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Hi,a question regarding Fourier transform and energy of a signals

  1. Oct 1, 2013 #1
    Somehow I have really hard time wrapping my head around the concept.I mean,I get it,but I can't seem to solve any questions regarding it.

    Here are some examples ,and I just get stuck.Its a part of test,so I think it shouldn't be that hard to solve,and if it looks hard,I know there are some tricks but I just don't know them.


    1)Need to find energy of a signal (why not power,and only energy)?

    2)Need to find energy and power of signal,and the third is the same.

    So,I know that energy of decaying signals is just integral of squared of the function over time.
    But that is one nasty integral,and as it is from exam,I understand there should be something trick that makes it more tolerable to solve.
    Now I know that it is possible using Parsevel theorem to do the same integral in frequency domain,and some time it is easier,but how do I transform that function ? (I think we can use some ready transformation tables).

    2 and 3,I'm just lost.I know I need to find some coeficients ,square them and sum them up,to get the power.How on earth I find them?
    Last edited: Oct 1, 2013
  2. jcsd
  3. Oct 1, 2013 #2


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    1) if the energy is finite then you can find it; else you must work with power, the rate of energy flow.

    Parseval's theorem has several corollaries which should be in your text book or course notes, based on convolutions.

    Here are some applicable notes: http://ocw.usu.edu/electrical_and_computer_engineering/signals_and_systems/5_10node10.html [Broken]
    Last edited by a moderator: May 6, 2017
  4. Oct 3, 2013 #3
    Ok,so I understood how to solve first one using Parseval theorem - I transformed the signal from time to w using "usefull transformation" formula page .
    Second one : I did integral from 0-8 seconds to find the energy.Which is just (2^2)*2+(1^2)*1=9
    The power is just energy divided by its period time = 9/8 ?

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