典型非周期信号的频谱
?3.5 典型非周期信号的频谱
• 矩形脉冲; 单边指数信号; 直流信号; 符号函数
• 升余弦脉冲信号
•
一(矩形脉冲信号 ,, ,,ft,2jt,E2,j,t,,F,,Eedt,e ,,,,2,j,,2 E ,,j,,j,22,, ,Ee,esin,,t2., ,0,E,,2,2,,22j,, 2 ,,,,E,Sa, ,,2,,
,,幅度频谱: ,,,,SaF,E,,2
,,,,4n22n,1,,0,, ,,,,相位频谱: ,,,,,n,0,1,2,?,,,,,,,22n,122n,2 ,,,,,,, ,,,
频谱图
,,F, E,
,2,,
,O2,,4,,
幅度频谱
,,F, ,,,,,,SaF,E,,E, 2
频宽:
,21, O,2,,2,,4,,B或B,,,f ,,
相位频谱
,,,, ,
,2,,
,2,,4,,0 ,,
二(单边指数信号
,,t,,,Eet00,,,ft ,,,ft,E 0 t,0,
F,ft,,,,,F()
,t,tj,t,,O,,,Eeutedt ,,,
,,,,,,,tjEet,d ,0 E , ,,j,
频谱图 E,,F,,F,,,幅度频谱: 22E ,,,, E, ,,,,,0,F,,, ,
,,,,,,,,F,,0,, 0, ,1,,,,,,tg,,,,相位频谱: , ,,2 ,,,,,,,0,,0 ,,, ,0,,,,,,,,,,,, 2, ,,2,, ,,,,,,,,,,, 2,
三(直流信号 ,,ft不满足绝对可积 f(t),E,,,,t,,,E 条件~不能直接
用定义求 ,,,,E,2,E,,F, t 0
,,, ,,ft1 E
,,, tO
推导
, ,j,t,,F,,Eedt lim,,,,,F,,,, ,j,t ,,,e,E,,2,E ,,lim,j,,,,,,,,
,j,,j,,O, ,ee,E lim,j,,,, ,,E,2,E,, ,,2sin 时域无限宽,频带无限窄 ,Elim,,,,
,,,sin ,,E2lim ,,,,,,
sgn(t),,,2,E,, 1四(符号函数
,1,t,0, f(t),sgn,,t,,tO ,1,t,0, ,1处理
:做一个双边函数
,,t,ftsgnte~求F~,,,,,,, 11
,,求极限得到F,.
0,jj,t,t,t,t,,, ,,F,,,eedt,eedt1,,0,,
,,,11j2 ,,,22,,,,,,,,,jj
,j22, ,,,,,,F,F,,1limlim22 ,,j,,,,,,00
F(,)频谱图 ,j,2222 ,,t,,,,esgnj2 ,,,j
,,,2,,22,,,,,F,,,,,,,O,,,,,, ,,F,是偶函数,,,, 2,,2,,,,/2, 0,, ,1,tg, 0,/2,,,0,,O
,,,,是奇函数,,2
五(升余弦脉冲信号 ,,ftE ,,,Et,, ,,ft,1,cos 0,t,,,,,, 2,,,,,E
, 2,j,t,,,,F,,ftedt ,,, tO,,,,,,, ,,Et,,,,j,t,,1cos edt,,22,, ,,,,2,,,,
tt,,,jj,,, EEE,,,jtjtjt,,,,,,edt,eedt,eedt ,,,,,,,,,244
,,,,,,,,EE,,,, ,,,,,,,E,Sa,,Sa,,Sa,,,,,,,,,, 22,,,,,,,,,,
频谱图
,,,,,,,,,EEsinSa ,,,F,,22 ,,,,,,,,,,1,,,,1,,, ,,,,,,,,,, ,,
,,F, E,
E,
2
O,,,,, 234,,,,
其频谱比矩形脉冲更集中。
,,,Sa,,,,(,) lim,,,,
Sa(,,)
,,,Sa,,,,Sa,,
,
,1
,
,,,,,
,,O2
,
,, ,,,,,曲线下的面积 减小。 ,, , ,,,,,能量压缩到,0~面积仍为,