Signals and Systms Fourir Sris Rprsntation of Priodic Signals Chang-Su Kim
Introduction
Why do W Nd Fourir Analysis? Th ssnc of Fourir analysis is to rprsnt a signal in trms of complx xponntials x t a a a a a a 0t 2 0t 0t 0t 2 0t...... 2 0 2 Many rasons: Almost any signal can b rprsntd as a sris or sum or intgral of complx xponntials Signal is priodic Fourir Sris DT, CT Signal is non-priodic Fourir Transform DT, CT W will larn ths CTFS, DTFS, CTFT, DTFT on by on Rspons of an LTI systm to a complx xponntial is also a complx xponntial with a scald magnitud.
Rspons of LTI systms to Complx Exponntials CT wt H w c H w h d c wt Proprty wt w2t x t a a a 2 3 w t 3 wt w2t y t a H w a H w a H w 2 2 3 3 w t 3 This is usful bcaus almost vry signal can b rprsntd as a sum of complx xponntial functions So w can asily comput th rspons of th systm to almost vry input signal using th abov quation
Rspons of LTI systms to Complx Exponntials DT wn H c wn c H w h[ ] w Proprty wn w2n x[ n] a a a 2 3 y t a H a H a H w n 3 w w n w2 w2n w3 3 2 3 w n This is usful bcaus almost vry signal can b rprsntd as a sum of complx xponntial functions So w can asily comput th rspons of th systm to almost vry input signal using th abov quation
Continuous Tim Fourir Sris How to rprsnt a priodic function xt as x t a a a a a a 0t 2 0t 0t 0t 2 0t...... 2 0 2 continuous tim discrt tim priodic sris CTFS DTFS apriodic transform CTFT DTFT
Harmonically Rlatd Complx Exponntials Basic priodic signal 0t Fundamntal frquncy: Fundamntal priod: 0 T 2 / 0 Th st of harmonically rlatd complx xponntials 0 t 2 / T t { 0,, 2, 3,...} t t Each of th signals is priodic with T Thus, a linar combination of thm is also priodic with priod T: 0t a
Continuous Tim Fourir Sris CTFS Most all in nginring sns functions with priod T=2/w 0 can b rprsntd as a CTFS x t 0t a, T 0t a x t dt T
Exampl T = /4T 0 0 T T 0 T T a x 0t 0t t dt dt T0 T 0 0 sin sin 0T 2. T -5-4 -3-2 - 0 2 3 4 5 a /5 0 -/3 0 / /2 / 0 -/3 0 /5 x t 3 2 3 2 2 cos0t cos 30t 2 3 3w0t w0t w0t 3w0t
Exampl continud Finit Approximation x N t and Gibbs phnomnon N 0 xn t a N t Ovrshoot is always about 9% of th hight of th discontinuity
Exampl 2
Exampl 3
Continuous Tim Fourir Sris CTFT Drivation of th formula x t 0t a, T 0t a x t dt T
Proprtis of CTFS Thr ar 5 proprtis in Tabl 3. in pag 206 of th txtboo Do w hav to rmmbr thm all? No Instad, familiariz yourslf with thm and b abl to driv thm whnvr ncssary Thy all com from th singl formula x t 0t a, T 0t a x t dt T
Slctd Proprtis Givn two priodic signals with sam priod T and fundamntal frquncy 0 =2/T: x t y t a b. Linarity: z t Ax t By t Aa Bb 2. Tim-Shifting: z t x t t a 0 t 00 3. Tim-Rvrsal Flip: z t x t a 4. Conugat Symmtry: z t x t a * *
Slctd Proprtis 5. xt is ral and vn a is ral and vn 6. xt is ral and odd a is purly imaginary and odd 7. Multiplication: z t x t y t alb l l 8. Parsval s Rlation: x t dt T T 2 a 2
Othr Forms of CTFS ral
Discrt Tim Fourir Sris How to rprsnt a priodic function x[n] with priod N as N x[ n] a a a a... a 0 2 / N n 2 / N n 22 / N n N 2 / N n 0 2 N continuous tim discrt tim priodic sris CTFS DTFS apriodic transform CTFT DTFT
Discrt Tim Priod Functions with Priod N x[n]=x[n+n] Fundamntal frquncy 0 =2/N { [n] = 0n = 2/Nn : =0, ±, ±2,...} is a st of signals, consisting of all discrt-tim complx xponntials that ar priodic with priod N [n] = [n+n] [n] = +N [n] = +2N [n] =... Only N distinct signals in th st Thrfor, whil an infinit numbr of complx xponntials ar rquird in CTFS, only N complx xponntials ar usd in DTFS.
Discrt Tim Fourir Sris DTFS All functions with priod N can b rprsntd as a DTFS x[ n] a N N n N a 2 N x[ n] n 2 N n n N : dnots th sum ovr any intrval of N succssiv valus of n. Driv it!
Proprtis
Exampl x[n]
Fourir Sris and LTI Systms
Frquncy Rspons CT DT wt H c wt wn H c wn w c H w h d, x t a t y t a H w 0 0 t 0 w w c H h[ ], x[ n] N 2 N 2 2 N N y[ n] a H a N n n w H w or H ar calld frquncy rsponss.
Exampl Exampl 3.6 in pp. 228 in txtboo
Filtring Filtring is a procss that changs th amplitud and phas of frquncy componnts of an input signal All LTI systms can b thought as filtrs Ex Diffrntiator H dx t y t H dt High frquncy componnt is magnifid, whil low frquncy componnt is supprssd It is a ind of highpass filtr /2 -/2 H
Lowpass, Bandpass and Highpass Filtrs CT stop band H c pass band stop band H c H Idal low-pass filtr LPF H = in pass band 0 in stop band Idal high-pass filtr HPF Idal band-pass filtr BPF c c2
Lowpass, Bandpass and Highpass Filtrs DT LPF H HPF H - c 2-2 BPF H - 2 H is priodic with priod 2 low frquncis: at around =0, 2,... high frquncis: at around =, 3,...
Lowpass Filtring
Highpass Filtring
Exampl of CT Filtr RC lowpass filtr V s + V r - V c + - : output : input t V t V t V t V dt t dv RC c s s c c tan 2 RC t t t t t t RC RC H H H RC H H dt d RC
Exampl of CT Filtr continud It is a lowpass filtr H RC RC 2 tan RC H H /2 /RC -/4 -/2
Exampl of DT Filtr y[n] - ay[n-] = x[n] W now if x[n]= n thn y[n]=h n n n n a H ah H
Exampl of DT Filtr Continud H H a H H H /2 -/2 a a=0.6 LPF b a=-0.6 HPF
Exampl of DT Filtr 2 M N M N n x n y ] [ ] [,, 0 ] [ othrwis M n N for n h M N M N M N h H ] [
Exampl of DT Filtr 2 Continud H a N=M=6 H b N=M=32 It is a lowpass filtr