Chebyshev Type II analog lowpass filter prototype
Design a 6th-order Chebyshev Type II analog lowpass filter with 70 dB of ripple in the stopband. Display its magnitude and phase responses.
[z,p,k] = cheb2ap(6,70); % Lowpass filter prototype [num,den] = zp2tf(z,p,k); % Convert to transfer function form freqs(num,den) % Frequency response of analog filter
n— Filter order
Filter order, specified as an integer.
Rs— Stopband ripple
Stopband ripple, specified as a scalar in decibels.
Chebyshev Type II filters are monotonic in the passband and equiripple in the stopband.
The pole locations are the inverse of the pole locations of the
cheb1ap function, whose poles are evenly spaced about an ellipse in the left
half plane. The Chebyshev Type II stopband edge angular frequency
ω0 is set to 1 for a normalized result. This value
is the frequency at which the stopband begins. The filter has a magnitude response of
Chebyshev Type II filters are sometimes called inverse Chebyshev filters
because of their relationship to Chebyshev Type I filters. The
function is a modification of the Chebyshev Type I prototype algorithm:
cheb2ap replaces the frequency variable
ω with 1/ω, turning the lowpass filter into a
highpass filter while preserving the performance at ω = 1.
cheb2ap subtracts the filter transfer function
The transfer function is given by
 Parks, Thomas W., and C. Sidney Burrus. Digital Filter Design. New York: John Wiley & Sons, 1987.
Usage notes and limitations:
All inputs must be constants. Expressions or variables are allowed if their values do not change.