Qucs is an open source circuit simulator that is, quite universal. It can solve from DC and AC analysis, to circuit S-Parameters. It can also design matching networks and filters. It's not yet finished (here's the roadmap to what is already available and what the project aims to include), there are some quirks with the GUI, and it is hard to troubleshoot some error messages. But as it is, I already find it useful.
For now, I find it most useful in designing and simulating my matching circuits. Just to test out whether the simulations give reliable results, I've designed a matching network for a diode, HSMS-286Y from Avago. The design is simple, a shorted shunt stub located a certain length from the diode. The transmission lines are Coplanar Waveguides on a 1.6mm thick FR-4 PCB. Electrical length and impedance of the lines of the said network were first solved using the smith chart. The diode equivalent model is taken from the datasheet including the package model.
This is the network S11 (SWR) from the S-Parameter simulation.
The board was fabricated with the exact measurements from the simulation schematic. Actual measurement (S11 - Return Loss in dB) seems to agree with simulation result.
With this, I now have peace of mind with results from Qucs. I am now waiting for Qucs to integrate an EM Simulator.
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