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Mentor's ModelSim HDL simulator has a GUI that is largely written in Tk. The ModelSim command console runs a Tcl/Tk shell, so that you can run your own Tcl scripts from within the simulator. It's also possible to modify the simulator's GUI using Tk.
Many communications-related functions involve modulation of a carrier signal both in phase and in amplitude. After demodulation, the signal is usually represented as an in-phase and a quadrature component, representing the amplitude of cosine and sine components respectively. It is very helpful to view these signals on a so-called "constellation" diagram, also known as a polar plot or Argand diagram - a 2-dimensional representation with in-phase on the X axis and quadrature on the Y axis. At any instant, the signal's two values can be represented by a point on this X/Y plot.
In a digital comms system, the in-phase and quadrature components are probably represented in multi-bit digital values. ModelSim's waveform display can easily display each of these values in a convenient analog format, giving an oscilloscope-like display. Our constellation plot add-in is an example of how to add custom GUI functionality to ModelSim, providing a display that's not possible in the standard simulator product.