Fourier Transform with both Time Delay and Frequency Shift

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$begingroup$


I know that the Fourier transform of a function with time delay can be written as: $$mathscrFbigx(t-t_0)big=X(f)e^-j2pi f t_0$$



The Fourier transform of a function with frequency shift can also be written as: $$mathscrFBigx(t)e^j2pi f_0 tBig=X(f-f_0)$$



So what if we have both shift and delay at the time domain, what will be the result in the frequency domain? E.g.:



$$mathscrFBigx(t-t_0)e^j2pi f_0 (t-t_0)Big$$



Will the result be: $$X(f-f_0)e^-j 2 pi f (t-t_0)$$



Also what will be the result of:



$$mathscrFBigx(t-t_0)e^j2pi f_0 t)Big$$



Is there an order to apply these properties?










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    it's really nice to see new contributors use $LaTeX$. thanks.
    $endgroup$
    – robert bristow-johnson
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4












$begingroup$


I know that the Fourier transform of a function with time delay can be written as: $$mathscrFbigx(t-t_0)big=X(f)e^-j2pi f t_0$$



The Fourier transform of a function with frequency shift can also be written as: $$mathscrFBigx(t)e^j2pi f_0 tBig=X(f-f_0)$$



So what if we have both shift and delay at the time domain, what will be the result in the frequency domain? E.g.:



$$mathscrFBigx(t-t_0)e^j2pi f_0 (t-t_0)Big$$



Will the result be: $$X(f-f_0)e^-j 2 pi f (t-t_0)$$



Also what will be the result of:



$$mathscrFBigx(t-t_0)e^j2pi f_0 t)Big$$



Is there an order to apply these properties?










share|improve this question











$endgroup$







  • 1




    $begingroup$
    it's really nice to see new contributors use $LaTeX$. thanks.
    $endgroup$
    – robert bristow-johnson
    Feb 5 at 20:37













4












4








4


2



$begingroup$


I know that the Fourier transform of a function with time delay can be written as: $$mathscrFbigx(t-t_0)big=X(f)e^-j2pi f t_0$$



The Fourier transform of a function with frequency shift can also be written as: $$mathscrFBigx(t)e^j2pi f_0 tBig=X(f-f_0)$$



So what if we have both shift and delay at the time domain, what will be the result in the frequency domain? E.g.:



$$mathscrFBigx(t-t_0)e^j2pi f_0 (t-t_0)Big$$



Will the result be: $$X(f-f_0)e^-j 2 pi f (t-t_0)$$



Also what will be the result of:



$$mathscrFBigx(t-t_0)e^j2pi f_0 t)Big$$



Is there an order to apply these properties?










share|improve this question











$endgroup$




I know that the Fourier transform of a function with time delay can be written as: $$mathscrFbigx(t-t_0)big=X(f)e^-j2pi f t_0$$



The Fourier transform of a function with frequency shift can also be written as: $$mathscrFBigx(t)e^j2pi f_0 tBig=X(f-f_0)$$



So what if we have both shift and delay at the time domain, what will be the result in the frequency domain? E.g.:



$$mathscrFBigx(t-t_0)e^j2pi f_0 (t-t_0)Big$$



Will the result be: $$X(f-f_0)e^-j 2 pi f (t-t_0)$$



Also what will be the result of:



$$mathscrFBigx(t-t_0)e^j2pi f_0 t)Big$$



Is there an order to apply these properties?







fourier-transform time-frequency






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edited Feb 5 at 20:36









robert bristow-johnson

10.9k31548




10.9k31548










asked Feb 5 at 13:08









tamunotamuno

235




235







  • 1




    $begingroup$
    it's really nice to see new contributors use $LaTeX$. thanks.
    $endgroup$
    – robert bristow-johnson
    Feb 5 at 20:37












  • 1




    $begingroup$
    it's really nice to see new contributors use $LaTeX$. thanks.
    $endgroup$
    – robert bristow-johnson
    Feb 5 at 20:37







1




1




$begingroup$
it's really nice to see new contributors use $LaTeX$. thanks.
$endgroup$
– robert bristow-johnson
Feb 5 at 20:37




$begingroup$
it's really nice to see new contributors use $LaTeX$. thanks.
$endgroup$
– robert bristow-johnson
Feb 5 at 20:37










2 Answers
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1












$begingroup$

As an alternative to going back to the definitions, as explained in Andy Walls' answer, you can also just apply the rules as you stated them:



$$mathcalFleftx(t-t_0)e^j2pi f_0(t-t_0)right=mathcalFleftx(t)e^j2pi f_0trighte^-j2pi ft_0=X(f-f_0)e^-j2pi ft_0$$



where $X(f)$ is the Fourier transform of $x(t)$.



