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Design a Butterworth digital IIRIow pass filter using Bilinear transformation by taking T=1

second, to satisfy the following specifications.

0,707|H(ejw)|1.0:0w0.2π

H(ejw)|0.08:0.4πwπ

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Given:

Desired Pass-band edge ωp=0.2π

Desired Stop-band edge ωs=0.4π

Gain at Pass-band edge Ap=0.707

Gain at Stop-band edge As=0.08

T=1sec

Step 1: Pre-warp Analog Frequency:

By Bilinear Transformation (BLT)

Ω=2Ttanω2

Ωp=21tanωp2=2tan0.2π2=0.6498

Ωs=21tanωs2=2tan0.4π2=1.4531

Step 2: Order of the Filter (N)

N1=log(1A2s1)log(1A2p1)2(logΩslogΩp)

=log(10.0821)log(10.70721)2(log1.4531log0.6498)=3.1340

since, NN1, let N=4,

Order of Butterworth filter =4

Step 3: 3dB Cut Off Analog Frequency

Ωc=Ωp(1A2p1)12N

=0.6498(10.70721)18

=0.6498

Step 4: T.F H(s) of the analog LPF

Since N=4 is even, Normalized T.F.

H(s)=N/2k=1BkΩ2cs2+bkΩcs+ckΩ2c

Here, bk=2sin[(2k1)π2N],Bk=1 and ck=1,k=0,1,2,

H(s)=2k=11×0.64982s2+2sin[(2k1)π2×4]×0.6498s+1×0.64982

=0.4222s2+1.2996ssinπ8+0.4222×0.4222s2+1.2996ssin3π8+0.4222

=0.42222(s2+0.4973s+0.4222)(s2+1.2007s+0.4222)(1)

Step 5: Digital Transfer Function

By BLT Method,

Put s=2(z1)T(z+1)=2(z1)1(z+1)=2z2z+1 in (1)

Digital Transfer Function

H(z)=0.1783[(2z2z+1)2+0.4973(2z2z+1)+0.4222]×[(2z2z+1)2+1.2007(2z2z+1)+0.4222]

=0.1783(z+1)4[(2z2)2+0.4973(2z2)(z+1)+0.4222(z+1)2]×[(2z2)2+1.2007(2z2)(z+1)+0.4222(z+1)2]

=0.1783(z+1)4[5.4168z27.1556z+3.4276][6.8236z27.1556z+2.0208]

=0.1783(z+1)45.4168[z21.3210z+0.6328]×6.8236[z21.0487z+0.2962]

=0.0048(z+1)4(z21.3210z+0.6328)(z21.0487z+0.2962)

Digital Transfer Function

H(z)=0.0048(z+1)4(z21.3210z+0.6328)(z21.0487z+0.2962)

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