Wilkinson divider example

Just a simple application example from the scikit-rf universe

Some month ago I found the python package scitkit-rf, so I was in need of a playground, python virtual environment fits best.

It took some time to get into it, after some month of playing around there are still oceans of the unknown…

Juypter notbooks seem to be almost universal for experimenting, see first results below.

# standard imports
%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
import skrf as rf
from skrf.circuit import Circuit

# Set global font size to 22
#plt.rcParams.update({'font.size': 14})


# frequency band
freq = rf.Frequency(start=0, stop=2, npoints=1001, unit='GHz')
# characteristic impedance of the ports
z0_ports = 50
# resistor
R = 100
line_resistor = rf.media.DefinedGammaZ0(frequency=freq, z0=R)
resistor = line_resistor.resistor(R, name='resistor')
# branches
z0_branches = np.sqrt(2)*z0_ports
beta = freq.w/rf.constants.c
line_branches = rf.media.DefinedGammaZ0(frequency=freq, z0=z0_branches, gamma=0+beta*1j)
d = line_branches.theta_2_d(90, deg=True)  # @ 90°(lambda/4)@ 1 GHz is ~ 75 mm
branch1 = line_branches.line(d, unit='m', name='branch1')
branch2 = line_branches.line(d, unit='m', name='branch2')
# ports
port1 = Circuit.Port(freq, name='port1', z0=50)
port2 = Circuit.Port(freq, name='port2', z0=50)
port3 = Circuit.Port(freq, name='port3', z0=50)
# Connection setup
# Note that the order of appearance of the port in the setup is important
connections = [
           [(port1, 0), (branch1, 0), (branch2, 0)],
           [(port2, 0), (branch1, 1), (resistor, 0)],
           [(port3, 0), (branch2, 1), (resistor, 1)]
        ]
# Building the circuit
C = Circuit(connections)

# plot its behavior
fig, (ax1,ax2) = plt.subplots(2, 1, sharex=False)
fig.suptitle('Ideal Wilkinson Divider @ 1 GHz')

C.network.plot_s_db(ax=ax1, m=0, n=0,  lw=1)  # S11
C.network.plot_s_db(ax=ax1, m=1, n=1,  lw=1)  # S22
ax1.set_ylim(-120, 0)

C.network.plot_s_db(ax=ax2, m=1, n=0,  lw=2)  # S21
C.network.plot_s_db(ax=ax2, m=2, n=0,  ls='--', lw=2)  # S31
ax2.set_ylim(-4, -2.5)

# fontsize seems to be ignored as it does not work at all
# text is almost unreadable, sorry, commented
#C.plot_graph(network_labels=True, edge_labels=True, port_labels=True, port_fontize=22)

Have Fun!

73 Johannes