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Could anyone tell me how to use NyquistPlot with a first argument that is not a transfer function model? The expression I'd like to pass to NyquistPlot is Exp[z]Cos[z].

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What does the expression represent? A sinusoidal transfer function, discrete-time transfer function, something else? eg, try Off[TransferFunctionModel::npnd] NyquistPlot[Exp[z] Cos[z], SamplingPeriod -> 1] if it's a discrete-time system with sampling period 1. – Suba Thomas Nov 22 '13 at 15:23

Nyquist plots usually require rational functions (i.e., ratios of polynimials). I suppose if you know what you are doing, you could approximate this with a Taylor series expansion:

NyquistPlot[Normal[Series[Exp[z] Cos[z], {z, 0, 6}]]]  

enter image description here

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