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Fourier transform for Discrete signal

  1. Feb 26, 2006 #1
    let us asuume this discrete signal:

    f(n)=a^n * u(n) ; where u(n) is unit step function
    ; u(n)=1 where n>=0
    u(n)=0 where n<0
    ;0=<a<1
    and the foruier transform for discrete signals is defined as :
    F(i)=sum ( f(n)*e^(-j2*pi*n/N) ;n=0 to inifinity

    i know that the sum is equal to:

    F(i)=sum(1/(1-a*e^-j2*pi*n/N)

    but actually i don't know why! could anyone help!


    thanks in advance!
     
  2. jcsd
  3. Feb 26, 2006 #2

    George Jones

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    Hint: think infinite geometric series. In this example, what is the common ratio r?

    Regards,
    George
     
  4. May 31, 2006 #3
    what do you mean by common ratio r?
     
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