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作业要求链接: 信号与系统 2022 春季学期第五次作业 https://zhuoqing.blog.csdn.net/article/details/123712642
§01 参考答案
1.1 傅里叶级数分解
1.1.1 计算傅里叶级数并画出频谱
(1)第一小题
本题的主要目的是联系傅里叶级数分解的基本公式的使用。
求解:
(1) 三角形式的傅里叶级数
周期方波是偶对称波形,而且满足奇谐对称,所以 b n = 0 , a 2 k = 0 b_n = 0,\,\,a_{2k} = 0 bn=0,a2k=0 。 a n = 4 T ∫ 0 T 2 f ( t ) cos 2 π n t T d t = 4 T { ∫ 0 T 4 cos 2 π n t T d t − ∫ T 4 T 2 cos 2 π n t T d t } a_n = {4 \over T}\int_0^{ {T \over 2}} {f\left( t \right)\cos { {2\pi nt} \over T}dt = {4 \over T}\left\{ {\int_0^{ {T \over 4}} {\cos { {2\pi nt} \over T}dt} - \int_{ {T \over 4}}^{ {T \over 2}} {\cos { {2\pi nt} \over T}dt} } \right\}} an=T4∫02Tf(t)cosT2πntdt=T4{ ∫04TcosT2πntdt−∫4T2TcosT2πntdt} = 4 T ⋅ T 2 π n ⋅ { sin 2 π n t T ∣ 0 T 4 − sin 2 π n t T ∣ T 4 T 2 } = 2 π n ⋅ [ sin π n 2 − ( sin π n − sin π n 2 ) ] = 4 π n sin n π 2 = {4 \over T} \cdot {T \over {2\pi n}} \cdot \left\{ {\left. {\sin { {2\pi nt} \over T}} \right|_0^{ {T \over 4}} - \left. {\sin { {2\pi nt} \over T}} \right|_{ {T \over 4}}^{ {T \over 2}} } \right\} = {2 \over {\pi n}} \cdot \left[ {\sin { {\pi n} \over 2} - \left( {\sin \pi n - \sin { {\pi n} \over 2}} \right)} \right] = {4 \over {\pi n}}\sin { {n\pi } \over 2} =T4⋅2πnT⋅{ sinT2πnt∣∣∣∣04T−sinT2πnt∣∣∣∣4T2T}=πn2⋅[sin2πn−(sinπn−sin2πn)]=πn4sin2nπ
结果也验证了,只有当 n = 2 k + 1 n = 2k + 1 n=2k+1 , a n ≠ 0 a_n \ne 0 an=0 。
对照 周期信号傅里叶级数表格 也能够验证上述推导公式是正确的。
(2) 复指数形式傅里叶级数
根据 F n = 1 2 ( a n − j b n ) F_n = {1 \over 2}\left( {a_n - jb_n } \right) Fn=21(an−jbn) 可以知道 F n = 1 n π sin ( n π 2 ) F_n = {1 \over {n\pi }}\sin \left( { { {n\pi } \over 2}} \right) Fn=nπ1sin(2nπ)
(3) 双边频谱(幅度谱和相位谱)
由于 F n F_n Fn 是实数,所以将幅度谱和相位谱绘制在一起
▲ 图1.1.1 傅里叶级数双边频谱
(2)第二小题
求解:
(1) 三角形式的傅里叶级数
观察三角波形,它是实偶函数,并且直流分量为0,因此 a 0 = 0 , b n = 0 a_0 = 0,b_n = 0 a0=0,bn=0 。这个信号 也是“奇谐对称”,所以它只有奇次谐波分量。
a n = 4 T ∫ 0 T 2 4 T t ⋅ cos n ω 1 t d t = 16 T 2 ∫ 0 T 2 t ⋅ cos n ω 1 t d t a_n = {4 \over T}\int_0^{ {T \over 2}} { {4 \over T}t \cdot \cos n\omega _1 tdt} = { {16} \over {T^2 }}\int_0^{ {T \over 2}} {t \cdot \cos n\omega _1 tdt} an=T4∫02TT4t⋅cosnω1tdt=T216∫02Tt⋅cosnω1tdt = 16 T 2 n ω 1 ( t ⋅ sin n ω 1 t ∣ 0 T 2 − ∫ 0 T 2 sin n ω 1 t ⋅ d t ) = 16 T 2 n ω 1 ( 0 + 1 n ω 1 cos n ω 1 t ∣ 0 T 2 ) = { {16} \over {T^2 n\omega _1 }}\left( {\left. {t \cdot \sin n\omega _1 t} \right|_0^{ {T \over 2}} - \int_0^{ {T \over 2}} {\sin n\omega _1 t \cdot dt} } \right) = { {16} \over {T^2 n\omega _1 }}\left( {0 + \left. { {1 \over {n\omega _1 }}\cos n\omega _1 t} \right|_0^{ {T \over 2}} } \right) =T2nω116(t⋅sinnω1t∣02T−∫02Tsinnω1t⋅dt)=T2nω116(0+nω11cosnω1t∣∣∣∣02T) = 4 n 2 π 2 ⋅ cos n ω 1 t ∣ 0 T 2 = 4 n 2 π 2 ( cos n π − 1 ) = {4 \over {n^2 \pi ^2 }} \cdot \left. {\cos n\omega _1 t} \right|_0^{ {T \over 2}} = {4 \over {n^2 \pi ^2 }}\left( {\cos n\pi - 1} \right) =n2π24⋅cosnω1t∣02T=n2π24(cosnπ−1)
