Create a Pole Zero Plot.
Transfer Function:¶
$ \begin{align} H(z)=\frac{z^2+0.2z+1}{z^2} \end{align} $
Define the two polynomials in python, and find their roots; which are the poles and zeros of the transfer function:
import numpy as np
b = [1, 0.2, 1]
a = [1, 0, 0]
poles = np.roots(a)
zeros = np.roots(b)
print("")
print("The poles are: ", poles)
print("The zeros are: ", zeros)
Test frequency is 1 KHz. Sampling rate is 32 KHz, which corresponds to one revoluton around the unit ciricle.
Find the complex value for 1 kHz in the Z plane. The location is on the unit circule, with an angle related to the ratio of 1 to 32.
angle = (1/32) * 2 * np.pi
frequency = np.exp(1j*angle)
print("")
print(frequency)
Calculate the distance from the frequency point to the zeros:¶
As we move around the unit circle, the distance from out currentl location to to the zeros determines the amplitude of the transfer function.
dist_one = frequency - zeros[0]
dist_two = frequency - zeros[1]
dist_one_mag = np.absolute(dist_one)
dist_two_mag = np.absolute(dist_two)
print("")
print("The distances from 1 KHz to the zeros are:")
print(dist_one_mag)
print(dist_two_mag)
print("")
print("And multiplied together we get")
print(dist_one_mag * dist_two_mag)
Create a Pole Zero Plot¶
%matplotlib inline
%config InlineBackend.figure_format = 'retina'
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns
plt.rcdefaults()
plt.xkcd()
# Reset default params
#sns.set()
# Set context to paper | talk | notebook | poster
# sns.set_context("poster")
fig = plt.figure(figsize=(8,8)) # sets size and makes it square
ax = plt.axes() # ax.set_aspect(1)
# plot unit circle
theta = np.linspace(-np.pi, np.pi, 201)
plt.plot(np.sin(theta), np.cos(theta), color = 'gray', linewidth=0.2)
# plot x-y axis
ax.axhline(y=0, color='gray', linewidth=1)
ax.axvline(x=0, color='gray', linewidth=1)
# plot poles and zeros
plt.plot(np.real(poles), np.imag(poles), 'Xb', label = 'Poles')
plt.plot(np.real(zeros), np.imag(zeros), 'or', label = 'Zeros')
# plot frequency point
angle = (1/32) * 2 * np.pi
frequency = np.exp(1j*angle)
plt.plot(np.real(frequency), np.imag(frequency), '.g', label = '1 KHz')
# annotations
ax.annotate('2 poles',
xy=(0,0),
xycoords='data',
xytext=(30,60),
textcoords='offset points',
va='top',
ha='left',
bbox=dict(facecolor='w',
edgecolor='none',
boxstyle='round, pad=0'
),
arrowprops=dict(facecolor='b',
edgecolor='b',
width=2,
linewidth=0,
shrink=0.2,
headwidth=6
#headlength=8
)
)
ax.annotate('1 KHz',
xy=(np.real(frequency), np.imag(frequency)),
xycoords='data',
xytext=(-60,-5),
textcoords='offset points',
va='top',
ha='right',
bbox=dict(facecolor='w',
edgecolor='none',
boxstyle='round, pad=0'
),
arrowprops=dict(facecolor='g',
edgecolor='g',
width=2,
linewidth=0,
shrink=0.2,
headwidth=6
#headlength=8
)
)
ax.annotate('zero',
xy=(np.real(zeros[0]), np.imag(zeros[0])),
xycoords='data',
xytext=(-40,-40),
textcoords='offset points',
va='top',
ha='right',
bbox=dict(facecolor='w',
edgecolor='none',
boxstyle='round, pad=0'
),
arrowprops=dict(facecolor='r',
edgecolor='r',
width=2,
linewidth=0,
shrink=0.2,
headwidth=6
#headlength=8
)
)
ax.annotate('zero',
xy=(np.real(zeros[1]), np.imag(zeros[1])),
xycoords='data',
xytext=(-40,40),
textcoords='offset points',
va='top',
ha='right',
bbox=dict(facecolor='w',
edgecolor='none',
boxstyle='round, pad=0'
),
arrowprops=dict(facecolor='r',
edgecolor='r',
width=2,
linewidth=0,
shrink=0.2,
headwidth=6
#headlength=8
)
)
# add lables
plt.title("Cool Pole-Zero Plot")
plt.xlabel("Real")
plt.ylabel("Imaginary")
plt.legend(loc='upper left')
plt.grid()
plt.show()
Plot the Transfer Function¶
from 0 to 32 KHz, which is the same as from 0 to $2\pi$.
%matplotlib inline
%config InlineBackend.figure_format = 'retina'
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns
plt.rcdefaults()
plt.xkcd()
# Reset default params
#sns.set()
# Set context to paper | talk | notebook | poster
#sns.set_context("poster")
fig = plt.figure(figsize=(8,6))
ax = plt.axes()
# H(z) = b/a
b = [1, 0.2, 1] # z^2 + 0.2z + 1
a = [1, 0, 0] # z^2
# create w array which travels around unit circle
w = np.linspace(0, 2*np.pi, 201) # for evauluating H(w)
z = np.exp(1j*w)
f = np.linspace(0, 32, 201) # for ploting H(w)
# evalue H over the w array
H = np.polyval(b, z) / np.polyval(a, z)
# plot the magnitude of H vs frequency. H is a complex number array.
plt.plot(f, np.absolute(H))
# plot frequency point
sample = 32 # KHz
freq = 1 # KHz
angle = (freq/sample) * 2 * np.pi
frequency = np.exp(1j*angle)
Hf = np.polyval(b, frequency) / np.polyval(a, frequency)
Hf = np.absolute(Hf)
plt.plot(freq, Hf, 'og')
# call out frequency point
text = "{0:0.4f} at {1:0.0f} KHz".format(Hf, freq)
ax.annotate(text,
xy=(freq, Hf),
xycoords='data',
xytext=(60, 5),
textcoords='offset points',
va='top',
ha='left',
bbox=dict(facecolor='w',
edgecolor='none',
boxstyle='round,pad=1'
),
arrowprops=dict(facecolor='g',
edgecolor = 'g',
width = 2,
linewidth = 0,
shrink=0.2,
headwidth = 6,
headlength = 8,
)
)
# add lables
plt.title("H(w) Frequency Response",y=1.03)
plt.xlabel("Frequency (KHz)")
plt.ylabel("Magnitude |H(w)|");
plt.xlim(0, 32)
plt.xticks(np.linspace(0, 32, 17)) # I want every 2 KHz
plt.ylim(ymin=0) # let ymax auto scale
plt.grid()
plt.show()
