Overview of Discrete-Time Filters First-order filters Ideal filters - - PowerPoint PPT Presentation

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Overview of Discrete-Time Filters First-order filters Ideal filters - - PowerPoint PPT Presentation

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Overview of Discrete-Time Filters First-order filters Ideal filters Practical filters Frequency-selective filter specifications Ripple versus filter order tradeoff Application example Portland State University ECE 223 DT

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Overview of Discrete-Time Filters

  • First-order filters
  • Ideal filters
  • Practical filters
  • Frequency-selective filter specifications
  • Ripple versus filter order tradeoff
  • Application example

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Discrete-Time Filters Overview

N

  • k=0

ak y[n − k] =

M

  • k=0

bk x[n − k] Y (ejω) = M

k=0 bke−jwk

N

k=0 ake−jwk X(ejω)

  • Discrete-time filters are divided into two categories

– Finite impulse response (FIR): h[n] = 0 for some a and b such that −∞ < a < n < b < +∞ – Infinite impulse response (IIR): not FIR

  • Filters that can be described with difference-equations

– FIR: N = 0 – IIR: N > 0

  • A simple FIR filter is the moving average filter
  • A simple IIR filter is the first-order lowpass filter

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Example 1: First-Order Filters Consider the following filter: y[n] − ay[n − 1] = (1 − a)x[n]

  • 1. Solve for the filter’s transfer function
  • 2. Find the cutoff frequency as a function of a

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Example 1: Workspace

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Example 1: Ωc versus a

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.5 1 1.5 2 2.5 3 3.5 ωc (rad/sample) a

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Example 1: H(ejω) for various a

−3 −2 −1 1 2 3 0.5 1 |H(ejω)| a=0.1 a=0.2 a=0.3 a=0.4 a=0.5 a=0.6 a=0.7 a=0.8 a=0.9 −3 −2 −1 1 2 3 −50 50 ∠ H(ejω) (o) Frequency (rads/sample)

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Example 1: Filtered Signals

50 100 150 200 20 22 24 26 28 30 32 34 36 x[n], y0.1[n], y0.6[n] Time (day) Intel Closing Daily Price Over 1 Year Raw signal Cutoff:0.56 Cutoff:0.11

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Example 1: MATLAB Code

%function [] = FirstOrderApplied(); close all; d = load(’Intel.txt’); % Closing daily price nd = length(d); x = d(nd:-1:1); % Reorder so first element is oldest data n = (0:nd-1)’; % Discrete time index %============================================================================== % Plot the relationship between cutoff frequency and a %============================================================================== a = 0.00001:0.01:1; wc = acos((1-4*a+a.^2)./(-2*a)); figure FigureSet(1,’LTX’); h = plot(a,wc,’LineWidth’,1.0); ylabel(’\omega_c (rad/sample)’); xlabel(’a’); grid on; box off; AxisSet(8); print -depsc FOCutoff; %============================================================================== % Plot the relationship between cutoff frequency and a %============================================================================== a = 0.1:0.1:0.9; w = -pi:0.01:pi; H = zeros(length(a),length(w)); for cnt = 1:length(a) [h,w] = freqz(1-a(cnt),[1 -a(cnt)],w); H(cnt,:) = h;

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end; figure FigureSet(1,’LTX’); subplot(2,1,1); h = plot(w,abs(H)); xlim([-pi pi]); ylim([0 1.05]); legend(’a=0.1’,’a=0.2’,’a=0.3’,’a=0.4’,’a=0.5’,’a=0.6’,’a=0.7’,’a=0.8’,’a=0.9’,2); ylabel(’|H(e^{j\omega})|’); grid on; box off; subplot(2,1,2); h = plot(w,rem(angle(H)*180/pi,180)); xlim([-pi pi]); ylim([-70 70]); ylabel(’\angle H(e^{j\omega}) (^o)’); xlabel(’Frequency (rads/sample)’); grid on; box off; AxisSet(8); print -depsc FOTransferFunctions; %============================================================================== % Filter & Plot %============================================================================== a = 0.58; % cutoff frequency approximately 0.1 w1 = acos((1-4*a+a.^2)./(-2*a)); y1 = zeros(nd,1); for cnt = 2:nd, y1(cnt) = a*y1(cnt-1) + (1-a)*x(cnt); end; a = 0.90; % cutoff frequency approximately 0.1 w2 = acos((1-4*a+a.^2)./(-2*a)); y2 = zeros(nd,1);