对照 周期信号傅里叶级数表格 也能够验证上述推导公式是正确的。
通过分析结果,可以看到只有当 n = 2 k + 1 n = 2k + 1 n=2k+1 , a n ≠ 0 a_n \ne 0 an=0 。
(2) 复指数形式傅里叶级数
根据 F n = 1 2 ( a n − j b n ) F_n = {1 \over 2}\left( {a_n - jb_n } \right) Fn=21(an−jbn) ,所以 F n = 2 n 2 π 2 ( cos n π − 1 ) F_n = {2 \over {n^2 \pi ^2 }}\left( {\cos n\pi - 1} \right) Fn=n2π22(cosnπ−1)
(3) 双边频谱(幅度谱和相位谱)
由于 F n F_n Fn 都是负实数,因此,对应的相位谱要么是 π \pi π ,要么是 − π - \pi −π 。
▲ 图1.1.2 傅里叶级数双边幅度谱
▲ 图1.1.3 傅里叶级数双边相位谱
(3)第三小题
求解:
(1) 三角形式的傅里叶级数
根据观察,周期锯齿波形是奇对称,所以 a n = 0 a_n = 0 an=0 。 b n = 4 T ∫ 0 T 2 f ( t ) sin n ω 1 t d t = 4 T ∫ 0 T 2 ( t − 1 ) sin n ω 1 t d t = 2 n π b_n = {4 \over T}\int_0^{ {T \over 2}} {f\left( t \right)\sin n\omega _1 tdt} = {4 \over T}\int_0^{ {T \over 2}} {\left( {t - 1} \right)\sin n\omega _1 tdt} = {2 \over {n\pi }} bn=T4∫02Tf(t)sinnω1tdt=T4∫02T(t−1)sinnω1tdt=nπ2 其中 n ≠ 0 n \ne 0 n=0 。
上面公式是根据 傅里叶级数表格 而得,省去了其中的推导过程。
(2) 复指数形式傅里叶级数
根据 F n = 1 2 ( a n − j b n ) F_n = {1 \over 2}\left( {a_n - jb_n } \right) Fn=21(an−jbn) ,所以 F n = − j n π , n ≠ 0 F_n = - {j \over {n\pi }},\,\,\,n \ne 0 Fn=−nπj,n=0
(3) 双边频谱(幅度谱和相位谱)
▲ 图1.1.4 双边幅度谱
▲ 图1.1.5 双边相位谱谱
(4)第四小题
求解:
(1) 三角形式的傅里叶级数
这是一个偶对称的周期信号,所以 b n = 0 b_n = 0 bn=0 。 a n = 4 2 ⋅ ∫ 0 1 + f ( t ) cos n π t d t = 2 × ( 1 − 2 ⋅ cos n π ) = 2 − 4 cos n π a_n = {4 \over 2} \cdot \int_0^{1_ + } {f\left( t \right)\cos n\pi tdt} = 2 \times \left( {1 - 2 \cdot \cos n\pi } \right) = 2 - 4\cos n\pi an=24⋅∫01+f(t)cosnπtdt=2×(1−2⋅cosnπ)=2−4cosnπ
(2) 复指数形式傅里叶级数
根据 F n = 1 2 ( a n − j b n ) F_n = {1 \over 2}\left( {a_n - jb_n } \right) Fn=21(an−jbn) ,所以 F n = 1 − 2 cos n π F_n = 1 - 2\cos n\pi Fn=1−2cosnπ
(3) 双边频谱(幅度谱和相位谱)
F n F_n Fn 是实数,所以将幅度谱和相位谱绘制在一起。
▲ 图1.1.6 傅里叶级数频谱
(5)第五小题
求解: 直接求取本题,过程略显复杂。可以先对单个周期信号进行傅里叶变换,得到单个周期内的傅里叶变换 F 0 ( ω ) F_0 \left( \omega \right) F0(ω) 。那么对应的傅里叶级数分解系数 F n = 1 T 1 F 0 ( ω ) ∣ ω = n ω 1 F_n = {1 \over {T_1 }}\left. {F_0 \left( \omega \right)} \right|_{\omega = n\omega _1 } Fn=T11F0(ω)∣ω=nω1
在本题中,抽取其中一个周期内的信号 f 0 ( t ) = { f ( t ) , − 1 / 2 ≤ t < 3 / 2 0 , o t h e r f_0 \left( t \right) = \left\{ \begin{matrix} {f\left( t \right),\,\,\,\, - 1/2 \le t < 3/2}\\{0,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,other}\\\end{matrix} \right. f0(t)={ f(t),−1/2≤t<3/20,other
假设窗口函数 R 1 / 2 ( t ) R_{1/2} \left( t \right) R1/2(t) 为 R 1 / 2 ( t ) = u ( t + 1 2 ) − u ( t − 1 2 ) R_{1/2} \left( t \right) = u\left( {t + {1 \over 2}} \right) - u\left( {t - {1 \over 2}} \right) R1/2(t)=u(t+21)−u(t−21) 所以 f 0 ( t ) = R 1 / 2 ( t ) ∗ [ 2 δ ( t ) + δ ( t − 1 ) ] f_0 \left( t \right) = R_{1/2} \left( t \right) * \left[ {2\delta \left( t \right) + \delta \left( {t - 1} \right)} \right] f0(t)=R1/2(t)∗[2δ(t)+δ(t−1)] 