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Qucs
A Tutorial
Component, compact device and circuit modelling using symbolic equations

Mike Brinson
Copyright c 2007 Mike Brinson Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation. A copy of the license is included in the section entitled”GNU Free Documentation License”.

Introduction
Qucs releases 0.0.11 and 0.0.12 mark a turning point in the development of the Qucs component and circuit modelling facilities. Release 0.0.11 introduced component values defined by equations and for the first time allowed subcircuits with parameters. Release 0.0.12 extends these features to add model development using symbolic equations that aresimilar to compact device code written in the Verilog-A modelling language. In designing the latest Qucs modelling features the Qucs team has made a central focus of their work the need to provide the package with an interactive and easy to use modelling system which allows fast model prototype construction. Much of these new aspects have up to now been undocumented and are likely to be very new tomost Qucs users. The aim of this tutorial note is to outline the background to these important package extensions and to provide real help to Qucs users who are interested in writing and experimenting with their own models. The text includes a number of illustrative examples for readers to try and experiment with.

Qucs electronic device and circuit modelling
Circuit simulation packages arecomplex software systems which often take years to mature to a stage where they are capable of analysing the current generation of integrated and discrete electronic circuits. Most circuit simulators have a number of common basic attributes; firstly circuits are represented by a textual netlist or a schematic diagram which contains all the information required by a simulator to analyse the performanceof a circuit, and secondly a simulation engine which undertakes the calculation of circuit performance in one or more different circuit domains such as DC, AC or transient, and thirdly a post simulation processing system which structures and displays the simulation data in both tabular and graphical forms. All circuit simulators have one other important attribute, namely that they representindividual electronic components by a model, or abstraction, in a way that can be understood and analysed by the simulation engine when undertaking a simulation task. Without component models the science of circuit simulation would not have developed to the stage it has today. From a users point of view component models are the key to simulator productivity; the greater the number of different models theeasier it becomes to analyse mixed analogue and digital electronic systems. Shown in Fig. 1 is a block diagram of the analogue component modelling and simulation facilities currently provided by the Qucs package. The diagram is structured as a flow chart which emphasises the different device modelling routes. When Qucs was first released only two of these were available for users to develop new devicemodels. The first of these has been used extensively by the package developers to construct the built-in models that are distributed with each Qucs release. This fundamental route involves hand coding the C++ code for a new model1 , its compilation and linking with the core Qucs C++
1

The technical details of the built-in models are described in: Qucs Technical Papers, Stefan Jahn,

1code. Obviously, this does require a specialised knowledge of the Qucs model programming interface2 , the necessary C++ skills, including a good working knowledge of the Trolltech Qt toolkit3 . At the time of writing these notes the latest device to be added to Qucs using this approach is the exponential pulse source4 . Models based on hand written C++ code are normally restricted to basic...