simulate or design distributed analog and mixed-mode circuits at high frequencies. 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, parts and devices are connected to one another using transmission line segments, which introduce additional phase shifts depending on their electrical lengths and may also alter the voltages and currents at different points of the circuit due to impedance mismatch.
[[RF.Spice A/D]] uses the same Berkeley SPICE and XSPICE simulation engines of its forerunner [[B2.Spice A/D]]. In other words, the high frequency AC analysis is carried out by the same analog and mixed-mode SPICE simulation engines based on nodal admittance analysis, which have been enhanced with additional RF simulation capabilities. As a result, you can mix the RF devices in your circuits with all the other analog and mixed-mode devices. You can also mix transmission-line-type RF devices with digital parts and perform mixed-mode time domain simulations.
The concepts of [[Transmission Lines|transmission lines]] and [[Multiport Networks|multiport networks]] are integral to any RF simulation. From a simulation point of view, an RF circuit is made up of a collection of [[Multiport Networks|multiport networks]] that are interconnected via [[Transmission Lines|transmission lines]] segments or components. 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 various circuit nodes, some of which may serve as external or internal ports of your circuit. Or you can calculate the port characteristics of the overall network by designating input and output ports to your RF circuit.
All the RF devices of [[RF.Spice A/D]] can be divided into two groups: devices based on transmission line models, and devices based on multiple networks. [[RF.Spice]]'s transmission line models are enhanced versions of SPICE's standard LTRA model. The transmission-line-based devices typically utilize a combination of LTRA and passive RLC models. [[Multiport Networks|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 their values are interpolated in between the frequency samples. [[RF.Spice A/D]] 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 A/D]] are primarily intended for use in two types of [[tests]]:
* AC Frequency Sweep Test
* Network Analysis Test
{{Note | S-parameter-based RF devices do not work with âLive Simulationâ or Transient Test as their models typically contain S-[[parameters]] at high frequencies only.}}
== RF Circuit Simulation vs. Electromagnetic Simulation ==