1 files changed, 66 insertions, 0 deletions
diff --git a/buch/papers/ellfilter/python/chebychef.py b/buch/papers/ellfilter/python/chebychef.py
new file mode 100644
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+++ b/buch/papers/ellfilter/python/chebychef.py
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+# %%
+
+import matplotlib.pyplot as plt
+import scipy.signal
+import numpy as np
+
+
+order = 5
+passband_ripple_db = 1
+omega_c = 1000
+
+a, b = scipy.signal.cheby1(
+    order,
+    passband_ripple_db,
+    omega_c,
+    btype='low',
+    analog=True,
+    output='ba',
+    fs=None,
+)
+
+w, mag, phase = scipy.signal.bode((a, b), w=np.linspace(0,2000,256))
+f, axs = plt.subplots(2,1, sharex=True)
+axs[0].plot(w, 10**(mag/20))
+axs[0].set_ylabel("$|H(\omega)| /$ db")
+axs[0].grid(True, "both")
+axs[1].plot(w, phase)
+axs[1].set_ylabel(r"$arg H (\omega) / $ deg")
+axs[1].grid(True, "both")
+axs[1].set_xlim([0, 2000])
+axs[1].set_xlabel("$\omega$")
+plt.show()
+
+
+# %% Cheychev filter F_N plot
+
+w = np.linspace(-1.1,1.1, 1000)
+plt.figure(figsize=(5.5,2.5))
+for N in [3,6,11]:
+    # F_N = np.cos(N * np.arccos(w))
+    F_N = scipy.special.eval_chebyt(N, w)
+    plt.plot(w, F_N, label=f"$N={N}$")
+plt.xlim([-1.2,1.2])
+plt.ylim([-2,2])
+plt.grid()
+plt.xlabel("$w$")
+plt.ylabel("$T_N(w)$")
+plt.legend()
+plt.tight_layout()
+plt.savefig("F_N_chebychev2.pgf")
+plt.show()
+
+# %% Build Chebychev polynomials
+
+N = 11
+
+zeros = (np.arange(N)+0.5) * np.pi
+zeros = np.cos(zeros/N)
+
+x = np.linspace(-1.2,1.2,1000)
+y = np.prod(x[:, None] - zeros[None, :], axis=-1)*2**(N-1)
+
+plt.plot(x, y)
+plt.ylim([-1,1])
+plt.grid()
+plt.show()