例2:从周期信号取单脉冲得到 Jf(t)· e Jno dt Et noT 2 f(t) E T f() F(O)=F,71 O=nO ET T 2丌 2丌 0 F(o)=ETAl 28 例2：从周期信号取单脉冲得到 −T1 T1 E f (t) t Fn  E ( ) 0 f t 0 2  2  − t 0 1 1 ( ) F  = Fn T =n   2  2 − 0 FT       =  = − − 2 ( ) 1 1 1 2 2 1 1 1 1  n Sa T E f t e dt T F T T j n t n       = 2 ( )  F  ESa E