How to Import S-Parameter Files into uSimmics (formerly QucsStudio) [2026]

Simulation
2024.01.31

uSimmics (formerly QucsStudio) can incorporate measured or manufacturer-supplied S-parameter files (Touchstone format) into circuit simulations. This guide uses a TDK low-pass filter as an example, covering the entire workflow from importing the file to running the simulation and evaluating the results.

What You’ll Learn

  • The fundamental concept of S-parameters and their applications
  • What the Touchstone file format is and how it is structured
  • How to import an S-parameter file into uSimmics (formerly QucsStudio)
  • How to build the simulation circuit and configure simulation settings
  • How to display and interpret simulation results

What Are S-Parameters?

S-parameters (Scattering Parameters) are a set of metrics used to characterize RF and microwave circuit behavior. In conventional circuit analysis, circuits are described in terms of voltage and current. At high frequencies, however, signals behave as traveling waves, so it is standard practice to characterize circuits in terms of the ratio of incident, reflected, and transmitted waves — that is, with S-parameters.

Parameter Name Meaning
S11 Input reflection coefficient Fraction of signal injected at Port 1 that is reflected back to Port 1. Used for antenna VSWR evaluation and input matching assessment.
S21 Forward transmission coefficient Fraction of signal injected at Port 1 that appears at Port 2. Indicates filter insertion loss or amplifier gain.
S12 Reverse transmission coefficient Fraction of signal injected at Port 2 that appears at Port 1 (isolation).
S22 Output reflection coefficient Fraction of signal injected at Port 2 that is reflected back to Port 2.

What Is the Touchstone Format?

The Touchstone format is an industry-standard text file format for storing S-parameter data. The file extension indicates the number of ports.

Extension Number of Ports
.s1p 1-port (e.g., antenna, capacitor)
.s2p 2-port (e.g., filter, amplifier)
.s3p 3-port
.s4p 4-port (e.g., balun, hybrid coupler)

S-parameter files can be generated from network analyzer measurements or computed by circuit simulators. Component manufacturers such as TDK, Murata, and AVX provide S-parameter files for their RF components on their official websites.

Sample Component Used in This Guide

This guide uses the TDK low-pass filter DEA070787LT-4002A1 as an example. The S-parameter file is available for download from the TDK product page’s technical support tools. The filename is dea070787lt-4002a1.s2p (a 2-port Touchstone file).

Step 1: Placing the S Parameter File Component

  1. Launch uSimmics (formerly QucsStudio) and create a new schematic.
  2. In the component panel on the left, select the Components tab.
  3. Click system components to expand the category.
  4. Select S parameter file from the list.
  5. Drag and drop (or click) to place the component on the schematic.

Step 2: Specifying the S-Parameter File

  1. Double-click the placed S parameter file component.
  2. The properties window opens.
  3. Click Browse.
  4. In the file selection dialog, locate and select the downloaded .s2p file (dea070787lt-4002a1.s2p).
  5. In the Ports section, confirm that the port count is set to 2. Change this value if using a component with a different port count.
  6. Click Apply to save the settings.

The S-parameter file is now associated with the component in uSimmics (formerly QucsStudio).

Step 3: Building the Circuit and Configuring Simulation Settings

3-1. Placing a Signal Source

  1. From Sources, select Power Source and place it on the schematic.
  2. Leave the source impedance at the default 50 Ω.

3-2. Wiring the Circuit

  1. Connect the output of the signal source (Port 1) to Port 1 (input) of the S parameter file component.
  2. Connect Port 2 (output) of the S parameter file component to a termination resistor or the next stage.
  3. Connect GND to each component.

3-3. Configuring the S-Parameter Simulation

  1. From Simulations, select S-parameter simulation and place it on the schematic.
  2. Double-click the simulation component and set the following parameters:
Setting Value Notes
Graph type Logarithmic A logarithmic scale is appropriate for the wide frequency range being analyzed.
Start frequency 100 MHz Set below the component’s lower operating frequency.
Stop frequency 10 GHz Set to cover the component’s upper operating frequency.
Number of points 201 A guideline of approximately 10 points per decade is reasonable.

Step 4: Running the Simulation

  1. Select Simulate → Simulate from the menu, or click the gear icon in the upper right corner.
  2. If prompted to save a file, specify a project folder and an appropriate filename.
  3. The simulation runs and results are displayed automatically.

Reviewing and Evaluating Simulation Results

After simulation completes, evaluate the results from the following perspectives.

S21 (Insertion Loss)

  • Passband: Verify that S21 is close to 0 dB and flat across the passband.
  • Stopband: Confirm that S21 drops sufficiently (adequate attenuation) above the cutoff frequency.
  • Cutoff frequency: Identify the frequency at which S21 equals −3 dB.

S11 (Return Loss)

  • Verify that S11 is low (below −15 dB or better) in the passband. This indicates that input-side reflections are small and the matching is adequate.

Important Notes

  • Verify that the frequency range of data in the S-parameter file covers the frequency range you intend to simulate. Results outside the file’s frequency range are extrapolated and accuracy is not guaranteed.
  • S-parameters represent device behavior measured under specific conditions (temperature, bias voltage, etc.). Use S-parameter files that correspond to your actual operating conditions.
  • Measured S-parameter data is also useful for validating the discrepancy between simulated and measured circuit performance. S-parameter files from other simulators such as LTspice or ADS can be imported into uSimmics (formerly QucsStudio) for comparative evaluation.

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