This guide walks through a complete low-pass filter (LPF) design workflow for Sub-GHz (915 MHz) communications using uSimmics (formerly QucsStudio). The process covers automated design with the Filter Synthesis tool, substitution of standard component values, and statistical tolerance evaluation via Monte Carlo analysis.
What You’ll Learn
- Why LPFs are required in Sub-GHz communications (harmonic suppression requirements)
- How to use the Filter Synthesis tool in uSimmics (formerly QucsStudio) for initial LPF design
- How to substitute ideal component values with standard (E-series) component values
- How to evaluate the effect of component tolerances using Monte Carlo analysis
- How to make cost-vs-performance trade-offs when selecting tolerance grades
Step 1: Define the Target Performance
Why an LPF Is Required
Power amplifiers used in wireless transmission are typically operated near their compression point to maximize power efficiency. This creates harmonic distortion — signal content at integer multiples of the fundamental frequency. Regulations in most jurisdictions require these harmonics to be suppressed below specified limits to prevent interference with other radio systems. A low-pass filter (LPF) is the standard solution for passing the fundamental while attenuating harmonics.
Design Targets
This guide targets performance equivalent to the TDK multilayer LPF DEA100915LT-6319A1:
| Parameter | Target | Frequency Band |
|---|---|---|
| Insertion Loss | ≤ 0.5 dB | 824–915 MHz |
| Attenuation | ≥ 18 dB | 1648–1830 MHz |
Step 2: Initial Design Using Filter Synthesis
uSimmics (formerly QucsStudio) includes a Filter Synthesis tool that automatically generates LC filter schematics from specification inputs. Reference: Filter Synthesis Guide
Filter Synthesis Settings
Design a Butterworth (maximally flat) LPF with the following parameters:
- Filter type: Butterworth LPF (minimum passband ripple)
- Order: 6th order
- Cutoff frequency: 1.1 GHz
The Filter Synthesis tool produces the following ideal component values:
| Component | Calculated Value |
|---|---|
| L1 | 10.23 nH |
| L2 | 13.94 nH |
| L3 | 3.745 nH |
| C1 | 1.498 pF |
| C2 | 5.59 pF |
| C3 | 4.092 pF |
Simulation with these ideal values confirms the target performance is met.
Step 3: Substitute Standard Component Values
Round the calculated values to the nearest standard (E-series) values available from component suppliers and re-confirm performance.
| Component | Calculated | Adopted |
|---|---|---|
| L1 | 10.23 nH | 10 nH |
| L2 | 13.94 nH | 14 nH |
| L3 | 3.745 nH | 3.7 nH |
| C1 | 1.498 pF | 1.5 pF |
| C2 | 5.59 pF | 5.6 pF |
| C3 | 4.092 pF | 4.1 pF |
Simulation with the rounded standard values still meets the target specification.
Step 4: Monte Carlo Tolerance Analysis
Real inductors and capacitors carry tolerances — their actual values deviate from the nominal. Monte Carlo analysis quantifies the impact of these deviations on filter performance. Reference: Monte Carlo Analysis Guide
Analysis Conditions
Start with the broadest tolerance grade (lowest cost) to determine whether tighter tolerances are actually needed.
| Component | Value | Tolerance |
|---|---|---|
| L1 | 10 nH | ±5% |
| L2 | 14 nH | ±5% |
| L3 | 3.7 nH | ±0.2 nH |
| C1 | 1.5 pF | ±0.25 pF |
| C2 | 5.6 pF | ±0.25 pF |
| C3 | 4.1 pF | ±0.25 pF |
Component variation is modeled as a normal distribution. The 4σ (worst case) is set equal to the tolerance percentage. For example, for a 10 nH inductor with ±5% tolerance, 4σ = 5%, so σ = 1.25%.
Analysis Results — Wide Tolerance Grade
| Measurement Point | Typical | Worst Case |
|---|---|---|
| Insertion loss at 915 MHz | −0.481 dB | −0.695 dB |
| Attenuation at 1650 MHz | −20.9 dB | −19.9 dB |
Even the worst case meets the target specification (insertion loss ≤ 0.5 dB, attenuation ≥ 18 dB). This means the widest (and least expensive) tolerance grade is acceptable.
Tighter Tolerance Comparison
If worst-case performance does not meet specification, tighten tolerances — for example, inductors to ±3% and capacitors to ±0.1 pF. In practice, balance performance requirements against component cost when making the final selection.
Comparing Simulated LPF Against the Reference Component
The simulated LPF S-parameters can be overlaid with the measured S-parameters of the DEA100915LT-6319A1 reference component:
- Red: simulated LPF
- Blue: measured DEA100915LT-6319A1 data
This comparison provides a quantitative assessment of the design’s validity relative to a production component.
Summary
uSimmics (formerly QucsStudio) enables a streamlined LPF design workflow: automated synthesis with Filter Synthesis, standard value substitution, and statistical tolerance validation with Monte Carlo analysis. For Sub-GHz harmonic suppression applications, this integrated simulation environment significantly reduces design iteration time while improving confidence in first-pass performance.
Related Articles
- LPF Optimization with Real Component S-Parameters in uSimmics (formerly QucsStudio) [2026]
- Monte Carlo Analysis in uSimmics (formerly QucsStudio) [2026]
- VSWR Analysis with uSimmics (formerly QucsStudio) [2026]
- Filter Synthesis Tool Guide
- Microstrip Line Characteristic Impedance Calculation in uSimmics (formerly QucsStudio) [2026]
Comment