LC Low Pass Filter Design Guide
An LC low pass filter passes slow signals and reduces fast signals. It uses an inductor in series and a capacitor to ground. The inductor resists sudden current change. The capacitor gives high frequency energy an easier path to ground. Together they create a second order response with a sharper slope than a simple RC filter.
Why This Calculator Helps
Manual LC work becomes difficult when units vary. A designer may enter microhenries, nanofarads, kilohertz, ohms, and tolerance values. This calculator converts them to base units, then returns practical values. It can solve cutoff frequency, missing inductance, missing capacitance, or a Butterworth style pair. It also checks load Q, impedance, reactance, and expected response at a test frequency.
Understanding Cutoff
The ideal resonant cutoff is based on one formula. Frequency equals one divided by two pi times the square root of L times C. Larger inductance lowers cutoff. Larger capacitance also lowers cutoff. Smaller parts raise cutoff. Real filters may shift because of load resistance, source resistance, coil resistance, capacitor loss, and part tolerance.
Q And Damping
Q describes how strongly the filter rings near cutoff. A low Q gives a soft knee. A Q near 0.707 is often used for a flat Butterworth response. A high Q may create gain peaking before attenuation begins. This can be useful in tuned circuits, but it can be risky in power lines or audio paths.
Practical Use
Start with the target cutoff and expected load resistance. Choose Butterworth mode for a balanced starting point. Then compare nearby standard parts. Check tolerance range before final selection. For switching noise, test the noise frequency. For audio work, test the highest wanted frequency and the first unwanted band. Always confirm results with a circuit simulator and real measurements.
Exporting Results
The CSV file stores each computed line for records. The PDF button creates a simple report from the displayed output. These exports help compare versions and share design choices.
Good layout also matters. Place the result near the inputs. This keeps every design step visible. When values change, compare the new cutoff, Q, and attenuation before saving the report. Document each assumption, especially load resistance and selected component tolerance.