# Plot the filter's frequency response, showing the critical points:

from scipy import signal
import matplotlib.pyplot as plt

b, a = signal.ellip(4, 5, 40, 100, 'low', analog=True)
w, h = signal.freqs(b, a)
plt.semilogx(w, 20 * np.log10(abs(h)))
plt.title('Elliptic filter frequency response (rp=5, rs=40)')
plt.xlabel('Frequency [radians / second]')
plt.ylabel('Amplitude [dB]')
plt.margins(0, 0.1)
plt.grid(which='both', axis='both')
plt.axvline(100, color='green') # cutoff frequency
plt.axhline(-40, color='green') # rs
plt.axhline(-5, color='green') # rp
plt.show()