Introduction ¶

Contents

Introduction

This is a brief introduction to skrf, aimed at those who are familiar with python. If you are unfamiliar with python, please see scipy’s Getting Started . This tutorial is most easily followed by using the ipython shell with the –pylab flag.

Note: All of the scripts and touchstone files used in these tutorials are provided along with this source code in the directory doc/source/pyplots/ (relative to the scikit-rf root).

Creating Networks ¶

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:

>>> import skrf as rf

If this produces an error, please see Installation.

skrf provides an object for a n-port microwave Network. Most commonly, a Network is constructed from data stored in a touchstone files, like so

>>> short = rf.Network('short.s1p')
>>> delay_short = rf.Network('delay_short.s1p')

Once created, the Network object will print out a short description if entered onto the command line:

>>> short
1-Port Network.  75-110 GHz.  201 points. z0=[ 50.]

Basic Network Properties ¶

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

Network.s : Scattering Parameter matrix.
Network.z0 : Characterisic Impedance matrix.
Network.frequency : Frequency Object.

These properties are represented internally as complex numpy.ndarray’s, (and the indexing starts at 0!). The Network class has numerous other properties and methods which can found in the Network docs. If you are using Ipython, then these properties and methods can be ‘tabbed’ out on the command line. Amongst other things, the methods of the Network class provide convenient ways to plot components of the s-parameters, below is a short list of common plotting commands,

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

For example, to create a 2-port Network from a touchstone file, and then plot all s-parameters on the Smith Chart.

import pylab
import skrf as rf

# create a Network type from a touchstone file
ring_slot = rf.Network('ring slot.s2p')
ring_slot.plot_s_smith()
pylab.show()

(Source code, png, hires.png, pdf)

For more detailed information about plotting see Plotting.

Network Operators ¶

Element-wise Operations ¶

Element-wise mathematical operations on the scattering parameter matrices are accessible through overloaded operators:

>>> short + delay_short
>>> short - delay_short
>>> short / delay_short
>>> short * delay_short

All of these operations return Network types, so the methods and properties of a Network are available on the result. For example, the difference operation (‘-‘) can be used to calculate the complex distance between two networks

>>> difference = (short- delay_short)

The result is stored in the returned Network object’s Network.s parameter matrix. So, to plot the magnitude of the complex difference between the networks short and delay_short:

>>> (short - delay_short).plot_s_mag()

Another common application is calculating the phase difference using the division operator. This can be done

>>> (delay_short/short).plot_s_deg()

Cascading and Embeding Operations ¶

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

>>> line = rf.Network('line.s2p')
>>> short = rf.Network('short.s1p')
>>> delay_short = line ** short

De-embedding can be accomplished either by using the de_embed() function, or by cascading the inverse of a network. The inverse of a network is accesible through the property Network.inv. So, to de-embed the short from delay_short:

>>> short = line.inv ** delay_short

Connecting Multi-ports ¶

skrf supports the connection of arbitrary ports of N-port networks. It accomplishes this using an algorithm call sub-network growth [1]. This algorithm, which is available through the function connect(), takes into account port impedances. Terminating one port of a ideal 3-way splitter can be done like so:

>>> tee = rf.Network('tee.s3p')
>>> delay_short = rf.Network('delay_short.s1p')

to connect port ‘1’ of the tee, to port 0 of the delay short:

>>> terminated_tee = rf.connect(tee,1,delay_short,0)

Sub-Networks ¶

Frequently, the one-port s-parameters of a multiport network’s are of interest. These can be accessed by theh sub-network properties, which return one-port Network objects

>>> port1_return = line.s11
>>> port1_insertion = line.s21

Convenience Functions ¶

Frequently there is an entire directory of touchstone files that need to be analyzed. The function load_all_touchstones() is meant deal with this scenario. It takes a string representing the directory, and returns a dictionary type with keys equal to the touchstone filenames, and values equal to Network types:

>>> ntwk_dict = rf.load_all_touchstones('.')
{'delay_short': 1-Port Network.  75-110 GHz.  201 points. z0=[ 50.],
'line': 2-Port Network.  75-110 GHz.  201 points. z0=[ 50.  50.],
'ring slot': 2-Port Network.  75-110 GHz.  201 points. z0=[ 50.  50.],
'short': 1-Port Network.  75-110 GHz.  201 points. z0=[ 50.]}

Other convenient functions, and pre-initialized objects are located in the convenience module

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