And, for your second example, with $tildeX(f)=mathcalFx(t-t_0)=X(f)e^-j2pi ft_0$ you get



$$mathcalFleftx(t-t_0)e^j2pi f_0tright=tildeX(f-f_0)=X(f-f_0)e^-j2pi (f-f_0)t_0$$



Of course, recognizing that the function in the second example just equals the first function scaled by $e^j2pi f_0t_0$, we could have written down its Fourier transform directly by scaling the Fourier transform of the first function by the same factor.






share|improve this answer









$endgroup$




















    5












    $begingroup$

    If you are ever unsure, just go back to the definition and work out the Fourier Transform property for the specific situation:



    $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0left(t-t_0right)right &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0left(t-t_0right) e^-j2pi f tdt\
    \
    &= int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f left(tau+t_0right)dtau \
    \
    &= e^-j2pi ft_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
    \
    &= e^-j2pi ft_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
    \
    &= e^-j2pi ft_0 X(f-f_0)\
    \
    endalign*$$



    $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0 tright &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0t e^-j2pi f tdt\
    \
    &= int_-infty^infty xleft(tauright)e^j2pi f_0(tau+t_0) e^-j2pi f left(tau+t_0right)dtau \
    \
    &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
    \
    &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
    \
    &= e^-j2pi (f-f_0)t_0 X(f-f_0)\
    \
    endalign*$$






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      2 Answers
      2






      active

      oldest

      votes








      2 Answers
      2






      active

      oldest

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      active

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      votes






      active

      oldest

      votes









      1












      $begingroup$

      As an alternative to going back to the definitions, as explained in Andy Walls' answer, you can also just apply the rules as you stated them:



      $$mathcalFleftx(t-t_0)e^j2pi f_0(t-t_0)right=mathcalFleftx(t)e^j2pi f_0trighte^-j2pi ft_0=X(f-f_0)e^-j2pi ft_0$$



      where $X(f)$ is the Fourier transform of $x(t)$.



      And, for your second example, with $tildeX(f)=mathcalFx(t-t_0)=X(f)e^-j2pi ft_0$ you get



      $$mathcalFleftx(t-t_0)e^j2pi f_0tright=tildeX(f-f_0)=X(f-f_0)e^-j2pi (f-f_0)t_0$$



      Of course, recognizing that the function in the second example just equals the first function scaled by $e^j2pi f_0t_0$, we could have written down its Fourier transform directly by scaling the Fourier transform of the first function by the same factor.






      share|improve this answer









      $endgroup$

















        1












        $begingroup$

        As an alternative to going back to the definitions, as explained in Andy Walls' answer, you can also just apply the rules as you stated them:



        $$mathcalFleftx(t-t_0)e^j2pi f_0(t-t_0)right=mathcalFleftx(t)e^j2pi f_0trighte^-j2pi ft_0=X(f-f_0)e^-j2pi ft_0$$



        where $X(f)$ is the Fourier transform of $x(t)$.



        And, for your second example, with $tildeX(f)=mathcalFx(t-t_0)=X(f)e^-j2pi ft_0$ you get



        $$mathcalFleftx(t-t_0)e^j2pi f_0tright=tildeX(f-f_0)=X(f-f_0)e^-j2pi (f-f_0)t_0$$



        Of course, recognizing that the function in the second example just equals the first function scaled by $e^j2pi f_0t_0$, we could have written down its Fourier transform directly by scaling the Fourier transform of the first function by the same factor.






        share|improve this answer









        $endgroup$















          1












          1








          1





          $begingroup$

          As an alternative to going back to the definitions, as explained in Andy Walls' answer, you can also just apply the rules as you stated them:



          $$mathcalFleftx(t-t_0)e^j2pi f_0(t-t_0)right=mathcalFleftx(t)e^j2pi f_0trighte^-j2pi ft_0=X(f-f_0)e^-j2pi ft_0$$



          where $X(f)$ is the Fourier transform of $x(t)$.



          And, for your second example, with $tildeX(f)=mathcalFx(t-t_0)=X(f)e^-j2pi ft_0$ you get



          $$mathcalFleftx(t-t_0)e^j2pi f_0tright=tildeX(f-f_0)=X(f-f_0)e^-j2pi (f-f_0)t_0$$



          Of course, recognizing that the function in the second example just equals the first function scaled by $e^j2pi f_0t_0$, we could have written down its Fourier transform directly by scaling the Fourier transform of the first function by the same factor.






          share|improve this answer









          $endgroup$



          As an alternative to going back to the definitions, as explained in Andy Walls' answer, you can also just apply the rules as you stated them:



          $$mathcalFleftx(t-t_0)e^j2pi f_0(t-t_0)right=mathcalFleftx(t)e^j2pi f_0trighte^-j2pi ft_0=X(f-f_0)e^-j2pi ft_0$$



          where $X(f)$ is the Fourier transform of $x(t)$.



