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I want to draw by hand the Nyquist plot of an RC filter's transfer function. I do not want to use the "tricks" that helps building it faster (evaluating just some limits points), I want to draw it like another function , namely I have x values (real part) and compute y values (imaginary part) . Ultimately I want to use Desmos graphing calculator.

My imaginary part is $$-\frac{\omega RC}{(1+(\omega RC)^2}$$ and real part is $$\frac{1}{1+(\omega RC)^2}$$ with \$\omega =2\pi f\$. The Nyquist plot plots the Imaginary part with Real part as a variable Should I replace \$\omega\$ in ]Im part with Real part, meaning $$\mathrm{Im}(\omega) = -\mathrm{Re}(\omega) \times \frac{RC}{1+(\mathrm{Re}(\omega)RC)^2}$$? I have tried that, it is not similar with the Matlab or Maple Nyquist plot.

This is what I get with Desmos:

enter image description here

How do I obtain this plot by "standard" function plotting techniques?

enter image description here

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    \$\begingroup\$ check this example desmos.com/calculator/sprwnkggss, you can plot the nyquist plot by adding a graph (r_e(t), i_m(t)), and you would have to give t the range of frequencies you want to plot, If you want the full nyquist plot that would be -Inf+Inf; as a recommendation however I would suggest you to learn matlab/python/octave or a language of choice, it is much more useful. In matlab you can simply do plot(re,im), you even have toolboxes which you can pass the transfer function and will do the nyquist plot \$\endgroup\$
    – MPA95
    Jan 2 at 16:59
  • \$\begingroup\$ Thank you. I did not want to use a tool because I have not understood how to plot this by myself. Now I know. I use either Maple, Matlab or Desmos for validation \$\endgroup\$
    – fjohn
    Jan 2 at 17:30
  • \$\begingroup\$ You're not really drawing it by hand if you're using Desmos, are you? \$\endgroup\$
    – Hearth
    Jan 6 at 4:45

2 Answers 2

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Your RC lowpass filter has the transfer function \$H(s) = \frac{1}{RCs+1} \$ and the frequency characteristic \$H(j\omega) = \frac{1}{j\omega RC+1} \$.

For the sake of understanding, let's say \$R=10\text{k}\Omega \$, \$C=27\mu\text{F}\$. So the frequency characteristic becomes $$H(j\omega) = \frac{1}{j\omega \cdot10\text{k}\Omega \cdot 27\mu\text{F}+1}=\frac{1}{\frac{j\omega}{3.70}+1} $$ From this, you can draw a straight line approximation of a Bode Plot, because we can read from \$H(j\omega) \$ that the cut-off frequency is \$3.70 \: \text{rad/s}\$.

enter image description here

Observations:

  1. At DC, the Bode Plot shows a gain of \$1 \frac{\text{V}}{\text{V}}\$ and \$0^\circ\$ phase shift \$\rightarrow\$ The Nyquist Plot starts at \$(1,0)\$.
  2. At \$0.37 \: \text{rad/s}\$ a linear negative phase shift starts but the gain is still \$ 1 \frac{\text{V}}{\text{V}}\$ \$\rightarrow\$ The Nyquist Plot goes clock-wise and maintains the same distance to the origin.
  3. At \$3.70 \: \text{rad/s}\$ the gain starts decreasing by 20dB/decade and the phase is still decreasing at the same rate \$\rightarrow\$ The distance to the origin in the Nyquist Plot is getting shorter and the plot still moves in the clock-wise direction.
  4. At \$37 \: \text{rad/s}\$ the phase is \$-90^\circ\$ and the gain is forever rolling off \$\rightarrow\$ The Nyquist Plot approaches the origin fast.

Below is a drawing is an approximate drawing I made in paint.net (I apologize for the bad penmanship), next to a plot of the Nyquist Plot made with MATLAB: -

enter image description here

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I managed to generate the plot with Excel and desmos. Excel :

  • in one column generetate omega data = 2pif, f = between 0 and 10000 (or whatever you want)
  • in 2nd column compute values for Re(f), for each f you have choosen
  • in 3rd column compute values for Im(f)
  • select 2nd and 3rd column, insert chart > XY chart.

Excel will plot each pair Re(f) - Im(f)

Desmos: https://www.desmos.com/calculator/iptojz0tpm

  • you can plot one expresion against another with the syntax (exp1(x), exp2(x)), min<x<max
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