LC Low Pass Filter Calculator

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Calculate an ideal second-order Butterworth LC low-pass filter from cutoff frequency and system impedance, including series inductor, shunt capacitor, and response notes.

Input Parameters

Cutoff Frequency

MHz

-3 dB cutoff frequency for the ideal 2nd-order Butterworth low-pass.

System Impedance

Source/load impedance for prototype scaling.

Results

Series Inductor

Shunt Capacitor

LC Resonant Check

Design Impedance

Ideal Roll-off

Design Note

Equations Used

2nd-Order Butterworth Prototype: g1 = g2 = 1.4142

Series Inductor: L = g1 × R / (2πfc)

Shunt Capacitor: C = g2 / (R × 2πfc)

Ideal Roll-off: 40 dB/decade after cutoff for a second-order filter.

Frequently Asked Questions (FAQ)

Q1: What does this LC low pass filter calculator design?
It calculates an ideal second-order Butterworth LC low-pass section using system impedance and cutoff frequency.

Q2: Can this be used for RF filters?
Yes as a starting point, but RF layout, component Q, parasitic capacitance, and self-resonance must be checked.

Q3: Why use a Butterworth response?
Butterworth filters provide a maximally flat passband with no ripple in the ideal model.

Q4: Does the load impedance matter?
Yes. The calculated L and C values assume a specific source/load impedance.

Q5: Why are real cutoff frequencies different?
Component tolerance, ESR, ESL, PCB parasitics, and load mismatch shift the actual response.

Disclaimer: This tool provides ideal LC filter starting values. Real filters require component tolerance checks, ESR/ESL review, load matching, PCB layout control, and measurement or simulation.

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