Plotting functions in MATLAB

MATLAB has built in functions that allow one to generate x-y polar,contour and three-dimentional plots and bar charts.MATLAB also allows one to give title to graph,label the x and y axis, and a grid to graph.In addition, there are commands for controlling the screen and scalling.

X-Y Plot and annotations:

The plot command generates a linear-y plot. This command has three variations .

Plot(x)

Plot(x,y)

Plot(x1,y1,x2,y2,x3,y3,……..,x_n,y_n)

If x is a vector, the command plot(x) is used.

If x and y are vectors of the same length, then  the command plot(x,y) is used.

 Plot(x1,y1,x2,y2,x3,y3,……..,x_n,y_n)  is used for multiple arguments.

To write text on a graphic screen beginning at a point (x,y) on the graphic screen ,one can use the command text(x,y,’any text’)

Example:  

t=0:0.25:7;

y = sin(t);

plot(t,y)

Example:

Response of an RC Circuit:

Solution:

MATLAB Script

T=0:0.5:4;

Y=6*exp(-2*t);

Plot(t,y);

Title(‘Response of an RC Circuit: ’)

Xlabel(‘ Time in seconds’)

Ylabel(‘Voltage in volts’)

Grid

Voltage and current of an RL Circuit:

For an RL Circuit, the voltage v(t) and current i(t) are given as:

V(t)=10cos(377t)

I(t)=5cos(377t+60⁰)

Solution:

MATLAB Script

% RL Circuit

% current i(t) and voltage v(t) are generated

% t is time

T=0:1E-3:20E-3;

V=10*cos(377*t);

A_rad=(60*pi/180); % angle in radians.

I=5*cos(377*t+a_rad);

Plot(t,v,t,v’*’,t,I,t,I,’o’)

Title(‘Voltage and current in RL Circuit : ’)

Xlabel(‘ Sec’)

Ylabel(‘Voltage(v) and current(mA)’)

Text(0.003,1.5,’v(t));

Text(0.009,2,’i(t));

Ø  Logarithmic Plots:

Logarithmic and semi logarithmic plots can be generated using the commands loglog,semilog,semilogy.

The descriptions of these commands are as follows:

Loglog(x,y)-generate a plot of log₁₀(x) vs log₁₀(y).

Semilogx(x,y)- generate a plot of log₁₀(x) vs linear axis of y.

Semilogy(x,y)- generate a plot of linear axis of x vs log₁₀(y).

Example:

Frequency(Hz)	Gain(dB)	Frequency(Hz)	Gain(dB)
20	5	2000	34
40	10	5000	34
80	30	8000	34
100	32	10000	32
120	34	12000	30

Draw a graph of gain vs frequency using logarithmic scale.

Solution:

MATLAB Script

% Gain vs frequency plot

F=[20 40 80 100 120 2000 5000 8000 10000 12000];

G=[5 10 30 32 34 34 34  34 32 30];

Semilogx(f,g)

Title(‘gain vs frequency plot ’)

Xlabel(‘ Frequency in hertz ’)

Ylabel(‘gain in dB’)

Basic Task: Plot the function sin(x) between 0≤x≤4π

Create an x-array of 100 samples between 0 and 4π.

Calculate sin(.) of the x-array

Plot the y-array

Solution:

MATLAB Script

x=linspace(0,4*pi,100);

y=sin(x);

plot(y)