matlab fft

dsa

Quote:

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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

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try this:

Code:

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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

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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.

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

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

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Have you tried executing this code?

.

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

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

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

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.

.