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Nyquist - shannon sampling theorem example

  1. Sep 4, 2011 #1
    1. The problem statement, all variables and given/known data
    What sampling frequency would you use to sample the following signal

    x(t) = 3sin(9πt) - 6cos(8πt)


    2. Relevant equations


    3. The attempt at a solution

    let T1 be the period of 3sin(9πt) and T2 the period of 6cos(8πt)

    T1 = 2π/9π = 2/9
    Τ2 = 2π/8π = 2/8

    thus, if f1 is the frequency of 3sin(9πt) and f2 the frequency of 6cos(8πt) we have

    f1 = 1/T1 = 9/2 = 4.5
    f2 = 1/T2 = 8/2 = 4

    now about the niquist rate,

    I'm not sure whether it will be 9(2*f1, since f1>f2) or the frequency that is 2 times larger than the overall frequency of x(t)

    can you please remind this to me? thanks
     
  2. jcsd
  3. Sep 5, 2011 #2

    lewando

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    Gold Member

    Re: niquist - shannon sampling theorem example

    Not sure what you mean by the "overall frequency of x(t)".

    When you sample at the Nyquist frequency, you need to sample at twice the highest frequency component of the signal.
     
  4. Sep 5, 2011 #3
    Re: niquist - shannon sampling theorem example

    thanks lewando

    What I was meaning is that

    x(t) may have two components but the overall frequency is the least common multiple of the frequencies of each of these 2 components right? so the result will be different

    thus if I understand correctly the result should be 2*f1 = 9 because f1 > f2 right?
     
  5. Sep 5, 2011 #4

    lewando

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    Gold Member

    Re: niquist - shannon sampling theorem example

    Honestly, I have never seen the term "overall frequency", as you have defined, used anywhere.

    Right! :smile:
     
  6. Sep 5, 2011 #5
    Re: niquist - shannon sampling theorem example

    thanks a lot ;)
     
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