The ‘Quite universal circuit simulator’ Qucs (pronounced: kju:ks) is an open source circuit simulator developed by a group of engineers, scientists and mathematicians under the GNU General Public License (GPL). Qucs is the brain-child of German Engineers Michael Margraf and Stefan Jahn. Since its initial public release in 2003 around twenty contributors, from all regions of the world, have invested their expertise and time to support the development of the software. Both binary and source code releases take place at regular intervals. Qucs numbered releases and day-to-day development code snapshots can be downloaded from ( Versions are available for Linux (Ubuntu and other distributions), Mac OS X © and the Windows © 32 bit operating system.

In the period since Qucs was first released it has evolved into an advanced circuit simulation and device modelling tool with a user friendly “graphical user interface” (GUI) for circuit schematic capture, for investigating circuit and device properties from DC to RF and beyond, and for launching other circuit simulation software, including the FreeHDL (VHDL) and Icarus Verilog digital simulators. Qucs includes built-in code for processing and visualising simulation output data. Qucs also allows users to process post-simulation data with the popular Octave numerical data analysis package. Similarly, circuit performance optimisation is possible using the A SPICE Circuit Optimizer (ASCO) package or Python code linked to Qucs.

Between 2003, and January 2015, the sourceforge Qucs download statistics show that over one million downloads of the software have been recorded. As well as extensive circuit simulation capabilities Qucs supports a full range of device modelling features, including non-linear and RF equation-defined device modelling and the use of the Verilog-A hardware description language (HDL) for compact device modelling and macromodelling. Recent extensions to the software aim to diversify the Qucs modelling facilities by running the Berkeley “Model and Algorithm Prototyping Platform” (MAPP) in parallel with Qucs, using Octave launched from the Qucs GUI. In the future, as the Qucs project evolves, the software will also provide circuit designers with a choice of simulation engine selected from the Qucs built-in code, ngspice and Xyce ©.

Qucs is a large software package which takes time to learn. Incidentally, this statement is also true for other GPL circuit simulators. New users must realise that to get the best from the software some effort is required on their part. In particular, one of the best ways to become familiar with Qucs is to learn a few basic user rules and how to apply them. Once these have been mastered users can move on with confidence to next level of understanding. Eventually, a stage will be reached which allows Qucs to be used productively to model devices and to investigate the performance of circuits. Qucs is equally easy to use by absolute beginners, like school children learning the physics of electrical circuits consisting of a battery and one or more resistors, as it is by cutting edge engineers working on the modelling of sub-nano sized RF MOS transistors with hundreds of physical parameters.

The primary purpose of these notes is to provide Qucs users with a source of reference for the operation and capabilities of the software. The information provided also indicates any known limitations and, if available, provides details of any work-arounds. Qucs is a high level scientific/engineering tool who’s operation and performance does require users to understand the basic mathematical, scientific and engineering principles underlying the operation of electronic devices and the design and analysis of electronic circuits. Hence, the individual sections of the Qucs-Help document include material of a technical nature mixed in with details of the software operation. Most sections introduce a number of worked design and simulation examples. These have been graded to help readers with different levels of understand get the best from the Qucs circuit simulator. Qucs-Help is a dynamic document which will change with every new release of the Qucs software. At this time, Qucs release 0.0.19, the document is far from complete but given time it will improve.