how to extend time after fft ?

I have a signal runs for 500 seconds ( 1:1:500 sec) after i did the fft, i want to go back to time domain and see how the signal looks like for 1000 Seconds (1:1:1000) ? Any advice how to extend the time ? Is it better to do it in frequency domain ? Is it simply to add the sines and cosines with loops ?

Thank you

Answer 1

[Vilnis Liepins]

You can try edft.m on fileexchange http://www.mathworks.com/matlabcentral/fileexchange/11020-extended-dft

ifft(edft(YourData,1500),1500)

Signal for 1000 sec will be ifft output 1:1000.

[omar thamer]

Thank you Vilnis, really helped a lot.Though, results not as expected for real signal but absolutely true for periodic computer generated one. I am wondering if you can advice on my signal which i attached here. As you can see a lot o discontinuities and no clear center frequency.

Any advice how to get periodicity to my signal ? I tried with STFT it also yields no results.

[omar thamer]

Its not working properly .. Have a look to my code:

t=1:1:400;
x=sin(2*pi*t*1/20);
X=edft(x,500);
Y=ifft(X,500);

the outcome for extended signal has different magnitude and frequency.

Answer 2

[Walter Roberson]

Try

ifft(fft(YourData), 1000)

[omar thamer]

it will be padded with zeros then and return a complex vector. Its not working

[Walter Roberson]

fft() assumes that its data is periodic, so to get 1000 seconds of output, ifft() to get the 500 second output and then replicate it.