# Ex2: Measured 190 GHz Active 2-Port

The Vector Fitting feature is demonstrated using a 2-port S-matrix of an active circuit measured from 140 GHz to 220 GHz. Additional explanations and background information can be found in the Vector Fitting tutorial.

```
[1]:
```

```
import numpy as np
import skrf
```

This example is a lot more tricky to fit, because the responses contain a few “bumps” and noise from the measurement. In such a case, finding a good number of initial poles can take a few iterations. A good alternative for such cases is the automatic vector fitting feature, `auto_fit()`

, which automatically refines the number of poles until a good fit is achieved.

Load the Network from a Touchstone file and create the Vector Fitting instance and start the automatic fit:

```
[2]:
```

```
nw = skrf.network.Network('./190ghz_tx_measured.S2P')
vf = skrf.VectorFitting(nw)
vf.auto_fit()
```

The function `plot_convergence()`

can be helpful to examine the convergence and see if something was going wrong. Important parameters to check are the number of iterations until convergence and the final model order.

```
[3]:
```

```
vf.plot_convergence()
```

```
[4]:
```

```
print(f'model order = {vf.get_model_order(vf.poles)}')
print(f'n_poles_real = {np.sum(vf.poles.imag == 0.0)}')
print(f'n_poles_complex = {np.sum(vf.poles.imag > 0.0)}')
```

```
model order = 15
n_poles_real = 1
n_poles_complex = 7
```

Checking the results by comparing the model responses to the original sampled data indicates a successful fit, which is also indicated by a small rms error (less than 0.05):

```
[5]:
```

```
vf.get_rms_error()
```

```
[5]:
```

```
np.float64(0.014023443772011032)
```

```
[6]:
```

```
# plot frequency responses
vf.plot_s_db()
```

It is a good idea to also check the model response well outside the original frequency range.

```
[7]:
```

```
freqs = np.linspace(0, 500e9, 501) # plot model response from dc to 500 GHz
vf.plot_s_db(freqs=freqs)
```