Ex4: Passivity Evaluation and Enforcement

To demonstrate the passivity evaluation and enforcement features of the vector fitting class, the ring slot example 2-port is used, once again. Have a look at the other vector fitting example notebooks for more general explanations of the fitting process.

[1]:
import skrf
import numpy as np
import matplotlib.pyplot as mplt

# load and fit the ring slot network with 3 poles
nw = skrf.data.ring_slot
vf = skrf.VectorFitting(nw)
vf.vector_fit(n_poles_real=3, n_poles_cmplx=0)

# plot fitting results
freqs = np.linspace(0, 200e9, 201)
fig, ax = mplt.subplots(2, 2)
fig.set_size_inches(12, 8)
vf.plot_s_mag(0, 0, freqs=freqs, ax=ax[0][0]) # s11
vf.plot_s_mag(0, 1, freqs=freqs, ax=ax[0][1]) # s12
vf.plot_s_mag(1, 0, freqs=freqs, ax=ax[1][0]) # s21
vf.plot_s_mag(1, 1, freqs=freqs, ax=ax[1][1]) # s22
fig.tight_layout()
mplt.show()
../../_images/examples_vectorfitting_vectorfitting_ex4_passivity_1_0.png

The fitting result does not look too bad, but is the model still passive at all frequencies?

[2]:
print(vf.is_passive())
False

It’s not passive? What’s going on? Was the original data of the ring slot representing a passive network?

[3]:
print(nw.is_passive())
True

The network data was passive, but the vector fitted model is not. Let’s investigate (and correct?) the problem.

[4]:
# plot singular values of vector fitted scattering matrix
freqs = np.linspace(0, 200e9, 201)
fig, ax = mplt.subplots(1, 1)
fig.set_size_inches(6, 4)
vf.plot_s_singular(freqs=freqs, ax=ax)
fig.tight_layout()
mplt.show()
../../_images/examples_vectorfitting_vectorfitting_ex4_passivity_7_0.png

One of the singular values of the fitted scattering matrix is greater than 1 at some frequencies. This indeed indicates a non-passive model. For further analysis, you can get a list of all frequency bands with passivity violations:

[5]:
print(vf.passivity_test())
[[0.00000000e+00 5.97242674e+09]
 [8.43154893e+10 9.83151113e+10]]

The network is not passive in two frequency bands: From dc to about 6 GHz, and from 84.3 GHz to 98.3 GHz. Luckily, passivity can be enforced, if a passive vector fitted model is desired:

[6]:
vf.passivity_enforce()

After passivity enforcement, the network should be passive at all frequencies. Let’s check ourselves:

[7]:
print(vf.is_passive())
True
[8]:
print(vf.passivity_test())
[]
[9]:
# plot singular values of vector fitted scattering matrix
freqs = np.linspace(0, 200e9, 201)
fig, ax = mplt.subplots(1, 1)
fig.set_size_inches(6, 4)
vf.plot_s_singular(freqs=freqs, ax=ax)
fig.tight_layout()
mplt.show()
../../_images/examples_vectorfitting_vectorfitting_ex4_passivity_15_0.png

Alright, the model is finally passive. But does it still fit the original network data?

[10]:
# plot fitting results again after passivity enforcement
freqs = np.linspace(0, 200e9, 201)
fig, ax = mplt.subplots(2, 2)
fig.set_size_inches(12, 8)
vf.plot_s_mag(0, 0, freqs=freqs, ax=ax[0][0]) # s11
vf.plot_s_mag(0, 1, freqs=freqs, ax=ax[0][1]) # s12
vf.plot_s_mag(1, 0, freqs=freqs, ax=ax[1][0]) # s21
vf.plot_s_mag(1, 1, freqs=freqs, ax=ax[1][1]) # s22
fig.tight_layout()
mplt.show()
../../_images/examples_vectorfitting_vectorfitting_ex4_passivity_17_0.png

Yes, the model still fits the original data very well and the differences to the first non-passive fit from above are insignificant.