所以 f 0 ( t ) f_0 \left( t \right) f0(t) 的傅里叶变换为 F 0 ( ω ) = 1 2 S a ( ω 4 ) ( 2 + e j ω ) F_0 \left( \omega \right) = {1 \over 2}Sa\left( { {\omega \over 4}} \right)\left( {2 + e^{j\omega } } \right) F0(ω)=21Sa(4ω)(2+ejω) 那么 F n = 1 2 ⋅ F 0 ( ω ) ∣ ω = n π = S a ( n π 4 ) ⋅ ( 1 + 1 2 e j n π ) = [ 1 + ( − 1 ) n 2 ] S a ( n π 4 ) F_n = {1 \over 2}\left. { \cdot F_0 \left( \omega \right)} \right|_{\omega = n\pi } = Sa\left( { { {n\pi } \over 4}} \right) \cdot \left( {1 + {1 \over 2}e^{jn\pi } } \right) = \left[ {1 + { {\left( { - 1} \right)^n } \over 2}} \right]Sa\left( { { {n\pi } \over 4}} \right) Fn=21⋅F0(ω)∣ω=nπ=Sa(4nπ)⋅(1+21ejnπ)=[1+2(−1)n]Sa(4nπ)
所以 a n = 2 F n = [ 2 + ( − 1 ) n ] S a ( n π 4 ) , b n = 0 a_n = 2F_n = \left[ {2 + \left( { - 1} \right)^n } \right]Sa\left( { { {n\pi } \over 4}} \right),\,\,b_n = 0 an=2Fn=[2+(−1)n]Sa(4nπ),bn=0
信号的双边频谱为
▲ 图1.1.7 傅里叶级数频谱
1.1.2 分析周期信号频谱
求解:
(1) 根据描述信号的表达式,可以看到信号的角频率包括有 { 2 , 3 , 5 } \left\{ {2,3,5} \right\} { 2,3,5} 等,它们的最大公约数为 1 r a d / s 1\,\,rad/s 1rad/s ,所以信号的基频等于 f 0 = 1 r a d / s f_0 = 1\,\,rad/s f0=1rad/s 。
(2) 为了绘制信号的频谱(幅度谱和相位谱),需要将信号每个分量表示成 c n cos ( ω n t + θ n ) c_n \cos \left( {\omega _n t + \theta _n } \right) cncos(ωnt+θn) 的形式。 f ( t ) = 2 + 5 cos ( 2 t − 53. 1 o ) + 2 cos ( 3 t − 6 0 0 ) + cos ( 5 t − 3 0 0 ) f\left( t \right) = 2 + 5\cos \left( {2t - 53.1^o } \right) + 2\cos \left( {3t - 60^0 } \right) + \cos \left( {5t - 30^0 } \right) f(t)=2+5cos(2t−53.1o)+2cos(3t−600)+cos(5t−300) 对应的幅度谱和相位谱为
▲ 图1.1.8 单边幅度谱
▲ 图1.1.9 单边相位谱
1.2 傅里叶级数分解对称性
1.2.1 信号中的谐波分量
(1)必做题
求解:
求解 由于信号是符合奇谐对称条件,所以信号中谐波只存在奇次谐波分量。原来信号的基频为 f 0 = 1 / T = 100 k H z f_0 = 1/T = 100kHz f0=1/T=100kHz ,所以它的谐波中只包括后 100 k , 300 k , 500 k 100k,300k,500k 100k,300k,500k 等,而 200 k , 250 k , 400 k 200k,250k,400k 200k,250k,400k 都不包括。
(2)选做题
求解:
为了能够使得信号具有二倍频(二次谐波),所以需要打破信号的奇谐对称特性。从可选择的三个电路中来看,只有电路(2),电路(3)才有可能。
另外,由于原来信号中不包括偶次谐波,所以信号经过电路系统处理后,希望出现新的频率分量(偶次谐波分量),则系统必然不再是线性时不变系统了。根据给定的三个电路来看,只有电路(2),电路(3)才为非线性系统。而电路(1)是线性电路(增量线性电路),所以电路(1)无法产生二次谐波。
1.2.2 补齐周期信号
求解: 根据给定的对称条件,补齐后的波形为。
(1)
▲ 图1.2.1 偶对称、奇谐对称
(2)
▲ 图1.2.2 奇对称,偶次谐波
(3)
▲ 图1.2.3 奇对称,奇次谐波
(4)
▲ 图1.2.4 偶对称,偶次谐波
1.2.3 判断谐波分量
(1)必做题
求解:
(1) 无直流分量、存在cos分量,无sin分量;只有奇次谐波分量;
(2) 无直流分量、无cos分量,存在sin分量;只有奇次谐波分量;
(3) 无直流分量、存在sin,cos分量; 只有奇次谐波分量;
(4) 无直流分量、无cos分量,存在sin分量; 存在奇次谐波、偶次谐波分量;
(5) 有直流分量, 存在cos分量,无sin分量;存在奇次谐波分量、偶次谐波分量;
(6) 有直流分量,存在cos分量,存在sin分量; 存在奇次谐波分量,偶次谐波分量;
(8) 有直流分量,存在cos分量,无sin分量; 存在奇次谐波分量,偶次谐波分量;
(7) 有直流分量,存在cos分量,无sin分量;存在奇次谐波、偶次谐波分量; 只有有限个谐波分量。
(2)选做题
根据信号的连续性,可以判断信号频谱衰减的规律:
- 如果信号不连续,信号频谱衰减是按照 1 / ω 1/\omega 1/ω 规律衰减;
- 如果信号连续,但一阶导数不连续,信号的频谱按照 1 / ω 2 1/\omega ^2 1/ω2 规律衰减;
- 如果信号连续,一阶导数连续,但二阶导数不连续,则信号频谱按照 1 / ω 3 1/\omega ^3 1/ω3 规律衰减。以此类推。
- 本题中最后一个是 ` sin c \sin c sinc 周期函数,它之后 有限个谐波分量。
