拉普拉斯变换:
正变换:F(s)=∫0∞f(t)e−stdt=L[f(t)]F(s)=\int_0^{\infty}f(t)e^{-st}dt=\mathscr{L}[f(t)]F(s)=∫0∞f(t)e−stdt=L[f(t)]
逆变换:F(t)=12πj∫σ−j∞σ+j∞F(s)estds=L−1[f(s)]F(t)=\frac{ 1 }{2\pi j }\int_{\sigma- j\infty}^{\sigma+j\infty}F(s)e^{s t}ds=\mathscr{L}^{-1}[f(s)]F(t)=2πj1∫σ−j∞σ+j∞F(s)estds=L−1[f(s)]
其中:s=σ+jω{\sigma+j \omega}σ+jω
常见信号的拉氏变换:
阶跃函数:ε(t)\varepsilon(t)ε(t)←\leftarrow←→\rightarrow→1s\frac{ 1 }{s }s1 Re{s}>0
单边指数信号:e−αtε(t)e^{-\alpha t}\varepsilon(t)e−αtε(t)←\leftarrow←→\rightarrow→1s+α\frac{ 1 }{s+\alpha }s+α1 Re{s}>−α-\alpha−α
单边正弦信号:sinωtε(t)\sin \omega t\varepsilon(t)sinωtε(t)←\leftarrow←→\rightarrow→ωs2+ω2\frac{ \omega }{s^2+\omega ^2}s2+ω2ω Re{s}>0
单边余弦信号:cosωtε(t)\cos \omega t \varepsilon(t)cosωtε(t)←\leftarrow←→\rightarrow→ss2+ω2\frac{ s }{s^2+\omega ^2}s2+ω2s Re{s}>0
单边衰减正弦:e−αtsinωtε(t)e^{-\alpha t} \sin\omega t \varepsilon(t)e−αtsinωtε(t)←\leftarrow←→\rightarrow→ω(s+α)2+ω2\frac{ \omega }{(s+\alpha)^2+\omega^2 }(s+α)2+ω2ω Re{s}>-α\alphaα
t的正幂信号:tnε(t)t^n \varepsilon(t)tnε(t)←\leftarrow←→\rightarrow→n!sn+1\frac{n! }{s^{n+1} }sn+1n! Re{s}>0
L[ε(t)]=1s\mathscr{L}[\varepsilon(t)]=\frac{1}{s}L[ε(t)]=s1
L[t]=1s2\mathscr{L}[t]=\frac{1}{s^2}L[t]=s21
L[tn]=∫0−∞tne−stdt=ns...L[tn−1]=...=n!sn+1\mathscr{L}[t^n]=\int_{0_-}^{\infty} t^n e^{-st} dt=\frac{n }{s}...\mathscr{L}[t^{n-1}]=...=\frac{n! }{s^{n+1} }L[tn]=∫0−∞tne−stdt=sn...L[tn−1]=...=sn+1n!
冲激信号:δ(t)\delta (t)δ(t)←\leftarrow←→\rightarrow→111 Re{s}:(-∞\infty∞,+∞\infty∞)
δ(t−t0)\delta (t-t_0)δ(t−t0)←\leftarrow←→\rightarrow→e−st0e^{-s t_0}e−st0
δ′(t)\delta{\prime}(t)δ′(t)←\leftarrow←→\rightarrow→s
拉氏变换性质:
线性:a1f1(t)+a2f2(t)a_1 f_1(t)+a_2 f_2(t)a1f1(t)+a2f2(t)←\leftarrow←→\rightarrow→a1F1(s)+a2F2(s)a_1 F_1(s)+a_2 F_2(s)a1F1(s)+a2F2(s)
时域微分:df(t)dt\frac{d f(t)}{dt }dtdf(t)←\leftarrow←→\rightarrow→sF(s)−f(0−)s F(s)-f(0_-)sF(s)−f(0−)
d2f(t)dt\frac{d^2 f(t)}{dt }dtd2f(t)←\leftarrow←→\rightarrow→s2F(s)−sf(0−)−f′(0−)s^2 F(s)-sf(0_-)-f{\prime}(0_-)s2F(s)−sf(0−)−f′(0−)
