1. ## matlab fft

dsa

2. 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.

3. 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

4. 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?

.

5. 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

6. ur code does not work anyhow
Undefined function or variable 'dt'.
why?

7. 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

8. 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.

.