1.3 傅里叶级数分解性质
1.3.1 傅里叶级数分解微分性质
证明:
f ( t ) = a 0 + ∑ n = 1 + ∞ a n cos ( n ω 1 t ) + b n sin ( n ω 1 t ) f\left( t \right) = a_0 + \sum\limits_{n = 1}^{ + \infty } {a_n \cos \left( {n\omega _1 t} \right) + b_n \sin \left( {n\omega _1 t} \right)} f(t)=a0+n=1∑+∞ancos(nω1t)+bnsin(nω1t) d f ( t ) d t = d d t [ a 0 + ∑ n = 1 + ∞ a n cos ( n ω 1 t ) + b n sin ( n ω 1 t t ) ] { {df\left( t \right)} \over {dt}} = {d \over {dt}}\left[ {a_0 + \sum\limits_{n = 1}^{ + \infty } {a_n \cos \left( {n\omega _1 t} \right) + b_n \sin \left( {n\omega _1 tt} \right)} } \right] dtdf(t)=dtd[a0+n=1∑+∞ancos(nω1t)+bnsin(nω1tt)] = 0 + ∑ n = 1 + ∞ − a n n ⋅ sin ( n ω 1 t ) + b n n ⋅ cos ( n ω 1 t ) = 0 + \sum\limits_{n = 1}^{ + \infty } { - a_n n \cdot \sin \left( {n\omega _1 t} \right)} + b_n n \cdot \cos \left( {n\omega _1 t} \right) =0+n=1∑+∞−ann⋅sin(nω1t)+bnn⋅cos(nω1t)
按照傅里叶级数分解系数,可以得到 a 0 ′ = 0 , a n ′ = n b n , b n ′ = − n a n , ( n = 1 , 2 , ⋯ ) a'_0 = 0,a'_n = nb_n ,b'_n = - na_n ,\left( {n = 1,2, \cdots } \right) a0′=0,an′=nbn,bn′=−nan,(n=1,2,⋯)
1.3.2 傅里叶级数分解时移性质
证明:
G n = 1 T 1 ∫ 0 T 1 g ( t ) e − j n ω 1 t d t = 1 T 1 ∫ 0 T 1 f ( t − t 0 ) e − j n ω 1 t d t G_n = {1 \over {T_1 }}\int_0^{T_1 } {g\left( t \right)e^{ - jn\omega _1 t} dt} = {1 \over {T_1 }}\int_0^{T_1 } {f\left( {t - t_0 } \right)e^{ - jn\omega _1 t} dt} Gn=T11∫0T1g(t)e−jnω1tdt=T11∫0T1f(t−t0)e−jnω1tdt = 1 T 1 ∫ t 0 T 1 + t 0 f ( k ) e − j n ω 1 ( k + t 0 ) t d k = 1 T 1 e − j n ω 1 t 0 ∫ 0 T 1 f ( k ) e − j n ω 1 k t d t = {1 \over {T_1 }}\int_{t_0 }^{T_1 + t_0 } {f\left( k \right)e^{ - jn\omega _1 \left( {k + t_0 } \right)t} dk} = {1 \over {T_1 }}e^{ - jn\omega _1 t_0 } \int_0^{T_1 } {f\left( k \right)e^{ - jn\omega _1 kt} dt} =T11∫t0T1+t0f(k)e−jnω1(k+t0)tdk=T11e−jnω1t0∫0T1f(k)e−jnω1ktdt = F n ⋅ e − j n ω 1 t 0 = F_n \cdot e^{ - jn\omega _1 t_0 } =Fn⋅e−jnω1t0
1.4 周期信号分析
1.4.1 选做题
求解: 根据条件(1),(2),(3),(4)可以知道给了函数只包含有一次谐波。根据条件(5)可以知道该谐波的功率为1/2,所以,该信号的有效值为 1 / 2 1/\sqrt 2 1/2 ,信号的峰值为1。
根据条件(6),可以知道该信号为偶实信号,所以信号是周期为6,峰值为1的余弦信号,它可以写为:
x ( t ) = cos ( π 3 t ) x\left( t \right) = \cos \left( { {\pi \over 3}t} \right) x(t)=cos(3πt)
A = 1 , B = 12 π , C = 0 A = 1,\,\,\,B = 12\pi ,\,\,C = 0 A=1,B=12π,C=0
1.5 傅里叶变换
1.5.1 半波余弦脉冲信号
求解: 下面给出三种求解方法。
- 求解方法1:
:
∫ − τ 2 τ 2 f ( t ) ⋅ e − j ω t d t = E ⋅ ∫ − τ 2 τ 2 cos ω 1 t ⋅ e − j ω t d t \int_{ - {\tau \over 2}}^{ {\tau \over 2}} {f\left( t \right) \cdot e^{ - j\omega t} dt} = E \cdot \int_{ { { - \tau } \over 2}}^{ {\tau \over 2}} {\cos \omega _1 t \cdot e^{ - j\omega t} dt} ∫−2τ2τf(t)⋅e−jωtdt=E⋅∫2−τ2τcosω1t⋅e−jωtdt = E 2 ∫ − τ 2 τ 2 ( e j ω 1 t + e − j ω 1 t ) ⋅ e − j ω t d t = {E \over 2}\int_{ - {\tau \over 2}}^{ {\tau \over 2}} {\left( {e^{j\omega _1 t} + e^{ - j\omega _1 t} } \right) \cdot e^{ - j\omega t} dt} =2E∫−2τ2τ(ejω1t+e−jω1t)⋅e−jωtdt = E 2 { ∫ − τ 2 τ 2 e j t ( ω 1 − ω ) d t + ∫ − τ 2 τ 2 e j t ( − ω 1 − ω ) d t } = {E \over 2}\left\{ {\int_{ - {\tau \over 2}}^{ {\tau \over 2}} {e^{jt\left( {\omega _1 - \omega } \right)} dt} + \int_{ - {\tau \over 2}}^{ {\tau \over 2}} {e^{jt\left( { - \omega _1 - \omega } \right)} dt} } \right\} =2E{ ∫−2τ2τejt(ω1−ω)dt+∫−2τ2τejt(−ω1−ω)dt} = E 2 [ 1 j ( ω 1 − ω ) e j t ( ω 1 − ω ) ∣ − τ 2 τ 2 + − j ω 1 + ω e − j t ( ω 1 + ω ) ∣ − τ 2 τ 2 ] = {E \over 2}\left[ {\left. { {1 \over {j\left( {\omega _1 - \omega } \right)}}e^{jt\left( {\omega _1 - \omega } \right)} } \right|_{ - {\tau \over 2}}^{ {\tau \over 2}} + \left. { { { - j} \over {\omega _1 + \omega }}e^{ - jt\left( {\omega _1 + \omega } \right)} } \right|_{ - {\tau \over 2}}^{ {\tau \over 2}} } \right] =2E[j(ω1−ω)1ejt(ω1−ω)∣∣∣∣−2τ2τ+ω1+ω−je−jt(ω1+ω)∣∣∣∣−2τ2τ] = E 2 [ e j ( ω 1 − ω ) τ 2 j ( ω 1 − ω ) − e − j ( ω 1 − ω ) τ 2 j ( ω 1 − ω ) + e − j ( ω 1 + ω ) τ 2 j ( ω 1 + ω ) − e − j ( ω 1 + ω ) − τ 2 j ( ω 1 + ω ) ] = {E \over 2}\left[ { { {e^{j\left( {\omega _1 - \omega } \right){\tau \over 2}} } \over {j\left( {\omega _1 - \omega } \right)}} - { {e^{ - j\left( {\omega _1 - \omega } \right){\tau \over 2}} } \over {j\left( {\omega _1 - \omega } \right)}} + { {e^{ - j\left( {\omega _1 + \omega } \right){\tau \over 2}} } \over {j\left( {\omega _1 + \omega } \right)}} - { {e^{ - j\left( {\omega _1 + \omega } \right){ { - \tau } \over 2}} } \over {j\left( {\omega _1 + \omega } \right)}}} \right] =2E[j(ω1−ω)ej(ω1−ω)2τ−j(ω1−ω)e−j(ω1−ω)2τ+j(ω1+ω)e−j(ω1+ω)2τ−j(ω1+ω)e−j(ω1+ω)2−τ] = E ⋅ [ sin ( ω 1 − ω ) τ 2 ω 1 − ω + sin ( ω 1 + ω ) τ 2 ω 1 + ω ] = E \cdot \left[ { { {\sin \left( {\omega _1 - \omega } \right){\tau \over 2}} \over {\omega _1 - \omega }} + { {\sin \left( {\omega _1 + \omega } \right){\tau \over 2}} \over {\omega _1 + \omega }}} \right] =E⋅[ω1−ωsin(ω1−ω)2τ+ω1+ωsin(ω1+ω)2τ] = E τ 2 [ S a ( τ ( ω 1 + ω ) 2 ) + S a ( τ ( ω 1 − ω ) 2 ) ] = { {E\tau } \over 2}\left[ {Sa\left( { { {\tau \left( {\omega _1 + \omega } \right)} \over 2}} \right) + Sa\left( { { {\tau \left( {\omega _1 - \omega } \right)} \over 2}} \right)} \right] =2Eτ[Sa(2τ(ω1+ω))+Sa(2τ(ω1−ω))]
使用MATLAB绘制升余弦曲线频谱图的命令:
>> o=linspace(-20,20,1000);
>> E=1;tao=1;
>> o1=2*pi/(2*tao);
>> F=E*tao/2*(sinc(tao*(o1+o)/2/pi)+sinc(tao*(o1-o)/2/pi));
>> plot(o,F)
下图显示了信号的频谱实际上是由两个Sinc函数叠加而成。
- 求解方法2:
根据傅里叶变换的频移特性可以简化求解傅里叶变换过程:
半波余弦脉冲可以写做:
矩形窗口的FT为:
根据FT的频移特性:
- 求解方法3:
