How to Use Filter Synthesis in uSimmics (formerly QucsStudio) [2026]

The Filter Synthesis tool in uSimmics (formerly QucsStudio) generates LC filter circuits automatically, without manual polynomial calculations. This guide uses a low-pass filter example to walk through parameter setup, circuit generation, simulation, and result analysis.

What You’ll Learn
What Is Filter Synthesis?
Step 1: Opening Filter Synthesis
Step 2: Setting Filter Parameters
Step 3: Generating the Circuit and Copying to Clipboard
Step 4: Pasting into the Schematic
Step 5: Running the Simulation
Step 6: Reviewing and Analyzing Results
Filter Approximation Types
Filter Type Selection Summary
Summary
Related Articles

What You’ll Learn

How to open Filter Synthesis and understand each parameter
Characteristics of Butterworth, Chebyshev, Bessel, and other filter approximations, with guidance on choosing among them
How to paste the generated circuit into a uSimmics (formerly QucsStudio) schematic
How to run an S-parameter simulation and inspect the filter response
Next steps for optimizing with real components

What Is Filter Synthesis?

Filter Synthesis is a passive filter design tool built into uSimmics (formerly QucsStudio). You specify the filter order, cutoff frequency, impedance, and topology, and the tool automatically computes element values for an LC ladder circuit and outputs the result as schematic data.

Manual filter synthesis requires complex polynomial arithmetic. This tool eliminates that overhead and dramatically reduces design time.

Step 1: Opening Filter Synthesis

Launch uSimmics (formerly QucsStudio).
In the menu bar, click Tools.
Select Filter Synthesis from the submenu.
The Filter Synthesis window opens.

Step 2: Setting Filter Parameters

Configure the following parameters in the Filter Synthesis window:

Parameter	Example Value	Description
Realization	LC ladder	Circuit implementation type. Specifies an LC ladder topology.
Filter type	Butterworth	Approximation function type (see below).
Filter Class	Low pass	Filter category: Low pass / High pass / Band pass / Band stop.
Order	3	Filter order. Higher order gives steeper roll-off but increases component count.
Corner Frequency	50 MHz	Cutoff frequency. For a low-pass filter, frequencies above this are attenuated.
Impedance	50 Ω	Reference impedance for input and output. Typically 50 Ω in RF circuits.
Topology	Pi type	Circuit topology: Pi-type or T-type.

Step 3: Generating the Circuit and Copying to Clipboard

Click Calculate and into Clipboard. This action:

Computes element values for the filter based on the specified parameters.
Copies the resulting schematic data to the clipboard.

Step 4: Pasting into the Schematic

Open a new or existing schematic in the uSimmics (formerly QucsStudio) main window.
Right-click in the schematic editor and select Paste, or press Ctrl + V.
The filter circuit is inserted into the schematic.

Step 5: Running the Simulation

Place an S-parameter Simulation component on the schematic.
From the menu, select Simulate.
If prompted, save the file with an appropriate name in your project folder.
The simulation runs and a results window appears.

Step 6: Reviewing and Analyzing Results

After the simulation completes, check the following:

S21 (transmission coefficient): Verify flatness in the passband and that attenuation in the stopband meets the design specification.
S11 (reflection coefficient): Confirm matching in the passband. The Smith chart view is also useful here.
Cutoff frequency: Place a marker on the graph and confirm the actual -3 dB point.

Filter Approximation Types

uSimmics (formerly QucsStudio) Filter Synthesis supports the following approximation types. Choosing the right type for your requirements is critical.

Butterworth

Characteristics: Maximally flat frequency response in the passband — no ripple. Attenuation increases monotonically beyond the cutoff frequency.
Use when: A smooth, ripple-free response is required. This is the most widely used general-purpose filter type.

Chebyshev

Characteristics: Introduces ripple in the passband in exchange for a steeper roll-off than Butterworth at the same order.
Use when: Steep roll-off is the priority and a defined level of passband ripple is acceptable.

Inverse Chebyshev

Characteristics: Flat passband (no ripple), with ripple in the stopband. Combines flat passband with reasonably sharp roll-off.
Use when: Passband flatness must be preserved while achieving sharper stopband rejection than Butterworth.

Cauer (Elliptic)

Characteristics: Ripple in both the passband and stopband. Achieves the steepest possible roll-off for a given order.
Use when: The sharpest possible roll-off is required and ripple in both bands is acceptable.

Bessel

Characteristics: Optimized for linear phase response (flat group delay) rather than amplitude. Minimizes signal waveform distortion.
Use when: Preserving pulse or digital signal waveforms with minimal distortion is the primary objective.

Legendre

Characteristics: Steeper roll-off than Butterworth while maintaining passband flatness. Some ripple may occur.
Use when: Intermediate characteristics between Butterworth and Chebyshev are needed.

Filter Type Selection Summary

Priority	Recommended Type
Passband flatness (no ripple)	Butterworth / Inverse Chebyshev
Steep roll-off	Chebyshev / Cauer
Flat group delay (minimal waveform distortion)	Bessel
Steepest possible roll-off (ripple acceptable)	Cauer (Elliptic)

Summary

The Filter Synthesis tool in uSimmics (formerly QucsStudio) is a powerful resource that makes complex filter design far more efficient. By entering the filter type, order, cutoff frequency, and impedance, the tool automatically generates a circuit that meets the target specification.

The generated circuit can be verified immediately via simulation and fed into a real-component optimization workflow. Use this tool as the starting point for filter design in RF circuit projects.