Plot a sine wave with decreasing frequency over time

Hi! I'm trying to plot a sine wave whose frequency decreases linearly over time (amplitude stays the same). This is my code:

clear all;
close all;
clc
period = 0.08;
for x = 0:4/1000:2
    a = 5;
    b = ((2*pi)/(period + 0.001));
    c = 300;
    d = 30;
    c = c * -1;

     output = a.*sin(b.*(x+c))+d;
    plot(x, output, 'Linewidth', 2);
end

When I run the code, my plot is blank. I know Matlab doesn't require the use of a "for" loop when plotting sinusoids, however I'm unsure of how else I can modify the frequency over time if I don't use a "for" loop. Any help is much appreciated!

Answer 1

[Eike Blechschmidt]

How about

f_upper = 50;
f_lower = 10;
t = 0:0.01:10;
f = linspace(f_upper, f_lower, numel(t));
a = 10;
x = a * sin(2*pi*f.*t);
plot(t,x)

[darova]

DOesn't work well

f_upper = 5;
f_lower = 1;
t = 0:0.01:10;
f = linspace(f_upper, f_lower, numel(t));
a = 10;
x = a * sin(2*pi*f.*t);
plot(t,x)

Answer 2

[darova]

Modify x coordinate

x = 0:.1:30;
y = sin(x);
x1 = x - 10*(x/max(x)).^2;  % shift last point 10 units
plot(x1,y)

[Eike Blechschmidt]

How do you know that your result is still a sin wave?

[darova]

It's not a sive wave anymore

[Alison Huffman]

@darova @Eike Blechschmidt Why is it not a sine wave?

[darova]

Because requency changes i think

[Eike Blechschmidt]

@darova changed the x-axis after calculating the sin. So I'm unsure (as in "I have not mathematically checked") if that solution is still expressable with a single sin function (you can always find multiple sin-waves to express any kind of time signal -> fourier transformation) which was my understanding of "is it still a sin function". My first solution does that as I only manipulate the input to the sin function. So I guess my question was not precise.

Answer 3

[Eike Blechschmidt]

The easiest and correct is probably to use:

f1 = 50;
f2 = 10;
t = 0:0.001:10;
y = chirp(t,f1,t(end),f2);
plot(t, y);

[Eike Blechschmidt]

A quick comparison of all answers:

[Alison Huffman]

@Eike Blechschmidt Thank you very much, this is helpful. What calculation does the chirp function do? Is it just modifying the period of the sine wave?

[Eike Blechschmidt]

Take a look at: Swept-frequency cosine - MATLAB chirp (mathworks.com).

You can also do

edit chirp

and scroll downt to

function yvalue = calculateChirp(f0,f1,t1,p,t,phi,isComplex)

It shows quite clear how the chirp is calculated.