F ( ω ) = ∫ − τ 2 τ 2 E ⋅ cos ( 2 π 2 τ t ) ⋅ e − j ω t d t F\left( \omega \right) = \int_{ - {\tau \over 2}}^{ {\tau \over 2}} {E \cdot \cos \left( { { {2\pi } \over {2\tau }}t} \right) \cdot e^{ - j\omega t} dt} F(ω)=∫−2τ2τE⋅cos(2τ2πt)⋅e−jωtdt = E ⋅ ∫ − τ 2 τ 2 e j π τ t + e − j π τ t 2 ⋅ e − j ω t d t = E \cdot \int_{ - {\tau \over 2}}^{ {\tau \over 2}} { { {e^{j{\pi \over \tau }t} + e^{ - j{\pi \over \tau }t} } \over 2} \cdot e^{ - j\omega t} dt} =E⋅∫−2τ2τ2ejτπt+e−jτπt⋅e−jωtdt = E 2 ⋅ ∫ − τ 2 τ 2 e j ( π τ − ω ) t + e − j ( π τ + ω ) t d t = {E \over 2} \cdot \int_{ - {\tau \over 2}}^{ {\tau \over 2}} {e^{j\left( { {\pi \over \tau } - \omega } \right)t} + e^{ - j\left( { {\pi \over \tau } + \omega } \right)t} dt} =2E⋅∫−2τ2τej(τπ−ω)t+e−j(τπ+ω)tdt
= E 2 ⋅ [ 1 j ( π τ − ω ) ( e j ( π τ − ω ) ⋅ τ 2 − e j ( π τ − ω ) ⋅ ( − τ 2 ) ) + j ( π τ + ω ) ( e − j ( π τ + ω ) ⋅ τ 2 − e − j ( π τ + ω ) ⋅ ( − τ 2 ) ) ] = {E \over 2} \cdot \left[ { {1 \over {j\left( { {\pi \over \tau } - \omega } \right)}}\left( {e^{j\left( { {\pi \over \tau } - \omega } \right) \cdot {\tau \over 2}} - e^{j\left( { {\pi \over \tau } - \omega } \right) \cdot \left( { - {\tau \over 2}} \right)} } \right) + {j \over {\left( { {\pi \over \tau } + \omega } \right)}}\left( {e^{ - j\left( { {\pi \over \tau } + \omega } \right) \cdot {\tau \over 2}} - e^{ - j\left( { {\pi \over \tau } + \omega } \right) \cdot \left( { - {\tau \over 2}} \right)} } \right)} \right] =2E⋅[j(τπ−ω)1(ej(τπ−ω)⋅2τ−ej(τπ−ω)⋅(−2τ))+(τπ+ω)j(e−j(τπ+ω)⋅2τ−e−j(τπ+ω)⋅(−2τ))] = E 2 ⋅ [ 1 j ( π τ − ω ) ⋅ ( j ⋅ e − ω τ 2 j − ( − j ) ⋅ e ω τ 2 j ) + j ( π τ + ω ) ⋅ ( − j ⋅ e − ω τ 2 j − j ⋅ e ω τ 2 j ) ] = {E \over 2} \cdot \left[ { {1 \over {j\left( { {\pi \over \tau } - \omega } \right)}} \cdot \left( {j \cdot e^{ - { {\omega \tau } \over 2}j} - \left( { - j} \right) \cdot e^{ { {\omega \tau } \over 2}j} } \right) + {j \over {\left( { {\pi \over \tau } + \omega } \right)}} \cdot \left( { - j \cdot e^{ - { {\omega \tau } \over 2}j} - j \cdot e^{ { {\omega \tau } \over 2}j} } \right)} \right] =2E⋅[j(τπ−ω)1⋅(j⋅e−2ωτj−(−j)⋅e2ωτj)+(τπ+ω)j⋅(−j⋅e−2ωτj−j⋅e2ωτj)] = E 2 [ ( 1 π τ − ω + 1 π τ + ω ) ⋅ e − τ j 2 ω + ( 1 π τ − ω + 1 π τ + ω ) ⋅ e j ω 2 τ ] = {E \over 2}\left[ {\left( { {1 \over { {\pi \over \tau } - \omega }} + {1 \over { {\pi \over \tau } + \omega }}} \right) \cdot e^{ - { {\tau j} \over 2}\omega } + \left( { {1 \over { {\pi \over \tau } - \omega }} + {1 \over { {\pi \over \tau } + \omega }}} \right) \cdot e^{ { {j\omega } \over 2}\tau } } \right] =2E[(τπ−ω1+τπ+ω1)⋅e−2τjω+(τπ−ω1+τπ+ω1)⋅e2jωτ] = E 2 ⋅ 2 π τ π 2 τ 2 − ω 2 ⋅ ( e − j ω τ 2 + e j ω τ 2 ) = 2 π E τ π 2 − τ 2 ω 2 ⋅ cos ( ω τ 2 ) = {E \over 2} \cdot { { { {2\pi } \over \tau }} \over { { {\pi ^2 } \over {\tau ^2 }} - \omega ^2 }} \cdot \left( {e^{ - { {j\omega \tau } \over 2}} + e^{ { {j\omega \tau } \over 2}} } \right) = { {2\pi E\tau } \over {\pi ^2 - \tau ^2 \omega ^2 }} \cdot \cos \left( { { {\omega \tau } \over 2}} \right) =2E⋅τ2π2−ω2τ2π⋅(e−2jωτ+e2jωτ)=π2−τ2ω22πEτ⋅cos(2ωτ)