          And, for your second example, with $tildeX(f)=mathcalFx(t-t_0)=X(f)e^-j2pi ft_0$ you get



          $$mathcalFleftx(t-t_0)e^j2pi f_0tright=tildeX(f-f_0)=X(f-f_0)e^-j2pi (f-f_0)t_0$$



          Of course, recognizing that the function in the second example just equals the first function scaled by $e^j2pi f_0t_0$, we could have written down its Fourier transform directly by scaling the Fourier transform of the first function by the same factor.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Feb 5 at 17:15









          Matt L.Matt L.

          50.6k23889




          50.6k23889





















              5












              $begingroup$

              If you are ever unsure, just go back to the definition and work out the Fourier Transform property for the specific situation:



              $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0left(t-t_0right)right &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0left(t-t_0right) e^-j2pi f tdt\
              \
              &= int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f left(tau+t_0right)dtau \
              \
              &= e^-j2pi ft_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
              \
              &= e^-j2pi ft_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
              \
              &= e^-j2pi ft_0 X(f-f_0)\
              \
              endalign*$$



              $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0 tright &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0t e^-j2pi f tdt\
              \
              &= int_-infty^infty xleft(tauright)e^j2pi f_0(tau+t_0) e^-j2pi f left(tau+t_0right)dtau \
              \
              &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
              \
              &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
              \
              &= e^-j2pi (f-f_0)t_0 X(f-f_0)\
              \
              endalign*$$






              share|improve this answer









              $endgroup$

















                5












                $begingroup$

                If you are ever unsure, just go back to the definition and work out the Fourier Transform property for the specific situation:



                $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0left(t-t_0right)right &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0left(t-t_0right) e^-j2pi f tdt\
                \
                &= int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f left(tau+t_0right)dtau \
                \
                &= e^-j2pi ft_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                \
                &= e^-j2pi ft_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                \
                &= e^-j2pi ft_0 X(f-f_0)\
                \
                endalign*$$



                $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0 tright &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0t e^-j2pi f tdt\
                \
                &= int_-infty^infty xleft(tauright)e^j2pi f_0(tau+t_0) e^-j2pi f left(tau+t_0right)dtau \
                \
                &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                \
                &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                \
                &= e^-j2pi (f-f_0)t_0 X(f-f_0)\
                \
                endalign*$$






                share|improve this answer









                $endgroup$















                  5












                  5








                  5





                  $begingroup$

                  If you are ever unsure, just go back to the definition and work out the Fourier Transform property for the specific situation:



                  $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0left(t-t_0right)right &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0left(t-t_0right) e^-j2pi f tdt\
                  \
                  &= int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f left(tau+t_0right)dtau \
                  \
                  &= e^-j2pi ft_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                  \
                  &= e^-j2pi ft_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                  \
                  &= e^-j2pi ft_0 X(f-f_0)\
                  \
                  endalign*$$



                  $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0 tright &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0t e^-j2pi f tdt\
                  \
                  &= int_-infty^infty xleft(tauright)e^j2pi f_0(tau+t_0) e^-j2pi f left(tau+t_0right)dtau \
                  \
                  &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                  \
                  &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                  \
                  &= e^-j2pi (f-f_0)t_0 X(f-f_0)\
                  \
                  endalign*$$






                  share|improve this answer









                  $endgroup$



                  If you are ever unsure, just go back to the definition and work out the Fourier Transform property for the specific situation:



                  $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0left(t-t_0right)right &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0left(t-t_0right) e^-j2pi f tdt\
                  \
                  &= int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f left(tau+t_0right)dtau \
                  \
                  &= e^-j2pi ft_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                  \
                  &= e^-j2pi ft_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                  \
                  &= e^-j2pi ft_0 X(f-f_0)\
                  \
                  endalign*$$



                  $$beginalign*mathscrFleftxleft(t-t_0right)e^j2pi f_0 tright &= int_-infty^infty xleft(t-t_0right)e^j2pi f_0t e^-j2pi f tdt\
                  \
                  &= int_-infty^infty xleft(tauright)e^j2pi f_0(tau+t_0) e^-j2pi f left(tau+t_0right)dtau \
                  \
                  &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright)e^j2pi f_0tau e^-j2pi f taudtau \
                  \
                  &= e^-j2pi (f-f_0)t_0int_-infty^infty xleft(tauright) e^-j2pi (f-f_0) taudtau \
                  \
                  &= e^-j2pi (f-f_0)t_0 X(f-f_0)\
                  \
                  endalign*$$







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Feb 5 at 14:20









                  Andy WallsAndy Walls

                  1,459127




                  1,459127



























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