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for cnt = 2:nd, y2(cnt) = a*y2(cnt-1) + (1-a)*x(cnt); end; figure FigureSet(1,’LTX’); h = plot(n,x,’b’,n,y1,’r’,n,y2,’g’); set(h,’LineWidth’,0.6); ylabel(’x[n], y_{0.1}[n], y_{0.6}[n]’); xlabel(’Time (day)’); title(’Intel Closing Daily Price Over 1 Year’); xlim([0 nd-1]); ylim([19 36]); box off; grid on; AxisSet(8); legend(’Raw signal’,sprintf(’Cutoff:%4.2f’,w1),sprintf(’Cutoff:%4.2f’,w2),4); print -depsc FOSignalFiltered;

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Ideal Filters

1 Lowpass 1 Highpass 1 Bandpass 1 Bandstop 1 Notch

Ω Ω Ω Ω Ω Ωc Ωc Ωc Ωc1 Ωc1 Ωc2 Ωc2

  • MATLAB can be used to design standard frequency selective

filters that meet user-specified requirements

  • These filters include: lowpass, highpass, bandpass, and bandstop
  • Unlike continuous-time filters, these must have cutoff frequencies

that range between 0 and π

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Practical Filters

  • Practical filters are usually designed to meet a set of specifications
  • Lowpass and highpass filters usually have the following

requirements – Passband range – Stopband range – Maximum ripple in the passband – Minimum attenuation in the stopband

  • If we know the specifications, we can ask MATLAB to generate

the filter for us

  • There are four popular types of standard filters

– Butterworth – Chebyshev Type I – Chebyshev Type II – Elliptic

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Ripple Tradeoff Filter Order Passband Stopband Butterworth Largest Smooth Smooth Chebyshev Type I Moderate Ripple Smooth Chebyshev Type II Moderate Smooth Ripple Elliptic Lowest Ripple Ripple

  • The four popular filter types differ in how they satisfy the

specifications

  • In the passband and stopband, each filter is either smooth or

contains ripple

  • Elliptic filters are also called equiripple filters and Cauer filters

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Example 2: Lowpass Filter Specifications Design a lowpass filter that meets the following specifications:

  • The passband ripple is no more than 0.4455 dB

(0.95 ≤ |H(ejω)| ≤ 1)

  • The stopband attenuation is at least 26.02 dB (|H(ejω)| ≤ 0.05)
  • The passband ranges from 0–0.2π rad/sample
  • The stopband ranges from 0.3π–π rad/sample

Plot the magnitude of the resulting transfer function on a linear-linear plot, the impulse response, and the step response. Try the Butterworth, Chebyshev I, Chebyshev II, and elliptic filters.

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Example 3: Butterworth Lowpass

0.5 1 1.5 2 2.5 3 0.2 0.4 0.6 0.8 1 |H(ejω)| Butterworth Lowpass Filter Transfer Function Order: 10 Frequency (rad/sample)

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Example 3: Butterworth Lowpass

10 20 30 40 50 60 70 80 90 100 −0.1 −0.05 0.05 0.1 0.15 0.2 0.25 h[n] Time (n) Butterworth Lowpass Filter Impulse Response Order:10

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Example 3: Butterworth Lowpass

10 20 30 40 50 60 70 80 90 100 0.2 0.4 0.6 0.8 1 1.2 1.4 h[n] Time (n) Butterworth Lowpass Filter Step Response Order:10

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Example 3: Chebyshev-I Lowpass

0.5 1 1.5 2 2.5 3 0.2 0.4 0.6 0.8 1 |H(ejω)| Chebyshev−I Lowpass Filter Transfer Function Order: 5 Frequency (rad/sample)

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Example 3: Chebyshev-I Lowpass

10 20 30 40 50 60 70 80 90 100 −0.1 −0.05 0.05 0.1 0.15 0.2 0.25 h[n] Time (n) Chebyshev−I Lowpass Filter Impulse Response Order:5

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Example 3: Chebyshev-I Lowpass

10 20 30 40 50 60 70 80 90 100 0.2 0.4 0.6 0.8 1 1.2 1.4 h[n] Time (n) Chebyshev−I Lowpass Filter Step Response Order:5

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Example 3: Chebyshev-II Lowpass

0.5 1 1.5 2 2.5 3 0.2 0.4 0.6 0.8 1 |H(ejω)| Chebyshev−II Lowpass Filter Transfer Function Order: 5 Frequency (rad/sample)

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Example 3: Chebyshev-II Lowpass

10 20 30 40 50 60 70 80 90 100 −0.1 −0.05 0.05 0.1 0.15 0.2 0.25 h[n] Time (n) Chebyshev−II Lowpass Filter Impulse Response Order:5

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Example 3: Chebyshev-II Lowpass

10 20 30 40 50 60 70 80 90 100 0.2 0.4 0.6 0.8 1 1.2 1.4 h[n] Time (n) Chebyshev−II Lowpass Filter Step Response Order:5

