第二章 离散傅里叶变换(DFT) 1. 设x(n)=R3(n) 求)(~kX,并作图表示)(~ nx,)(~kX。 解:102)(~)(~NnknNjenxkX =)7sin()73sin(722072kkeekjnknj )(~ nx -7 1 2 7 8 9 n |)(~kX| k )7()(~rnxnx2.设 求:)(~ nx,)(~ ny的周期卷积序列)(~ nf,以及)(~ kF。 解: rrnfnf)7()(~ )6(3)5(2)4()3(0)2()1(2)(3)(nnnnnnnnf )7sin()73sin()(~)7sin()74sin()(~7106472733072kkeekYkkeekXkjnknjkjnknj )7(sin)73sin()74sin()(~)(~)(~2713kkkekYkXkFkj 2. 用封闭形式表达以下有限长序列的DFT[x(n)]。 解: (1) nnny其他,064,1)(rrnxnx)7()(~rrnyny)7()(~nnnx其它,030,1)()()(0nRenxNnj X(k)=DFT[x(n)] )()2s i n ()2s i n ()(11)(00)21(100000kRNkNekRWeWekRWeNNkNjNkNjkNNNjNnNknNnj (2) )(]11211121[)]([)()]()([21)]([][cos)(cos)(0000000kRWeeWeenxDFTkXkXkXnxReRnnnRnxNkNjNjkNjNjenjeN有:由关系: )(c o s21)1c o s (c o sc o s120000kRWWNWWNNkNkNkNkN (3) )]()([21)](Im[]Im[sin)(sin)(000kXkXjnxennnRnxnjN由关系: 有:X(k)=DFT[x(n)] )(]11211121[)(0000kRWeejWeejkXNkNjNjkNjNj )(c o s21)1s i n (s i ns i n20000kRWWWNNWNkNkNkNkN (4) )()sin(2)()1()()()()()2(10kRkNNekRWNkRnWkXnnRnxNkNjNkNNnNknNN 4.已知以下X(k),求IDFT[X(k)],其中m 为某一正整数,0