第第11页页■卷积性质例3()()htft例:f1(t),f2(t)如图,求f1(t)*f2(t)t11-1f1(t)t102f2(t)0解:f1(t)=2ε(t)–2ε(t–1)f2(t)=ε(t+1)–ε(t–1)f1(t)*f2(t)=2ε(t)*ε(t+1)–2ε(t)*ε(t–1)–2ε(t–1)*ε(t+1)+2ε(t–1)*ε(t–1)由于ε(t)*ε(t)=tε(t)据时移特性,有f1(t)*f2(t)=2(t+1)ε(t+1)-2(t–1)ε(t–1)–2tε(t)+2(t–2)ε(t–2)
第第11页页■卷积性质例2()()htft图(a)系统由三个子系统构成,已知各子系统的冲激响应如图(b)所示。求复合系统的冲激响应,并画出它的波形。thth21,thtth1O11tth2O112tthO1123(a)(b)解:hthththt211如图(c)所示th1th1th2ftty(c)
第第11页页■卷积性质例1()()htft例1:f1(t)如图,f2(t)=e–tε(t),求f1(t)*f2(t)e)()1(()e()ed()de()00)12(ttttfttttf1(t)t201解:f1(t)*f2(t)=f1’(t)*f2(–1)(t)f1’(t)=δ(t)–δ(t–2)f1(t)*f2(t)=(1-e–t)ε(t)–[1-e–(t-2)]ε(t-2)注意:当f1(t)=1,f2(t)=e–tε(t),套用f1(t)*f2(t)=f1’(t)*f2(–1)(t)=0*f2(–1)(t)=0显然是错误的。
第第11页页■卷积定理举例Forexample?)(sin2FjttAns:2Sa()2()tgUsingsymmetry,)(22Sa()2gt()Sa()g2t()()*2()]()]*[2[1sin22222ggggttg2(ω)*g2(ω)22-20ωF(jω)π2-20ω