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Example 3: Elliptic Lowpass

0.5 1 1.5 2 2.5 3 0.2 0.4 0.6 0.8 1 |H(ejω)| Elliptic Lowpass Filter Transfer Function Order: 4 Frequency (rad/sample)

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Example 3: Elliptic Lowpass

10 20 30 40 50 60 70 80 90 100 −0.1 −0.05 0.05 0.1 0.15 0.2 0.25 h[n] Time (n) Elliptic Lowpass Filter Impulse Response Order:4

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Example 3: Elliptic Lowpass

10 20 30 40 50 60 70 80 90 100 0.2 0.4 0.6 0.8 1 1.2 1.4 h[n] Time (n) Elliptic Lowpass Filter Step Response Order:4

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Example 3: MATLAB Code

%function [] = Lowpass(); clear all; close all; Wp = 0.20; % Passband ends Ws = 0.30; % Stopband begins Rp = -20*log10(0.95); % Maximum deviation from 1 in the passband (dB) Rs = -20*log10(0.05); % Minimum attenuation in the stopband (dB) for cnt = 1:4, if cnt==1, [od,wn] = buttord(Wp,Ws,Rp,Rs); [B,A] = butter(od,wn); stFilter = ’Butterworth’; elseif cnt==2, [od,wn] = ellipord(Wp,Ws,Rp,Rs); [B,A] = ellip(od,Rp,Rs,wn); stFilter = ’Elliptic’; elseif cnt==3, [od,wn] = cheb1ord(Wp,Ws,Rp,Rs); [B,A] = cheby1(od,Rp,wn); stFilter = ’Chebyshev-I’; elseif cnt==4, [od,wn] = cheb2ord(Wp,Ws,Rp,Rs); [B,A] = cheby2(od,Rs,wn); stFilter = ’Chebyshev-II’; else break; end; sys = tf(B,A,-1); wp = Wp*pi; ws = Ws*pi;

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%============================================================================== % Plot Magnitude on Linear Scale %============================================================================== ymax = 1.05; pbmax = 1; pbmin = 10^(-Rp/20); sbmax = 10^(-Rs/20); wmax = pi; w = 0:0.001:wmax; [H,w] = freqz(B,A,w); mag = abs(H); phs = angle(H); figure; FigureSet(1,’LTX’); h = patch([0 wp wp 0],[0 0 pbmin pbmin],0.5*[1 1 1]); set(h,’LineWidth’,0.0001); hold on; h = patch([0 ws ws wmax wmax 0],[pbmax pbmax sbmax sbmax ymax ymax],0.5*[1 1 1]); set(h,’LineWidth’,0.0001); h = plot(w,mag,’r’); set(h,’LineWidth’,1.0); hold off; ylim([0 ymax]); xlim([0 wmax]); grid on; ylabel(’|H(e^{j\omega})|’); title(sprintf(’%s Lowpass Filter Transfer Function Order: %d’,stFilter,od)); xlabel(’Frequency (rad/sample)’); box off; AxisSet(8); st = sprintf(’print -depsc Lowpass%s’,stFilter); eval(st); %============================================================================== % Impulse Response %============================================================================== figure;

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FigureSet(1,’LTX’); n = 0:100; [x,t] = impulse(sys,n); h = stem(t,x,’b’); set(h(1),’MarkerFaceColor’,’b’); set(h(1),’MarkerSize’,2); ylabel(’h[n]’); xlabel(’Time (n)’); title(sprintf(’%s Lowpass Filter Impulse Response Order:%d’,stFilter,od)); box off; hold on; h = plot(xlim,[0 0],’k:’); hold off; AxisSet(8); st = sprintf(’print -depsc Lowpass%sImpulse’,stFilter); eval(st); %============================================================================== % Step Response %============================================================================== figure; FigureSet(1,’LTX’); n = 0:100; [x,t] = step(sys,n); h = stem(t,x,’b’); set(h(1),’MarkerFaceColor’,’b’); set(h(1),’MarkerSize’,2); ylabel(’h[n]’); xlabel(’Time (n)’); title(sprintf(’%s Lowpass Filter Step Response Order:%d’,stFilter,od)); box off; hold on; h = plot(xlim,[1 1],’k:’); hold off; AxisSet(8); st = sprintf(’print -depsc Lowpass%sStep’,stFilter); eval(st);

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end;

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Practical Filter Tradeoffs

  • Butterworth
  • Highest order H(ejω)

+ No passband or stopband ripple

  • Chebyshev Type I

+ No stopband ripple

  • Chebyshev Type II

+ No passband ripple

  • Elliptic

+ Lowest order H(ejω)

  • Passband and stopband ripple

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Application Example 1: Microelectrode Recording Filter An engineer wishes to detect action potentials in a microelectrode recording with a simple threshold detector. The signal contains significant baseline drift. Action potentials typically last about 1 ms.

