Fourier Transforms of Special Functions http www google

Fourier Transforms of Special Functions http: //www. google. com/search? hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=unit+step+fourier+transform&spell=1 主講者：虞台文

Content l l l Introduction More on Impulse Function Fourier Transform Related to Impulse Function Fourier Transform of Some Special Functions Fourier Transform vs. Fourier Series

Introduction l Sufficient condition for the existence of a Fourier transform l That is, f(t) is absolutely integrable. However, the above condition is not the necessary one. l

Some Unabsolutely Integrable Functions: cos t, sin t, … l Unit Step Function: u(t). l Sinusoidal l Generalized – – Functions: Impulse Function (t); and Impulse Train.

Fourier Transforms of Special Functions More on Impulse Function

Dirac Delta Function and Also called unit impulse function. 0 t

Generalized Function l The value of delta function can also be defined in the sense of generalized function: (t): Test Function l l We shall never talk about the value of (t). Instead, we talk about the values of integrals involving (t).

Properties of Unit Impulse Function Pf) Write t as t + t 0

Properties of Unit Impulse Function Pf) Write t as t/a Consider a>0 Consider a<0

Properties of Unit Impulse Function Pf)

Properties of Unit Impulse Function Pf)

Properties of Unit Impulse Function

Generalized Derivatives The derivative f’(t) of an arbitrary generalized function f(t) is defined by: Show that this definition is consistent to the ordinary definition for the first derivative of a continuous function. =0

Derivatives of the -Function

Product Rule Pf)

Product Rule Pf)

Unit Step Function u(t) l Define u(t) 0 t

Derivative of the Unit Step Function l Show that

Derivative of the Unit Step Function (t) u(t) Derivative 0 t

Fourier Transforms of Special Functions Fourier Transform Related to Impulse Function

Fourier Transform for (t) F(j ) (t) 0 F t 1 0

Fourier Transform for (t) Show that The integration converges to in the sense of generalized function.

Fourier Transform for (t) Show that Converges to (t) in the sense of generalized function.

Two Identities for (t) These two ordinary integrations themselves are meaningless. They converge to (t) in the sense of generalized function.

Shifted Impulse Function Use the fact |F(j )| (t t 0) F 0 t 1 0

Fourier Transforms of Special Functions Fourier Transform of a Some Special Functions

Fourier Transform of a Constant

Fourier Transform of a Constant F(j ) 0 A 2 ( ) F A t 0

Fourier Transform of Exponential Wave

Fourier Transforms of Sinusoidal Functions F(j ) ( + 0) ( 0) f(t)=cos 0 t t F 0 0 0

Fourier Transform of Unit Step Function Let F(j )=? Can you guess it?

Fourier Transform of Unit Step Function Guess 0 B( ) must be odd

Fourier Transform of Unit Step Function Guess 0

Fourier Transform of Unit Step Function Guess

Fourier Transform of Unit Step Function |F(j )| f(t) 1 0 F t ( ) 0

Fourier Transforms of Special Functions Fourier Transform vs. Fourier Series

Find the FT of a Periodic Function l Sufficient condition --- existence of FT l Any periodic function does not satisfy this condition. How to find its FT (in the sense of general function)? l

Find the FT of a Periodic Function We can express a periodic function f(t) as:

Find the FT of a Periodic Function We can express a periodic function f(t) as: The FT of a periodic function consists of a sequence of equidistant impulses located at the harmonic frequencies of the function.

Example: Impulse Train 3 T 2 T T 0 T 2 T 3 T t Find the FT of the impulse train.

Example: Impulse Train 3 T 2 T T 0 T 2 T 3 T t Find the FT of the impulse train. cn

Example: Impulse Train 3 T 2 T T 0 0 T 2 T 3 T t Find the FT of the impulse train. cn

Example: Impulse Train 3 T 2 T T 0 0 T 2 T 3 T 0 2 0 3 0 t F 2 /T 3 0 2 0 0 0

Find Fourier Series Using Fourier Transform f(t) T/2 t T/2 fo(t) T/2 t

Sampling the Fourier Transform of fo(t) with period 2 /T, we can find the Fourier Series of f (t). Find Fourier Series Using Fourier Transform f(t) T/2 t T/2 fo(t) T/2 t

Example: The Fourier Series of a Rectangular Wave f(t) 1 1 d 0 fo(t) t 0 t

Example: The Fourier Transform of a Rectangular Wave f(t) 1 d 0 t F [f(t)]=?