Chapter: Digital Signal Processing - IIR Filter Design

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Method For Designing Digital Filters Using BZT

Normalized analog transfer function is frequency scaled by replacing s by s/ωp*.

METHOD FOR DESIGNING DIGITAL FILTERS USING BZT


 

step 1. Find out the value of ωc*,.



step 2. Find out the value of frequency scaled analog transfer function

 

 

Normalized analog transfer function is frequency scaled by replacing s by s/ωp*.

 

step 3. Convert into digital filter

 

Apply BZT. i.e Replace s by the ((z-1)/(z+1)). And find out the desired transfer function of digital function.

 

Example:

 

Q) Design first order high pass butterworth filter whose cutoff frequency is 1 kHz at sampling frequency of 104 sps. Use BZT Method

 

Step 1. To find out the cutoff frequency

 

ωc = 2∏f

= 2000 rad/sec

 

Step 2. To find the prewarp frequency

ωc* = tan (ωc Ts/2) = tan(∏/10)

 

Step 3. Scaling of the transfer function

 

For First order HPF transfer function H(s) = s/(s+1) Scaled transfer function H*(s) = H(s) |s=s/ωc*

H*(s)= s/(s + 0.325)


Step 4. Find out the digital filter transfer function. Replace s by (z-1)/(z+1)




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

 

Q) First order low pass butterworth filter whose bandwidth is known to be 1 rad/sec . Use BZT method to design digital filter of 20 Hz bandwidth at sampling frequency 60 sps.

 

Q) Second order low pass butterworth filter whose bandwidth is known to be 1 rad/sec . Use BZT method to obtain transfer function H(z) of digital filter of 3 DB cutoff frequency of 150 Hz and sampling frequency 1.28 kHz.

 

Q) The transfer function is given as s2+1 / s 2+s+1 The function is for Notch filter with frequency 1 rad/sec. Design digital Notch filter with the following specification


(1) Notch Frequency= 60 Hz

 

(2) Sampling frequency = 960 sps.

 

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