Help with plotting

Part I Basic Plotting The RC circuit shown below forms a low pass filter. The transfer function of this filter is given as

 Vout / Vin = (1 + j?RC ) -1

Picture here of simple Capacitor Circuit R=50Ohms and C=1.0*10^-9 Farads

Plot the transfer function over a frequency range from 10 kHz to 10 MHz.

Case A. Use a linearly spaced frequency vector.

• Add labels to the x- and y-axis; include units here!
• Add a title to the figure.
• Add a horizontal line at the value Vout/Vin = 0.5.
• Add an arrow pointing to the intersection of the two lines, insert text near the arrow to explain.
• Turn the grid on.
• Adjust all line widths and text sizes to make them easily visible.
• Collect these commands into a “dot M” file.
• Execute the file to produce the plot, submit the plot with your report.

I have developed this incoplete code Answered a certain amount of the problem but my problem is my plot errors in which I do not know how to fix.

x = linspace(10000, 10000000);
w=2*pi*x;
c=1.0*10^-9;
r=50;
y=(1+i*w*c*r).^-1
plot(x,y);
title( 'Case A' );
xlabel( ' Linearly Spaced Frequency Domain' );
ylabel( ' V_in / V_out' );
grid on;

Also I need to learn how to do a vertical line and text arrow but the plot is the most important first.

Answer 1

[Walter Roberson]

Your formula for y involves constructing complex numbers and taking their reciprocal. The result for non-zero x is always going to be complex (otherwise there would have to be a real number whose reciprocal was a complex number.) Thus your y is going to be mostly complex. You cannot use plot(x,y) to plot a complex y.

You will need to decide what you want to plot. You might want to plot real(y) or imag(y) or abs(y).

If what you want is a 3D plot with the real y on one axis and the imaginary y on another axis, then you would use

plot3(x, real(y), imag(y))

Answer 2

[Sean de Wolski]

help annotation
help line
help grid

a few hints...