RC/LC Filter Calculator
Design passive and active filters. Calculate component values for lowpass, highpass, bandpass, and notch filters.
Calculator
How to Use This Calculator
This filter calculator helps you design passive RC/LC filters and active op-amp filters for audio, RF, and general signal conditioning applications.
- Select Filter Type — Low-pass, high-pass, band-pass, or notch
- Choose Topology — Passive (RC/LC) or active (op-amp based)
- Enter Cutoff Frequency — The -3dB point for LP/HP filters
- Set Impedance — Determines resistor values
- For Bandpass — Also specify bandwidth to determine Q factor
- Click Calculate — Get component values
Filter Types Explained
Low-pass Filter
Passes frequencies below the cutoff frequency and attenuates higher frequencies. Used for anti-aliasing before ADCs, removing high-frequency noise, and audio bass extraction.
High-pass Filter
Passes frequencies above the cutoff frequency and attenuates lower frequencies. Used for DC blocking, removing low-frequency rumble, and audio treble extraction.
Band-pass Filter
Passes frequencies within a specific band and attenuates frequencies outside. The Q factor determines selectivity: higher Q means narrower bandwidth.
Notch Filter (Band-stop)
Attenuates a specific frequency while passing all others. Commonly used to remove 50/60Hz mains hum or specific interference frequencies.
Filter Response Comparison
| Filter | Roll-off (1st order) | Phase Shift | Common Use |
|---|---|---|---|
| Low-pass | -20 dB/decade | 0° to -90° | Anti-aliasing, noise removal |
| High-pass | +20 dB/decade | +90° to 0° | DC blocking, rumble removal |
| Band-pass | ±20 dB/decade | +90° to -90° | Radio tuning, tone detection |
| Notch | Deep null at fc | ±180° at fc | Hum rejection, interference |
Design Tips
Passive vs Active Filters
| Aspect | Passive | Active |
|---|---|---|
| Power | No power needed | Requires supply |
| Gain | Loss only | Can amplify |
| Q Factor | Limited by components | Can be very high |
| Size | Can be bulky (inductors) | Compact |
| Bandwidth | DC to very high freq | Limited by op-amp |
Component Selection
- Resistors: Use 1% tolerance metal film for precision
- Capacitors: C0G/NP0 for stability, X7R acceptable for less critical
- Inductors: Consider DCR, saturation current, and self-resonance
- Op-amps: Choose based on bandwidth, noise, and supply requirements
Common Impedance Values
- Audio: 10kΩ-100kΩ (high impedance for low noise)
- RF: 50Ω or 75Ω (standard transmission line impedances)
- Instrumentation: 1kΩ-10kΩ (balance between noise and loading)
Avoiding Common Mistakes
- Consider source and load impedance effects on filter response
- Account for capacitor parasitic inductance at high frequencies
- Use low-ESR capacitors for better Q factor
- Buffer high-impedance filters to prevent loading
Frequently Asked Questions
What is the -3dB cutoff frequency?
The -3dB point is where the output power is half the input power (voltage is ~70.7% of input). This is the standard definition for filter cutoff frequency. Above/below this point, attenuation increases at the filter's roll-off rate.
How do I get steeper roll-off?
Use higher-order filters. Each order adds 20 dB/decade of roll-off. Cascade multiple first-order stages or use second-order Sallen-Key, Butterworth, or Chebyshev topologies. Higher orders require more components and careful design.
Why use active filters instead of passive?
Active filters can provide gain, don't require inductors (which are bulky at low frequencies), and can achieve high Q factors. They're ideal for audio and low-frequency applications. However, they're limited by op-amp bandwidth and require power.
What Q factor should I use for bandpass filters?
Q = center frequency / bandwidth. For general filtering, Q of 1-10 is common. Higher Q (10-100) for selective frequency detection. Very high Q (100+) requires precision components and may be unstable.
How accurate are these calculated values?
The formulas are exact for ideal components. Real-world accuracy depends on component tolerances (1% resistors, 5% capacitors = ~6% frequency error). For precision applications, use trimmer components or select matched values.
Verify Your Component Selections
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