uSimmics (formerly QucsStudio) provides a Subcircuit feature that enables hierarchical organization of large and complex circuit designs. This article walks through the complete process of designing a 3rd-order LC low-pass filter as a subcircuit and integrating it into a main schematic.
What You Will Learn
- The concept of Subcircuits and their advantages in circuit design
- How to create a sub-schematic and place ports
- The procedure for embedding a subcircuit into the main schematic
- How to specify the Sub.sch file and troubleshoot common issues
- How to place multiple instances of the same subcircuit for reuse
What Is a Subcircuit?
A Subcircuit is a self-contained circuit block defined as an independent module within a larger electronic design. Incorporating subcircuits into your workflow offers the following benefits:
- Improved schematic readability through hierarchical design
- Reuse of the same sub-block at multiple points in the design
- Global propagation of design changes by modifying only the subcircuit file
Procedure
1. Prepare Two Schematic Files in the Project
Begin by creating two schematic files within the same project.
| File Name | Role |
|---|---|
| Main.sch | Main schematic (the schematic that uses the subcircuit) |
| Sub.sch | Sub-schematic (the schematic defined as the subcircuit) |
2. Design the Circuit in the Sub-Schematic (Sub.sch)
In this example, a 3rd-order LC low-pass filter is designed in Sub.sch.
Note on LC Low-Pass Filters: A filter circuit combining inductors (L) and capacitors (C) to pass only low-frequency components. The Filter Synthesis feature in uSimmics (formerly QucsStudio) automatically calculates component values from a specified filter order and cutoff frequency.
For detailed instructions on filter design using Filter Synthesis, refer to this article.
3. Place Ports in the Sub-Schematic
Remove any elements not required for connection to the main schematic (such as termination resistors) from the designed filter, and place Ports at the connection nodes.
- Place one Port at the input terminal.
- Place one Port at the output terminal.
- Save the file as
Sub.sch.
Important: If ports are not correctly placed and saved, the main schematic will not recognize the subcircuit terminals. If the subcircuit does not expose two ports, verify that
Sub.schhas been saved correctly.
4. Add the Subcircuit to the Main Schematic (Main.sch)
- Open
Main.sch. - From the Components → Devices menu, select Subcircuit and place it on the schematic canvas.
- Double-click the placed Subcircuit component to open its properties dialog.
- Click the Browse button and select the
Sub.schfile designed in the previous step. - Close the dialog. The subcircuit will now appear as a block with two ports on the main schematic.
5. Run a Simulation with the Subcircuit
Execute the simulation using the same procedure as any standard circuit simulation. Components inside the subcircuit behave exactly as if they were placed directly in the main schematic.
Advanced use: Multiple instances of the same subcircuit can be placed in a single design. For example, to cascade the same filter in multiple stages, simply place additional Subcircuit blocks referencing the same
Sub.schfile.
Troubleshooting
| Symptom | Check |
|---|---|
| Subcircuit does not display two ports | Verify that ports are correctly placed and saved in Sub.sch |
| Simulation error occurs | Confirm that the Sub.sch path is correctly specified and the file exists |
| Changes to the sub-schematic are not reflected in the main schematic | Save Sub.sch, then re-run the simulation in Main.sch |
Summary
The Subcircuit feature in uSimmics (formerly QucsStudio) enables hierarchical organization of complex circuits, significantly improving design reusability and maintainability. Even a simple workflow—designing a filter with Filter Synthesis and encapsulating it as a subcircuit—directly translates to a more efficient practical design process. Start with a simple sub-schematic to build familiarity, then apply the technique to larger, more complex designs.
Related Articles
- How to Use Filter Synthesis in uSimmics (formerly QucsStudio)
- DC Analysis in uSimmics (formerly QucsStudio) [2026]
- Beginner’s Guide to Digital Simulation in uSimmics (formerly QucsStudio) [2026]
- How to Import S-Parameter Files in uSimmics (formerly QucsStudio)
- Basic Operations of the Circuit Editor in uSimmics (formerly QucsStudio) [2026]
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