LC Filter Calculator

About the LC Filter Calculator

LC filters are fundamental building blocks of electronics, using inductors and capacitors to separate desired signal frequencies from unwanted ones. From RF communications to audio crossover networks to power supply filtering, LC filters provide efficient frequency-selective behavior without the power losses associated with resistive elements.

This calculator designs both low-pass and high-pass LC filters using the Butterworth (maximally flat) response. You specify the cutoff frequency, characteristic impedance, and filter order, and the calculator determines the exact inductor and capacitor values needed. Butterworth filters are the most commonly used type because they provide the flattest possible passband response with no ripple, making them ideal for applications where signal fidelity is important.

The calculator supports filter orders from 1st through 5th, with each additional order providing an extra 20 dB per decade of rolloff. It displays the Butterworth normalized coefficients, per-stage component values, a visual frequency response plot, and an attenuation table at key frequencies — everything you need to go from specification to component selection.

When This Page Helps

Designing an LC filter by hand requires looking up Butterworth coefficients, performing impedance scaling, and calculating denormalized component values for each stage. This calculator automates the entire process, giving you ready-to-use component values in seconds.

The built-in frequency response visualization and attenuation table let you verify that the design meets your rejection requirements before ordering components, saving time and preventing costly prototyping iterations.

How to Use the Inputs

1. Select a preset for a common application, or configure manually.
2. Choose Low-Pass (passes below cutoff) or High-Pass (passes above cutoff).
3. Enter the desired cutoff frequency (−3 dB point) and select the unit.
4. Set the characteristic impedance (50Ω for RF, 600Ω for telecom, 8Ω for audio speakers).
5. Select the filter order — higher orders give steeper rolloff but need more components.
6. Read the inductance and capacitance values for each filter stage.
7. Check the attenuation table to verify the design meets your rejection requirements.

Example Calculation

Tips & Best Practices

• Start with 2nd order and increase only if the rolloff is insufficient — more stages add complexity and potential for parasitics.
• Use 5% tolerance capacitors and 10% tolerance inductors as a minimum — tighter tolerances improve filter accuracy.
• At RF frequencies, consider parasitic resistance (ESR) of real components — it limits achievable Q and adds insertion loss.
• For audio crossover networks, the impedance is the nominal speaker impedance (typically 4Ω or 8Ω).
• Place the inductor first (series element) for a π-section low-pass, or the capacitor first for a T-section.
• Real inductors have self-resonant frequencies — ensure your operating frequency is well below this to avoid unexpected behavior.

LC Filter Fundamentals

An LC filter exploits the frequency-dependent impedance of inductors and capacitors. An inductor\'s impedance (XL = 2πfL) increases with frequency, while a capacitor\'s impedance (XC = 1/(2πfC)) decreases. By combining them, you create a frequency-selective network that passes some frequencies while attenuating others.

In a low-pass configuration, the inductor is in series (blocking high frequencies) and the capacitor is in shunt (shorting high frequencies to ground). In a high-pass configuration, these positions are swapped — the capacitor is in series (blocking DC and low frequencies) and the inductor is in shunt.

Butterworth Filter Design

The Butterworth approximation provides a maximally flat magnitude response in the passband. The normalized transfer function has poles equally spaced on the left half of the unit circle in the s-plane. For an nth-order Butterworth filter, the magnitude-squared response is |H(jω)|² = 1 / (1 + ω^(2n)), which gives exactly −3 dB at the normalized cutoff frequency ω = 1.

The normalized element values (g-values) are denormalized by frequency and impedance scaling: L = g × Z₀ / ω_c and C = g / (Z₀ × ω_c). The calculator performs this scaling automatically for each stage.

Practical Component Selection

After calculating ideal values, you must select real components. Ceramic capacitors work well for RF but have voltage-dependent capacitance. Film capacitors are more stable for audio. For inductors, air-core types offer the best Q at RF, while ferrite or iron-powder cores provide higher inductance in smaller packages for lower frequencies. Always verify that your inductor\'s self-resonant frequency is at least 5-10× above your operating frequency.

Frequently Asked Questions

• What is the advantage of an LC filter over an RC filter?

  LC filters have virtually no power loss in the passband because ideal inductors and capacitors dissipate no energy. RC filters always waste power in the resistor. For high-power applications (RF, power supplies, audio), LC is strongly preferred.

• Why use Butterworth design?

  Butterworth filters provide the flattest possible magnitude response in the passband — no ripple or peaking. This makes them ideal when you need consistent signal level across the passband. Other types like Chebyshev offer steeper rolloff but with passband ripple.

• What filter order do I need?

  Each order adds 20 dB/decade of rolloff. If you need 40 dB rejection at 10× the cutoff frequency, a 2nd-order filter suffices. For 60 dB rejection at 10×, use 3rd order. More stages mean more components, tighter tolerances, and higher cost.

• How do I choose the impedance?

  Use the source/load impedance of your system: 50Ω for RF systems, 75Ω for video/cable, 600Ω for telecom, 8Ω for speakers, and 10kΩ+ for high-impedance audio. Impedance mismatch causes reflections and incorrect filtering.

• What if the calculated component values are not standard?

  LC filters are tolerant of 5-10% component variation. Choose the nearest standard value. For critical applications, use series or parallel combinations of standard values to get within 1-2% of the target.

• Can I build a bandpass filter with this?

  A bandpass filter combines a low-pass and high-pass filter. Design a high-pass at your lower cutoff frequency and a low-pass at your upper cutoff. Cascade them for a bandpass response.