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.
