Networks

Introduction

This tutorial gives an overview of the microwave network analysis features of skrf. For this tutorial, and the rest of the scikit-rf documentation, it is assumed that skrf has been imported as rf. Whether or not you follow this convention in your own code is up to you.

If this produces an import error, please see Installation .

Creating Networks

skrf provides an object for a N-port microwave Network. A Network can be created in a number of ways. One way is from data stored in a touchstone file.

A short description of the network will be printed out if entered onto the command line

Networks can also be created by directly passing values for the frequency, s-paramters and port impedance z0.

See network for more information on network creation.

Basic Properties

The basic attributes of a microwave Network are provided by the following properties :

• Network.s : Scattering Parameter matrix.
• Network.z0 : Port Characteristic Impedance matrix.
• Network.frequency : Frequency Object.

The Network object has numerous other properties and methods. If you are using IPython, then these properties and methods can be 'tabbed' out on the command line.

In [1]: ring_slot.s<TAB>
ring_slot.line.s              ring_slot.s_arcl         ring_slot.s_im
ring_slot.line.s11            ring_slot.s_arcl_unwrap  ring_slot.s_mag
...

All of the network parameters are represented internally as complex numpy.ndarray. The s-parameters are of shape (nfreq, nport, nport)

Slicing

You can slice the Network.s attribute any way you want.

Slicing by frequency can also be done directly on Network objects like so

or with a human friendly string ,

Notice that slicing directly on a Network returns a Network. So, a nice way to express slicing in both dimensions is

Ploting

Amongst other things, the methods of the Network class provide convenient ways to plot components of the network parameters,

• Network.plot_s_db : plot magnitude of s-parameters in log scale
• Network.plot_s_deg : plot phase of s-parameters in degrees
• Network.plot_s_smith : plot complex s-parameters on Smith Chart
• ...

If you would like to use skrf's plot styling,

To plot all four s-parameters of the ring_slot on the Smith Chart.

Combining this with the slicing features,

For more detailed information about plotting see Plotting.

Operators

Arithmetic Operations

Element-wise mathematical operations on the scattering parameter matrices are accessible through overloaded operators. To illustrate their usage, load a couple Networks stored in the data module.

All of these operations return Network types. For example, to plot the complex difference between short and delay_short,

Another common application is calculating the phase difference using the division operator,

Linear operators can also be used with scalars or an numpy.ndarray that ais the same length as the Network.

Cascading and De-embedding

Cascading and de-embeding 2-port Networks can also be done though operators. The cascade function can be called through the power operator, **. To calculate a new network which is the cascaded connection of the two individual Networks line and short,

De-embedding can be accomplished by cascading the inverse of a network. The inverse of a network is accessed through the property Network.inv. To de-embed the short from delay_short,

Comparison operators also work with networks.

Connecting Multi-ports

skrf supports the connection of arbitrary ports of N-port networks. It accomplishes this using an algorithm called sub-network growth[1], available through the function connect(). Terminating one port of an ideal 3-way splitter can be done like so,

To connect port 1 of the tee, to port 0 of the delay short,

Note that this function takes into account port impedances. If two connected ports have different port impedances, an appropriate impedance mismatch is inserted.

Interpolation and Concatenation

A common need is to change the number of frequency points of a Network. To use the operators and cascading functions the networks involved must have matching frequencies, for instance. If two networks have different frequency information, then an error will be raised,

line1+line

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-49-82040f7eab08> in <module>()
----> 1 line1+line

/home/alex/code/scikit-rf/skrf/network.py in __add__(self, other)
    500 
    501         if isinstance(other, Network):
--> 502             self.__compatable_for_scalar_operation_test(other)
    503             result.s = self.s + other.s
    504         else:

/home/alex/code/scikit-rf/skrf/network.py in __compatable_for_scalar_operation_test(self, other)
    701         '''
    702         if other.frequency  != self.frequency:
--> 703             raise IndexError('Networks must have same frequency. See `Network.interpolate`')
    704 
    705         if other.s.shape != self.s.shape:

IndexError: Networks must have same frequency. See `Network.interpolate`

This problem can be solved by interpolating one of Networks allong the frequency axis using Network.resample.

And now we can do things

You can also interpolate from a Frequency object. For example,

A related application is the need to combine Networks which cover different frequency ranges. Two Netwoks can be concatenated (aka stitched) together using stitch, which concatenates networks along their frequency axis. To combine a WR-2.2 Network with a WR-1.5 Network,

Reading and Writing

For long term data storage, skrf has support for reading and partial support for writing touchstone file format. Reading is accomplished with the Network initializer as shown above, and writing with the method Network.write_touchstone().

For temporary data storage, skrf object can be pickled with the functions skrf.io.general.read and skrf.io.general.write. The reason to use temporary pickles over touchstones is that they store all attributes of a network, while touchstone files only store partial information.

Frequently there is an entire directory of files that need to be analyzed. rf.read_all creates Networks from all files in a directory quickly. To load all skrf files in the data/ directory which contain the string 'wr2p2'.

Other Parameters

This tutorial focuses on s-parameters, but other network represenations are available as well. Impedance and Admittance Parameters can be accessed through the parameters Network.z and Network.y, respectively. Scalar components of complex parameters, such as Network.z_re, Network.z_im and plotting methods are available as well.

Other parameters are only available for 2-port networks, such as wave cascading parameters (Network.t), and ABCD-parameters (Network.a)

Conclusion

There are many more features of Networks that can be found in networks

References

[1] Compton, R.C.; , "Perspectives in microwave circuit analysis," Circuits and Systems, 1989., Proceedings of the 32nd Midwest Symposium on , vol., no., pp.716-718 vol.2, 14-16 Aug 1989. URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=101955&isnumber=3167