# matlab fft

• Jan 12th 2009, 03:44 AM
qwerty321
matlab fft
dsa
• Jan 12th 2009, 06:28 AM
Constatine11
Quote:

Originally Posted by qwerty321
Hello

can someone help me with this assignement?

Find in MATLAB the frequency components in the following
signal:

y
(t) = (cos(300πt) + sin(500πt))3

Use

.^3 (with the dot) in MATLAB to calculate the third power.

how do I do this?

thank you

try this:

Code:

```fs=2000;  %sampling frequency dt=1/fs;  %sampling interval t=0:dt:1-dt/2; y=(cos(300*pi*t)+cos(500*pi*t)).^3;  %1 seconds worth ofsignal sp=fft(y);      %fft spp=abs(sp);  %absolute value of frequency component               %(you could square this as well to get something like a power) df=1;            %frequency resolution (in Hz) f=0:df:999;    %frequency scale for spectrum spp=spp(1:1000); %cut out the part of spectrum below Nyquist frequenct plot(f,spp);    %plot```
This is not normalised (since the normalisation is different for different FFT implementations). It also gives an absolute amplitude plot. I would prefere a dB plot re arbitary myself.
• Jan 12th 2009, 09:15 AM
qwerty321
ok..someone told me I could also use this:
Y = fft(y,512);
Pyy = Y.* conj(Y)/512;
f = 20000*(0:256)/512;
plot(f,Pyy(1:257))
title('Frequency content of y')
xlabel('frequency (Hz)')

can you expalin to me what each one does?

y is a function
• Jan 12th 2009, 12:34 PM
Constatine11
Quote:

Originally Posted by qwerty321
ok..someone told me I could also use this:
Y = fft(y,512);
Pyy = Y.* conj(Y)/512;
f = 20000*(0:256)/512;
plot(f,Pyy(1:257))
title('Frequency content of y')

xlabel('frequency (Hz)')

can you expalin to me what each one does?

y is a function

Have you tried executing this code?

.
• Jan 13th 2009, 05:58 AM
qwerty321
well in fact this code was use to find the frequency components of a signal buried inside a noisy time domain signal using the fft function in MATLAB.
the assignement says :
Using the same technique, find in MATLAB the frequency components in the following
signal:
y(t) = (cos(300πt) + sin(500πt))^3

so i think i need to use the same code
thank you
• Jan 13th 2009, 06:24 AM
qwerty321
ur code does not work anyhow
Undefined function or variable 'dt'.
why?
• Jan 13th 2009, 06:35 AM
qwerty321
btw is there a command that can clear the command window?becauase each time i want to write something new i have to exit then reenter it
• Jan 13th 2009, 07:11 AM
Constatine11
Quote:

Originally Posted by qwerty321
ur code does not work anyhow
Undefined function or variable 'dt'.
why?

Because of an error in copying it over, the previous line's comment where it says:

"dt=1/fs;"

should have been on a new line as a command.

.