Chapter: Digital Signal Processing - IIR Filter Design

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Frequency Response Characteristic

The frequency response characteristic of |Ha(Ω)|-Power(2) is as shown. As the order of the filter N increases, the butterworth filter characteristic is more close to the ideal characteristic.

FREQUENCY RESPONSE CHARACTERISTIC

 

The frequency response characteristic of |Ha()|2 is as shown. As the order of the filter N increases, the butterworth filter characteristic is more close to the ideal characteristic. Thus at higher orders like N=16 the butterworth filter characteristic closely approximate ideal filter characteristic. Thus an infinite order filter (N ) is required to get ideal characteristic.




Ap= attenuation in passband.

As= attenuation in stopband.

Ωp = passband edge frequency

Ωs = stopband edge frequency

 

Specification for the filter is


To determine the poles and order of analog filter consider equalities. 




Q) Design a digital filter using a butterworth approximation by using impulse invariance. 


  Filter Type - Low Pass Filter           

Ap - 0.89125                

As  - 0.17783                

Ωp -  0.2∏          

Ωs -  0.3∏          

 

Step 1) To convert specification to equivalent analog filter.

 

(In impulse invariance method frequency relationship is given as ω= Ω T while in Bilinear transformation method frequency relationship is given as Ω= (2/T) tan (ω/2) If Ts is not specified consider as 1)

 

|Ha(Ω)| ≥ 0.89125 for Ω ≤ 0.2∏/T and       |Ha(Ω)| ≤ 0.17783 for    Ω ≥ 0.3∏/T.

 

Step 2) To determine the order of the filter.


N= 5.88

 

1.   Order of the filter should be integer.

 

2.   Always go to nearest highest integer vale of N.

 

Hence N=6

 

Step 3) To find out the cutoff frequency (-3DB frequency)


cutoff frequency Ωc = 0.7032

 

Step 4) To find out the poles of analog filter system function.


For stable filter all poles lying on the left side of s plane is selected. Hence

 

S1  = -0.182 + j 0.679               S1* = -0.182 - j 0.679

S2  = -0.497 + j 0.497                       S2* = -0.497 - j 0.497

S3  = -0.679 + j 0.182                         S3* = -0.679 - j 0.182

 

Step 5) To determine the system function (Analog Filter)



Step 6) To determine the system function (Digital Filter)

 

(In Bilinear transformation replace s by the term ((z-1)/(z+1)) and find out the transfer function of digital function)

 

Step 7) Represent system function in cascade form or parallel form if asked.

 

 

Q) Given for low pass butterworth filter


 

Ap= -1 db at 0.2∏

As= -15 db at 0.3∏

 

a)     Calculate N and Pole location

 

b)    Design digital filter using BZT method.

 

Q) Obtain transfer function of a lowpass digital filter meeting specifications

Cutoff 0-60Hz

Stopband > 85Hz

Stopband attenuation > 15 db

Sampling frequency= 256 Hz . use butterworth characteristic.

 

Q) Design second order low pass butterworth filter whose cutoff frequency is 1 kHz at sampling frequency of 104 sps. Use BZT and Butterworth approximation.


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