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Re: ParametricPlot problem

On 3/21/09 at 5:17 AM, Serych at (Serych Jakub) wrote:

>Dear M users, I'm trying to draw phasor characteristic of simple RC
>low pass filter. Its transfer function is:

>r = 100; c = 0.00001; transfer[f_] := 1/(1 + I 2 \[Pi] f r c);

>The phasor characteristic should be drawn with:

>ParametricPlot[{Re[transfer[f]], Im[transfer[f]]}, {f, 0, Infinity}]

>I can understand, that Mathematica has problem with calculation of
>the transfer function for frequency from 0 to Infinity, so I tried
>to use 10^8 in the place of infinity. But as the ParametricPlot IMHO
>subdivides the frequency range  into constant intervals, it draws
>piece of the curve only for frequencies near to 10^8. I found that
>the relatively reasonable result is with frequency range 0 to 10^3:

>ParametricPlot[{Re[transfer[f]], Im[transfer[f]]}, {f, 0, 10^3}]

>but there is missing the part of curve near to coordinates origin.

Are you sure something is missing? Looking at your transfer
function I see

In[15]:= {Re@#, Im@#} &@transfer[f] /. f -> 0

Out[15]= {1,0}


In[16]:= {Re@#, Im@#} &@transfer[f] /. f -> 1000

Out[16]= {0.0247045,-0.155223}

Looking at the plot, I see the right end of the curve at {1,0}
and a gap between where the axes cross and the left hand end of
the curve. But the left hand end does seem to be at
{0.0247045,-0.155223} which indicates Mathematica did plot the
curve over the entire range you specified. This is using version
7.01 on Mac OS X

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