  • What type of filter should the engineer use?
  • What should the filter specifications be?
  • What should the cutoff frequency(ies) be?

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Application Example 1: Frequency-Selective Filters

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 −0.6 −0.4 −0.2 0.2 0.4 0.6 0.8 Time (sec) Microelectrode Recording

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Application Example 1: Frequency-Selective Filters

500 1000 1500 2000 10 20 30 40 50 FT Magnitude 50 100 150 200 250 300 100 200 300 400 500 FT Magnitude

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Application Example 1: Frequency-Selective Filters

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 −0.5 0.5 Time (sec) Microelectrode Recording 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 −0.5 0.5 Time (sec)

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Application Example 1: Frequency-Selective Filters

500 1000 1500 2000 10 20 30 40 50 FT Magnitude 500 1000 1500 2000 10 20 30 40 50 FT Magnitude Filtered

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Application Example 1: MATLAB Code

%function [] = MER(); close all; [x,fs,nbits] = wavread(’Henderson2.wav’); x = decimate(x,2); fs = fs/2; k = round(fs*1):round(fs*3); % Look at only 5 s x = x(k); nx = length(x); figure; FigureSet(1,’LTX’); t = (k-1)/fs; h = plot(t,x,’b’); set(h,’LineWidth’,0.6); xrng = max(x)-min(x); xlim([min(t) max(t)]); ylim([min(x)-0.01*xrng max(x)+0.01*xrng]); AxisLines; xlabel(’Time (sec)’); ylabel(’’); title(’Microelectrode Recording’); box off; AxisSet(8); print -depsc MERSignal; X = fft(x,2^12); nX = length(X); k = 1:floor((length(X)+1)/2); f = (k-1)*(fs)./(nX+1); figure; FigureSet(1,’LTX’);

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subplot(2,1,1); h = plot(f,abs(X(k)),’r’); set(h,’LineWidth’,0.6); xlim([min(f) max(f)]); ylim([0 50]); set(gca,’XTick’,[0:500:max(f)]); box off; ylabel(’FT Magnitude’); subplot(2,1,2); h = plot(f,abs(X(k)),’r’); set(h,’LineWidth’,0.6); xlim([0 300]); ylim([0 500]); %set(gca,’XTick’,[0:500:max(f)]); S box off; ylabel(’FT Magnitude’); AxisSet(6); print -depsc MERSpectralDensity; Wp = 210/(fs/2); % Passband ends Ws = 190/(fs/2); % Stopband begins Rp = -20*log10(0.95); % Maximum deviation from 1 in the passband (dB) Rs = -20*log10(0.05); % Minimum attenuation in the stopband (dB) [od,wn] = ellipord(Wp,Ws,Rp,Rs); [B,A] = ellip(od,Rp,Rs,wn,’high’); stFilter = ’Elliptic’; y = filtfilt(B,A,x); figure; FigureSet(1,’LTX’); k = 1:length(x); t = (k-1)/fs; subplot(2,1,1); t = (k-1)/fs; h = plot(t,x,’b’);

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set(h,’LineWidth’,0.6); xrng = max(x)-min(x); xlim([min(t) max(t)]); ylim([min(x)-0.01*xrng max(x)+0.01*xrng]); AxisLines; xlabel(’Time (sec)’); ylabel(’’); title(’Microelectrode Recording’); box off; subplot(2,1,2); h = plot(t,y,’g’); set(h,’LineWidth’,0.6); xlim([min(t) max(t)]); ylim([min(x)-0.01*xrng max(x)+0.01*xrng]); AxisLines; xlabel(’Time (sec)’); ylabel(’’); box off; AxisSet(8); print -depsc MERSignalFiltered; Y = fft(y,2^12); nY = length(Y); k = 1:floor((length(Y)+1)/2); f = (k-1)*(fs)./(nY+1); figure; FigureSet(1,’LTX’); subplot(2,1,1); h = plot(f,abs(X(k)),’r’); set(h,’LineWidth’,0.6); xlim([min(f) max(f)]); ylim([0 50]); set(gca,’XTick’,[0:500:max(f)]); box off; ylabel(’FT Magnitude’); subplot(2,1,2);

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h = plot(f,abs(Y(k)),’g’); set(h,’LineWidth’,0.6); xlim([min(f) max(f)]); ylim([0 50]); set(gca,’XTick’,[0:500:max(f)]); box off; ylabel(’FT Magnitude Filtered’); AxisSet(6); print -depsc MERSpectralDensityFiltered;

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