3rd Order Filter Design

Simulated response and found cutoff low

4:19

Started: Simulating AC response around 120 MHz.
Ran simulation.
Ran simulation.
Ran simulation.
Evaluated results for 18 sec
Ran simulation.
Evaluated results for 58 sec
Simulation complete.
Running calculation.
Running calculation.

Completed your request.

12:25

Started: Tuning L and C to hit 120 MHz.
Ran simulation.
Evaluated results for 8 sec
Ran simulation.
Ran simulation.
Evaluated results for 42 sec
Ran simulation.
Evaluated results for 5 sec
Ran simulation.
Evaluated results for 14 sec
Simulation complete.
Running calculation.
Running calculation.
Running calculation.
Ran simulation.
Evaluated results for 9 sec
Ran simulation.
Evaluated results for 4 sec
Ran simulation.
Evaluated results for 12 sec
Ran simulation.
Evaluated results for 1 min 2 sec
Ran simulation.
Evaluated results for 43 sec
Simulation complete.

flux

  • Set L1's Inductance to "103nH"
  • Set L2's Inductance to "103nH"
  • Set C1's Capacitance to "48.2pF"

flux

  • Set L1's Inductance to "85.1nH"
  • Set L2's Inductance to "85.1nH"
  • Set C1's Capacitance to "58.3pF"

flux

Work done
  • Added and wired a 3rd order LC ladder low-pass filter: L1 (series), C1 (shunt to GND), L2 (series) with named nets IN, OUT, GND.
  • Documented the architecture in the existing project-specification (includes Mermaid block diagram).
  • Ran AC simulations with 50 ohm source/load.
Success criteria review
  • ✅ Filter schematic shows correct 3rd order ladder topology with labeled L/C values.
  • ❌ Simulation cutoff did not reach 120 MHz: multiple tuned attempts still yielded ~109.6 MHz (-3 dB).
  • ✅ Topology is layout-ready (0402 footprints, single shunt-to-ground element).
Next steps
To hit 120 MHz, I need either (1) to redefine how cutoff is measured (e.g., match -3 dB of insertion loss vs ideal transfer definition), or (2) to change the network (e.g., Pi topology with 2 shunt caps) and re-optimize values. Tell me which definition/topology you want and I can re-tune accordingly.
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Schematic

Layout

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Project Specification
Version: 0.1.0 Date: ... Author: ... Status: ... Tags: ...
1. Executive Summary
What board we're building and the intended outcome in one paragraph. This project implements a 3rd order LC low-pass filter for RF signal conditioning with a target cutoff frequency of 120 MHz and a nominal 50 ohm source/load impedance.
2. Goals and Non-Goals
What must be true in this rev, and what is explicitly out of scope. ...
2.1 Goals
  • Implement a 3rd order LC ladder low-pass filter with cutoff frequency 120 MHz.
  • Use a passive L/C ladder suitable for 50 ohm RF systems.
  • Verify frequency response in simulation.
2.2 Non-Goals
  • This revision does not include active gain stages or impedance transformation.
  • This revision does not guarantee a specific insertion loss at 120 MHz beyond the ideal LC target (real-world Q and parasitics depend on final selected MPNs and layout).
3. System Overview
High-level description of the system and how the board fits into it. ...
3.1 Block Diagram
A space for a simple block diagram (power + data paths).
Diagram



  
    
  
  
    
  














"RF IN node_50ohm"
"Series Inductor L1"
"Node"
"Shunt Capacitor C1 to GND"
"Series Inductor L2"
"RF OUT node_50ohm"
"GND"
4. Requirements
What the design must do (behaviorally and electrically), independent of implementation details. ...
4.1 Functional Requirements
...
4.2 Electrical Requirements
Power input, rails, currents, interfaces, sensing accuracy, etc.
  • Filter type: passive LC low-pass ladder
  • Order: 3rd
  • Response: Butterworth (maximally flat)
  • Nominal impedance: 50 ohm
  • Cutoff frequency: 120 MHz
4.3 Environmental and Reliability Requirements
Operating ranges, expected lifetime, handling constraints, safety/compliance. ...
5. Constraints
Hard limits that shape the design choices. ...
5.1 Mechanical Constraints
Size, mounting, connector access, enclosure interactions. ...
5.2 Power and Thermal Constraints
Power source limits, dissipation ceilings, heat-sensitive components. ...
5.3 Layout / Implementation Constraints
High-level layout rules (keepouts, grounding intent, routing classes), without detailed PCB guidance.
  • Use small packages (0402 or 0201 preferred) for L/C to reduce parasitics at 120 MHz.
  • Keep the series path short and straight from IN to OUT.
  • Shunt capacitor must return to a solid ground with minimum loop inductance (adjacent ground via).
  • Maintain controlled impedance (50 ohm) on the IN and OUT traces if routed as transmission lines.
5.4 Cost and Sourcing Constraints
Target cost range, availability requirements, preferred vendors/alternates policy. ...
6. Assumptions and Dependencies
What we assume is true, and what the design depends on (parts, standards, external systems).
  • The filter is terminated in 50 ohm at both input and output.
  • Ideal L and C values will be approximated with standard values; final cutoff depends on tolerance and parasitics.
7. Validation Criteria
What “done” means in terms of checks and evidence (not task lists).
  • Schematic contains a complete 3rd order LC ladder filter between IN and OUT with GND reference.
  • AC sweep / frequency response simulation shows cutoff close to 120 MHz.
  • Layout guidance documented to minimize parasitics (short path, solid ground return).
7.1 Acceptance Tests
...
7.2 Review Checklist
...
Appendix (Optional)
Any supporting urls, analysis, datasets, or taxonomy notes that informed this spec. ...

  • 1. Executive Summary

  • 2. Goals and Non-Goals

  • 2.1 Goals

  • 2.2 Non-Goals

  • 3. System Overview

  • 3.1 Block Diagram

  • 4. Requirements

  • 4.1 Functional Requirements

  • 4.2 Electrical Requirements

  • 4.3 Environmental and Reliability Requirements

  • 5. Constraints

  • 5.1 Mechanical Constraints

  • 5.2 Power and Thermal Constraints

  • 5.3 Layout / Implementation Constraints

  • 5.4 Cost and Sourcing Constraints

  • 6. Assumptions and Dependencies

  • 7. Validation Criteria

  • 7.1 Acceptance Tests

  • 7.2 Review Checklist

  • Appendix (Optional)