Qucs - open-source CAD for electronic circuit modeling.

Currently there are not so many open-source CAD systems. Nevertheless, among CAD for electronics (EDA) there are very worthy products. This post will be devoted to the Qucs open source electronic circuit simulator. Qucs is written in C ++ using the Qt4 framework. Qucs is cross-platform and released for Linux, Windows and MacOS.

The development of this CAD began in 2004 by the Germans Michael Margraf and Stefan Jahn (currently not active). Now Qucs is being developed by an international team that I belong to. The project managers are Frans Schreuder and Guilherme Torri. Under the cat will be told about the key features of our circuit designer, its advantages and disadvantages compared to analogues.

The main program window is shown in the screenshot. There, a resonant amplifier was modeled at a field-effect transistor and oscillograms of the input and output voltage and also the frequency response were obtained.


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As you can see, the interface is intuitive. The central part of the window is occupied by the actual simulated scheme. Components are placed on the scheme by dragging from the left side of the window. Types of modeling and equations are also special components. The principles of editing schemes are described in more detail in the program documentation.

The Qucs schema file format is XML based and documentation is supplied to it. Therefore, the Qucs scheme can be easily generated by third-party programs. This allows you to create circuit synthesis software that is an extension of Qucs. Proprietary software typically uses binary formats.

Here are the main components available in Qucs:

1. Passive RCL components
2. Diodes
3. Bipolar transistors
4. Field effect transistors (JFET, MOSFET, MESFET and microwave transistors)
5. Ideal op amps
6. Coaxial and microstrip lines
7. Library components: transistors, diodes and chips
8. File components: subcircuits, spice subschemes, Verilog components

The component library uses its own XML-based format. But it is possible to import existing libraries of components based on Spice (given in datasheets for electronic components).

The following types of modeling are supported:

1. DC operation point simulation
2. Simulation in the frequency domain on alternating current
3. Simulation of the transition process in the time domain
4. Modeling S-Parameters
5. Parametric analysis

The simulation results can be exported to Octave / Matlab and post-processed data there.

Qucs is based on the newly developed schematic modeling engine. A distinctive feature of this engine is the built-in ability to simulate S-parameters and CWS, which is important for the analysis of RF circuits. Qucs can recalculate S-parameters into Y- and Z-parameters.

The screenshots show an example of modeling the S-parameters of a wideband high-frequency amplifier.





So, the distinctive feature of Qucs is the ability to analyze complex frequency characteristics (CFC), plot graphs on the complex plane and Smith diagrams, analyze complex resistances and S-parameters. These features are absent in the proprietary systems MicroCAP and MultiSim, and here Qucs even exceeds commercial software and allows you to get results unavailable for electronic circuit simulators based on Spice.

The disadvantage of Qucs is the low number of library components. But this disadvantage is not an obstacle to use, since Qucs is compatible with the Spice format, which contains models of electronic components in datasheets. Also, the modeler works slower than similar Spice-compatible modelers (for example, MicroCAP (proprietary) or Ngspice (open-source)).

Currently we are working on the possibility of providing the user with a choice of engine for modeling the circuit. You can use the built-in Qucs engine, Ngspice (spice-compatible console simulator, similar to PSpice) or Xyce (simulator with parallel computing support via OpenMPI)

Now consider the list of innovations in the recent release of Qucs 0.0.18 promising areas in the development of Qucs:

1. Improved compatibility with Verilog
2. Porting interface to Qt4 continues
3. Implemented a list of recent open documents in the main menu.
4. The export of graphs, schemes in raster and vector formats: PNG, JPEG, PDF, EPS, SVG, PDF + LaTeX. This feature is useful when preparing articles and reports containing simulation results.
5. The ability to open a schema document from a future version of the program.
6. Fixed bugs related to the hangup of the modeler under certain conditions.
7. An active filter synthesis system for Qucs is under development (expected in version 0.0.19)
8. The development of interfacing with other open-source engines for modeling electronic circuits ( Ngspice , Xyce , Gnucap). In future versions, the ability to select the engine for modeling the circuit will be added.

It can be concluded that, despite its shortcomings, Qucs is a very worthy alternative to proprietary CAD systems for modeling electronic circuits.

Qucs Resources:

Project website: qucs.sourceforge.net
Repository on Github: github.com/Qucs/qucs
Library of domestic components for Qucs: github.com/ra3xdh/qucs-rus-complib