How to Export S-Parameters from uSimmics (formerly QucsStudio) [2026]

This guide walks through the complete workflow for exporting S-parameters from uSimmics (formerly QucsStudio) in Touchstone SnP format. Converting simulation results to SnP is an essential skill for sharing data with other simulators, measurement systems, and downstream RF design tools.

What You’ll Learn

• What S-parameters (S11, S21) mean and how they are used in RF circuit analysis
• How to create a project and schematic in uSimmics (formerly QucsStudio)
• How to run an S-parameter simulation and generate a .dat file
• How to export a .dat file to Touchstone SnP format
• How to troubleshoot common simulation and export issues

S-Parameter Fundamentals

S-parameters (Scattering Parameters) describe the electrical behavior of RF and microwave circuits. Unlike conventional circuit analysis that uses voltages and currents, S-parameters express signal behavior in terms of incident, reflected, and transmitted waves — a representation better suited to GHz-range design where signals behave as traveling waves.

Key S-Parameters

S11 (Reflection Coefficient)
Represents the fraction of the signal incident on Port 1 that reflects back to Port 1. Used to evaluate antenna VSWR and input impedance matching. A lower S11 (e.g., below −20 dB) indicates less reflection and better matching.

S21 (Transmission Coefficient)
Represents the fraction of the signal incident on Port 1 that passes through to Port 2. Used to evaluate filter insertion loss and amplifier gain. For ideal lossless transmission, S21 = 0 dB.

Overall Workflow

The complete process for exporting S-parameters from uSimmics (formerly QucsStudio) is:

1. Create a new project
2. Build the schematic (place and connect components)
3. Configure the S-parameter simulation (frequency range and points)
4. Run the simulation (generate a .dat file)
5. Review the simulation results (graphs, Smith chart)
6. Export the .dat file to SnP format

Step 1: Create a New Project

uSimmics (formerly QucsStudio) organizes schematics and simulation results by project. Creating a project keeps all related files in a single folder.

1. Launch uSimmics (formerly QucsStudio).
2. Select the Project tab.
3. Click New.
4. Enter a project name and specify a save location.
5. Click OK to create the project.

A folder with the project name is created automatically. All subsequent files — schematics, simulation data, and exported files — are saved there.

Step 2: Build the Schematic

This guide uses a 3rd-order Butterworth LC low-pass filter designed with the Filter Synthesis tool (see https://denki-sim.blog/filter-synthesis_guide/). You can paste the Filter Synthesis output directly, or build the schematic manually:

1. From Components → Lumped Components, place inductors and capacitors.
2. From Sources, place a Power Source at the input.
3. Place a 50 Ω termination resistor at the output.
4. Connect all components with wires and add GND symbols.

Step 3: Configure the S-Parameter Simulation

The frequency range and resolution of the exported S-parameter data are determined entirely by the simulation settings. Set the range to fully cover the frequencies of interest.

1. Double-click the S-parameter simulation component on the schematic.
2. Configure the following parameters:

1. Click Apply then OK to save.

Important: The frequency range and point count set here define the frequency axis in the exported SnP file. If you need different coverage, you must re-run the simulation.

Step 4: Run the Simulation

1. Select Simulation → Simulate from the menu (or press F2).
2. If prompted to save, choose a filename within the project folder.
3. After successful simulation, a .dat file is generated in the project folder.

The .dat file is uSimmics’s native simulation results format. This is the file you will export to SnP in the next step.

Step 5: Review Simulation Results

Before exporting, verify that the simulation results look correct.

1. Graphs should display automatically after the simulation completes.
2. Check the S21 (insertion loss) graph for filter passband flatness, cutoff frequency, and stopband attenuation.
3. Check the S11 (return loss) graph or Smith chart for input matching.
4. Confirm the .dat file appears in the Datasets section of the Content tab.

If the .dat file does not appear: Save the project (Ctrl + S), close uSimmics (formerly QucsStudio), and reopen the project.

Step 6: Export S-Parameters to SnP Format

1. In the Content tab, locate the .dat file under Datasets.
2. Right-click the .dat file.
3. Select Export to SnP from the context menu.
4. The SnP file is written to the project folder:
5. 2-port circuits: .s2p
6. 1-port circuits: .s1p

The exported SnP file is in the industry-standard Touchstone format, compatible with:

• Commercial RF simulators such as Keysight ADS and Microwave Office
• LTspice (with limited import support)
• Vector network analyzers (VNAs) such as NanoVNA for measurement comparison
• Python’s scikit-rf library for numerical analysis

Practical Applications

S-parameter export is particularly valuable in the following scenarios:

Filter Verification
Export simulated S21 and compare it against VNA measurements of a fabricated board to quantify design-to-implementation deviation.

Antenna Design
Export antenna S11 (VSWR) for use in external tools to analyze radiation efficiency and patterns.

Amplifier Stability Analysis
Export amplifier S-parameters for stability analysis (µ-factor, K-factor) in external tools.

Troubleshooting

Related Articles

• How to Import an S-Parameter File into uSimmics (formerly QucsStudio) [2026]
• Using the Filter Synthesis Tool in uSimmics (formerly QucsStudio) [2026]
• Using the Create Matching Circuit Tool in uSimmics (formerly QucsStudio) [2026]
• Using the Transmission Line Calculator in uSimmics (formerly QucsStudio) [2026]
• Using the Qucsattenuator Tool in uSimmics (formerly QucsStudio) [2026]