F()ae品n 1 -x2 2a√rt a-s 2ta e4a't 2Nπt 33 2 2 2 2 4 2 1 1 1 1 j e exp d 2 π 1 2 2 2 2π 2 x a t x a t ta a t t a 2 2 2 2 2 2 2 j ( ) 1 4 2 1 F e e e d 2π x x a t a t a t a t 2 2 4 1 e 2 π x a t a t