LC Filter Calculator
Calculate the cutoff frequency of an LC low-pass or high-pass filter circuit using fc = 1/(2π√(LC)). Free online LC filter calculator with frequency response chart, gain analysis, and detailed parameter breakdowns for electronics engineers and physics students.
About This Calculator
The LC Filter Calculator helps electronics engineers, hobbyists, and physics students calculate the cutoff frequency of an LC filter circuit. Whether you are designing a low-pass or high-pass filter, this calculator provides instant results with interactive frequency response charts and detailed parameter breakdowns.
An LC filter is a second-order passive filter that uses an inductor (L) and a capacitor (C) to selectively pass or block signals based on their frequency. The cutoff frequency is calculated using the formula fc = 1 / (2π √(LC)) where L is the inductance in henries and C is the capacitance in farads. Unlike first-order RC or RL filters with a -20 dB/decade roll-off, LC filters achieve a sharper -40 dB/decade slope, making them ideal for applications requiring more precise frequency separation.
In a low-pass LC filter configuration, the inductor is placed in series with the load while the capacitor is connected in parallel. This arrangement allows low-frequency signals to pass through while attenuating high-frequency signals. In a high-pass LC filter, the capacitor is placed in series and the inductor in parallel, achieving the opposite effect. Both configurations use the same fundamental cutoff frequency formula.
The optional input frequency field lets you compute the exact gain (in dB and percentage) at any specific frequency, helping you understand how your filter will perform at frequency of interest. The frequency response chart visualizes the gain across a wide frequency range from 0.01× to 100× the cutoff frequency.
Regional Notes
Global: LC filter design is universal across all regions since it is based on fundamental physics. The calculator uses SI units (henries and farads) which are the international standard for electrical engineering. Component availability and preferred unit conventions may vary — for example, US manufacturers often label capacitors in μF and pF, while European datasheets may use nF more commonly. The unit selectors support all common submultiples to accommodate any standard.
Frequently Asked Questions
What is an LC filter circuit?
An LC filter is a second-order electronic filter circuit consisting of an inductor (L) and a capacitor (C) that selectively passes or blocks signals based on frequency. Low-pass LC filters allow frequencies below the cutoff frequency to pass while attenuating higher frequencies. High-pass LC filters do the opposite, allowing high frequencies to pass while blocking low frequencies.
How do you calculate the cutoff frequency of an LC filter?
The cutoff frequency of an LC filter is calculated using the formula fc = 1/(2π√(LC)), where L is the inductance in henries (H) and C is the capacitance in farads (F). This formula applies to both low-pass and high-pass LC filter circuits since both configurations use the same resonant frequency equation.
What is the difference between LC low-pass and high-pass filters?
An LC low-pass filter has the inductor in series with the load and the capacitor in parallel, blocking high frequencies and passing low frequencies. An LC high-pass filter has the capacitor in series and the inductor in parallel, blocking low frequencies and passing high frequencies. Both use the same cutoff frequency formula fc = 1/(2π√(LC)).
What is the roll-off slope of an LC filter?
An LC filter is a second-order filter with a roll-off slope of -40 dB per decade (-12 dB per octave) for both low-pass and high-pass configurations. This is twice the slope of a first-order RC or RL filter, making LC filters more effective at separating frequencies near the cutoff point.
What is the gain at the cutoff frequency of an LC filter?
At the cutoff frequency, the gain of an LC filter drops to -3 dB (approximately 70.7% of the passband amplitude). This -3 dB point is the standard definition of the cutoff frequency for all filter types, including LC, RC, and RL filters. The -3 dB point represents the frequency at which the output power has fallen to half the passband power.
What are typical applications of LC filters?
LC filters are widely used in audio crossover networks, radio frequency (RF) circuits, power supply filtering, impedance matching networks, speaker systems, and communication equipment. They are preferred over RC filters in high-frequency applications due to their sharper roll-off and better performance at RF frequencies, though inductors are more expensive and bulkier than resistors.
Can I design an LC band-pass filter with this calculator?
Yes, an LC band-pass filter combines a low-pass and a high-pass LC filter in series. You can use this calculator to determine the cutoff frequency for each section independently. First calculate the low-pass filter for the upper cutoff frequency, then calculate the high-pass filter for the lower cutoff frequency, and combine both stages in series.
How does the input frequency affect LC filter output?
For a low-pass LC filter, frequencies below the cutoff pass with minimal attenuation (near 0 dB gain), while frequencies above are attenuated at -40 dB/decade. For a high-pass LC filter, frequencies above the cutoff pass freely, while frequencies below are attenuated. The optional input frequency field lets you compute the exact gain at any specific frequency of interest.