dnf(t)dt\frac{d^n f(t)}{dt }dtdnf(t)←\leftarrow←→\rightarrow→snF(s)s^n F(s)snF(s)-sn−1f(0−)s^{n-1}f(0_-)sn−1f(0−)-…-f(n−1)(0−)f^{(n-1)}(0_-)f(n−1)(0−)
时域积分:∫−∞tf(τ)dτ\int_{-\infty}^{t}f(\tau)d\tau∫−∞tf(τ)dτ←\leftarrow←→\rightarrow→F(s)s\frac{F(s)}{s }sF(s)+f(−1)(0−)s\frac{f^{(-1)}(0_-)}{s }sf(−1)(0−)
有始函数:df(t)ε(t)dt\frac{df(t)\varepsilon(t)}{dt }dtdf(t)ε(t)←\leftarrow←→\rightarrow→SF(s)SF(s)SF(s)
∫0−tf(τ)dτ\int_{0_-}^{t}f(\tau)d\tau∫0−tf(τ)dτ←\leftarrow←→\rightarrow→F(s)s\frac{F(s)}{s }sF(s)
=>tε(t)=∫0−tε(τ)dτ=>t \varepsilon(t)=\int_{0_-}^{t}\varepsilon(\tau)d\tau=>tε(t)=∫0−tε(τ)dτ←\leftarrow←→\rightarrow→1s2\frac{1}{s^2 }s21
延时特性(时域平移):f(t−t0)ε(t−t0)f(t-t_0)\varepsilon(t-t_0)f(t−t0)ε(t−t0)←\leftarrow←→\rightarrow→e−st0F(s),t0>0e^{-s t_0}F(s),t_0>0e−st0F(s),t0>0
S域平移:f(t)e−st0f(t)e^{-s t_0}f(t)e−st0←\leftarrow←→\rightarrow→F(s+s0)F(s+s_0)F(s+s0)
尺度变换:f(at)f(at)f(at)←\leftarrow←→\rightarrow→1aF(sa),(a>0)\frac{1}{a }F(\frac{s}{a }),(a>0)a1F(as),(a>0)
初值定理:f(0+)=limt→0+f(t)=lims→∞sF(s)(当F(s)是真分式时成立)f(0^+)=\lim_{t\rightarrow0_+}f(t)=\lim_{s\rightarrow\infty}sF(s) (当F(s)是真分式时成立)f(0+)=limt→0+f(t)=lims→∞sF(s)(当F(s)是真分式时成立)
终值定理:f(∞)=limt→∞f(t)=lims→0sF(s)(F(s)极点在复频域左半平面)f(\infty)=\lim_{t\rightarrow\infty}f(t)=\lim_{s\rightarrow 0}sF(s) (F(s)极点在复频域左半平面)f(∞)=limt→∞f(t)=lims→0sF(s)(F(s)极点在复频域左半平面)
卷积定理:时域 f1(t)∗f2(t)f_1(t)*f_2(t)f1(t)∗f2(t)←\leftarrow←→\rightarrow→F1(s).F2(s)F_1(s).F_2(s)F1(s).F2(s)
卷积定理:频域 f1(t).f2(t)f_1(t).f_2(t)f1(t).f2(t)←\leftarrow←→\rightarrow→12πjF1(s)∗F2(s)\frac{1}{2\pi j }F_1(s)*F_2(s)2πj1F1(s)∗F2(s)
复频域微分: −tf(t)-tf(t)−tf(t)←\leftarrow←→\rightarrow→d(F(s)ds\frac{d(F(s)}{ds}dsd(F(s)
复频域微分: f(t)t\frac{f(t)}{t}tf(t)←\leftarrow←→\rightarrow→∫s∞F(η)dη\int_s^{\infty}F(\eta)d\eta∫s∞F(η)dη
序列傅里叶变换(DTFT:discrete time Fourier transform)
正变换: $$
逆变换:
性质:序列位移:
性质:频域位移:
性质:线性加权: DTFT[nx(n)]=j[ddωXe(jω)]DTFT [nx(n)]=j[\frac{d}{d \omega}X e^{(j\omega)}]DTFT[nx(n)]=j[dωdXe(jω)]
性质:序列反褶:DTFT[x(−n)]=Xe(−jω)DTFT [x(-n)]=X e^{(-j\omega)}DTFT[x(−n)]=Xe(−jω)