The Frequency Response of a Time-Delay System 

The NyquistPlot of a model with an internal time delay .

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X Manipulate[ NyquistPlot[ TransferFunctionModel[ Unevaluated[{{500/(-500 k SystemsModelDelay[\[Tau]] - s)}}], s, SamplingPeriod ->None, SystemsModelLabels -> None], MaxRecursion -> 12, PlotRange -> {{-2, 2}, {-2, 2}}], {k, 0.5, 3}, {\[Tau], 0.005, 0.02}, ContinuousAction -> False]

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