Filter Calculator

Design RC and LC passive filters

Passes frequencies below cutoff, attenuates higher frequencies

kΩ
nF

Filter Formulas

1st Order RC Filter

fc = 1 / (2π × R × C)

2nd Order LC Filter

fc = 1 / (2π × √(L × C))

Q = (1/R) × √(L/C)

Understanding Passive Filters

Passive filters use resistors, capacitors, and inductors to shape the frequency response of signals. They require no power supply and are fundamental building blocks in analog circuit design.

Filter Types

Low-Pass Filter

Passes low frequencies, attenuates high frequencies

  • • Anti-aliasing before ADC
  • • Noise reduction
  • • Audio bass control
  • • Power supply filtering

High-Pass Filter

Passes high frequencies, attenuates low frequencies

  • • DC blocking
  • • Audio treble control
  • • AC coupling
  • • Removing 50/60Hz hum

Band-Pass Filter

Passes frequencies within a specific range

  • • Radio tuning circuits
  • • Audio equalizers
  • • Signal isolation
  • • Frequency selection

Filter Order and Rolloff

OrderRolloff RateComponentsCharacteristics
1st Order-20 dB/decadeRCGentle slope, no resonance
2nd Order-40 dB/decadeLC or 2×RCSteeper slope, can have resonance
3rd Order-60 dB/decadeMultiple stagesSharp cutoff
4th Order+-80+ dB/decadeMultiple stagesVery sharp cutoff, complex design

Q Factor and Damping

The Q factor (Quality factor) describes how "peaky" the filter response is at the cutoff frequency. Higher Q means sharper resonance but can cause ringing.

Q < 0.5

Overdamped - slow, no overshoot

Q = 0.707

Butterworth (maximally flat) - optimal for most uses

Q > 0.707

Underdamped - peaking at cutoff, potential ringing

Design Considerations

  • Source/Load Impedance: Filter performance depends on proper impedance matching. Consider source and load impedances in your design.
  • Component Tolerance: Use 1% or better resistors and 5% or better capacitors for accurate cutoff frequencies.
  • Inductor Quality: Real inductors have resistance (DCR) and parasitic capacitance that affect high-frequency performance.
  • Capacitor Type: Choose appropriate capacitor types - ceramic for high frequency, film for precision, electrolytic for bulk filtering.
  • Active Filters: For better performance, consider active filters using op-amps (Sallen-Key, Multiple Feedback, State Variable).