= π E τ 2 ( π 2 ) 2 − ( ω τ 2 ) 2 ⋅ cos ( ω τ 2 ) = { {\pi E{\tau \over 2}} \over {\left( { {\pi \over 2}} \right)^2 - \left( { { {\omega \tau } \over 2}} \right)^2 }} \cdot \cos \left( { { {\omega \tau } \over 2}} \right) =(2π)2−(2ωτ)2πE2τ⋅cos(2ωτ) = E τ 2 ⋅ ( 1 π 2 + ω τ 2 + 1 π 2 − ω τ 2 ) ⋅ cos ( ω τ 2 ) = { {E\tau } \over 2} \cdot \left( { {1 \over { {\pi \over 2} + { {\omega \tau } \over 2}}} + {1 \over { {\pi \over 2} - { {\omega \tau } \over 2}}}} \right) \cdot \cos \left( { { {\omega \tau } \over 2}} \right) =2Eτ⋅(2π+2ωτ1+2π−2ωτ1)⋅cos(2ωτ) = E τ 2 1 π 2 + ω τ 2 sin ( π 2 + ω τ 2 ) + 1 π 2 − ω τ 2 sin ( π 2 − ω τ 2 ) = { {E\tau } \over 2}{1 \over { {\pi \over 2} + { {\omega \tau } \over 2}}}\sin \left( { {\pi \over 2} + { {\omega \tau } \over 2}} \right) + {1 \over { {\pi \over 2} - { {\omega \tau } \over 2}}}\sin \left( { {\pi \over 2} - { {\omega \tau } \over 2}} \right) =2Eτ2π+2ωτ1sin(2π+2ωτ)+2π−2ωτ1sin(2π−2ωτ) = E τ 2 [ S a ( π 2 + ω τ 2 ) + S a ( π 2 + ω τ 2 ) ] = { {E\tau } \over 2}\left[ {Sa\left( { {\pi \over 2} + { {\omega \tau } \over 2}} \right) + Sa\left( { {\pi \over 2} + { {\omega \tau } \over 2}} \right)} \right] =2Eτ[Sa(2π+2ωτ)+Sa(2π+2ωτ)]
▲ 频谱图
1.5.2 五个型号的傅里叶变换
求解:
(1)
f ( t ) = e − 2 ( t − 1 ) u ( t − 1 ) f\left( t \right) = e^{ - 2\left( {t - 1} \right)} u\left( {t - 1} \right) f(t)=e−2(t−1)u(t−1)
F ( ω ) = ∫ 1 ∞ e − 2 ( t − 1 ) ⋅ e − j ω t d t = e 2 ⋅ ∫ 1 ∞ e − ( j ω + 2 ) t d t F\left( \omega \right) = \int_1^\infty {e^{ - 2\left( {t - 1} \right)} \cdot e^{ - j\omega t} dt} = e^2 \cdot \int_1^\infty {e^{ - \left( {j\omega + 2} \right)t} dt} F(ω)=∫1∞e−2(t−1)⋅e−jωtdt=e2⋅∫1∞e−(jω+2)tdt = e 2 − ( j ω + 2 ) ⋅ e − ( j ω + 2 ) t ∣ 1 ∞ = { {e^2 } \over { - \left( {j\omega + 2} \right)}} \cdot e\left. {^{ - \left( {j\omega + 2} \right)t} } \right|_1^\infty =−(jω+2)e2⋅e−(jω+2)t∣∣∣1∞ = e 2 2 + j ω e − j ω − 2 = e − j ω 2 + j ω = { {e^2 } \over {2 + j\omega }}e^{ - j\omega - 2} = { {e^{ - j\omega } } \over {2 + j\omega }} =2+jωe2e−jω−2=2+jωe−jω
(2)
f ( t ) = e − 2 ∣ t − 1 ∣ f\left( t \right) = e^{ - 2\left| {t - 1} \right|} f(t)=e−2∣t−1∣
F ( ω ) = ∫ − ∞ ∞ e − 2 ∣ t − 1 ∣ ⋅ e − j ω t d t F\left( \omega \right) = \int_{ - \infty }^\infty {e^{ - 2\left| {t - 1} \right|} \cdot e^{ - j\omega t} dt} F(ω)=∫−∞∞e−2∣t−1∣⋅e−jωtdt = ∫ − ∞ 1 e 2 ( t − 1 ) ⋅ e − j ω t d t + ∫ 1 ∞ e − 2 ( t − 1 ) ⋅ e − j ω t d t = \int_{ - \infty }^1 {e^{2\left( {t - 1} \right)} \cdot e^{ - j\omega t} dt} + \int_1^\infty {e^{ - 2\left( {t - 1} \right)} \cdot e^{ - j\omega t} dt} =∫−∞1e2(t−1)⋅e−jωtdt+∫1∞e−2(t−1)⋅e−jωtdt = e − 2 ⋅ ∫ − ∞ 1 e ( 2 − j ω ) t d t + e 2 ⋅ ∫ 1 ∞ e − ( 2 + j ω ) t d t = e^{ - 2} \cdot \int_{ - \infty }^1 {e^{\left( {2 - j\omega } \right)t} dt} + e^2 \cdot \int_1^\infty {e^{ - \left( {2 + j\omega } \right)t} dt} =e−2⋅∫−∞1e(2−jω)tdt+e2⋅∫1∞e−(2+jω)tdt = [ e − 2 2 − j ω e ( 2 − j ω ) t ∣ − ∞ 1 + − e 2 2 + j ω ⋅ e − ( 2 + j ω ) t ∣ 1 ∞ ] = \left[ { { {e^{ - 2} } \over {2 - j\omega }}\left. {e^{\left( {2 - j\omega } \right)t} } \right|_{ - \infty }^1 + { { - e^2 } \over {2 + j\omega }} \cdot \left. {e^{ - \left( {2 + j\omega } \right)t} } \right|_1^\infty } \right] =[2−jωe−2e(2−jω)t∣∣∣−∞1+2+jω−e2⋅e−(2+jω)t∣∣∣1∞] = [ e − j ω 2 − j ω + e − j ω 2 + j ω ] = 4 4 + ω 2 e − j ω = \left[ { { {e^{ - j\omega } } \over {2 - j\omega }} + { {e^{ - j\omega } } \over {2 + j\omega }}} \right] = {4 \over {4 + \omega ^2 }}e^{ - j\omega } =[2−jωe−jω+2+jωe−jω]=4+ω24e−jω
(3)
f ( t ) = u ( t ) − u ( t − 1 ) f\left( t \right) = u\left( t \right) - u\left( {t - 1} \right) f(t)=u(t)−u(t−1) F ( ω ) = ∫ − ∞ + ∞ f ( t ) e − j ω t d t = ∫ 0 1 1 ⋅ e − j ω t d t = 1 − j ω ⋅ e − j ω t ∣ 0 1 F\left( \omega \right) = \int_{ - \infty }^{ + \infty } {f\left( t \right)e^{ - j\omega t} dt} = \int_0^1 {1 \cdot e^{ - j\omega t} dt} = {1 \over { - j\omega }}\left. { \cdot e^{ - j\omega t} } \right|_0^1 F(ω)=∫−∞+∞f(t)e−jωtdt=∫011⋅e−jωtdt=−jω1⋅e−jωt∣∣01 = 1 − j ω ( e − j ω − 1 ) = 1 − e − j ω j ω = S a ω 2 ⋅ e − j ω 2 = {1 \over { - j\omega }}\left( {e^{ - j\omega } - 1} \right) = { {1 - e^{ - j\omega } } \over {j\omega }} = Sa{\omega \over 2} \cdot e^{ - j{\omega \over 2}} =−jω1(e−jω−1)=jω1−e−jω=Sa2ω⋅e−j2ω
(4)
f ( t ) = δ ( t + 1 ) + δ ( t − 1 ) f\left( t \right) = \delta \left( {t + 1} \right) + \delta \left( {t - 1} \right) f(t)=δ(t+1)+δ(t−1)
F ( ω ) = ∫ − ∞ + ∞ [ δ ( t + 1 ) + δ ( t − 1 ) ] e − j ω t d t F\left( \omega \right) = \int_{ - \infty }^{ + \infty } {\left[ {\delta \left( {t + 1} \right) + \delta \left( {t - 1} \right)} \right]e^{ - j\omega t} dt} F(ω)=∫−∞+∞[δ(t+1)+δ(t−1)]e−jωtdt = e j ω + e − j ω = 2 cos ω = e^{j\omega } + e^{ - j\omega } = 2\cos \omega =ejω+e−jω=2cosω
(5)
f ( t ) = d d t { u ( − 2 − t ) + u ( t − 2 ) } f\left( t \right) = {d \over {dt}}\left\{ {u\left( { - 2 - t} \right) + u\left( {t - 2} \right)} \right\} f(t)=dtd{
u(−2−t)+u(t−2)}
而:
d d t [ u ( − 2 − t ) + u ( t − 2 ) ] {d \over {dt}}\left[ {u\left( { - 2 - t} \right) + u\left( {t - 2} \right)} \right] dtd[u(−2−t)+u(t−2)] = − δ ( t + 2 ) + δ ( t − 2 ) = - \delta \left( {t + 2} \right) + \delta \left( {t - 2} \right) =−δ(t+2)+δ(t−2)
所以:
F T [ − δ ( t + 2 ) + δ ( t − 2 ) ] FT\left[ { - \delta \left( {t + 2} \right) + \delta \left( {t - 2} \right)} \right] FT[−δ(t+2)+δ(t−2)] = − e j 2 ω + e − j 2 ω = − 2 j sin 2 ω = - e^{j2\omega } + e^{ - j2\omega } = - 2j\sin 2\omega =−ej2ω+e−j2ω=−2jsin2ω
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