[[RF.Spice A/D]] provides an extensive library of black-box virtual blocks that allow you to quickly test and verify new system concepts without getting into the details of particular circuit realizations. A virtual block typically has one or more input pins and one or more output pins. It models a system-level function or behavior in the form of a relationship between the input and output voltages. For example, a multiplier block takes two input voltages and outputs their product. An analog frequency doubler takes a single sinusoidal input voltage and produces a sinusoidal output voltage whose frequency is twice as large as the input frequency. A real practical frequency doubler typically involves a sophisticated circuit either with discrete components or in the form of an integrated circuit. In many situations like system design, you simply need a "black box" that exhibits the behavior of a frequency doubler just to try out and verify a system concept. You would worry about the hardware implementation of your block at a later time after the proof of concept has been established. [[RF.Spice]]'s virtual blocks will prove very useful when you design complex systems that contain many subcircuits and subsystems.
One of the great strengths of [[RF.Spice A/D]] is the ease of creating new user defined models. Most of [[RF.Spice]]'s virtual blocks are indeed based on preloaded parameterized subcircuit models. You can open the Netlist code of these subcircuits, copy or modify them and package them into your own new virtual blocks. You can even convert an entire circuit into a model complete with your now symbol. An important difference between black-box virtual blocks and models is that virtual blocks typically use ideal devices and a combination of linear and nonlinear sources to minimize circuit complexity regardless of the feasibility of their hardware realization. In other words, they primarily represent a functional behavior rather than a practical circuit.
== The Stages of a Macromodel Virtual Block ==