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MathGroup Archive 2005

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RE: Making a phase plot

  • To: mathgroup at
  • Subject: [mg62530] RE: [mg62516] Making a phase plot
  • From: "David Park" <djmp at>
  • Date: Sun, 27 Nov 2005 02:40:06 -0500 (EST)
  • Sender: owner-wri-mathgroup at


The DrawGraphics package contains a BodePlot routine in the Examples
section. It allows the breakpoint for ArcTan to be specified.

Be aware that there is a problem with LogLinearPlot, and also LogLogPlot, in
that the automatic point sampling is performed on a linear scale and not on
a log scale. If you are plotting more than a few decades this causes serious
undersampling of the low points and will result in an incorrect curve. It is
difficult to overcome this simply by increasing the PlotPoints. The BodePlot
corrects this and the Example section shows how to correct this in general.

David Park
djmp at

From: Milind Gupta [mailto:milind.gupta at]
To: mathgroup at

     I wanted to plot the phase of a transfer function. I was doing it like

LogLinearPlot[Phase[w], {w, 0, wmax}]

    Where Phase is calculated using the ArcTan function. The problem is tha=
the ArcTan function rounds up the result to positive values when the phase
goes below -pi. But I want the phase to keep going negative values upto
infinity. Is there a way to do this using some standard functions?


Milind <Milind at>

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