Low Pass / High Pass Filter Calculator
-3 dB cutoff for RC and RL filters
Required Parameters
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Quick Answer
The -3dB cutoff frequency (fc) is where the signal power drops by half. For RC filters: fc = 1 / (2πRC). For RL filters: fc = R / (2πL).
RC Filter Calculator
Design first-order RC filters and compute the -3 dB cutoff frequency from resistor and capacitor values — or solve for R, C, or fc when you know any two.
Quick start: Select RC Low Pass or RC High Pass, enter R and C → instant fc. Use Target Variable to find C for a desired cutoff.
RC filter formula
fc = 1 / (2π × R × C)
| Symbol | Meaning |
|---|---|
| fc | Cutoff frequency (Hz) |
| R | Resistance (Ω) |
| C | Capacitance (F) |
Rearranged:
- C = 1 / (2π fc R)
- R = 1 / (2π fc C)
- τ = RC = 1 / (2π fc) (time constant)
RC low pass vs RC high pass
| Type | Passes | Blocks | Typical use |
|---|---|---|---|
| RC Low Pass | Below fc | Above fc | Noise filtering, ADC anti-aliasing, debounce |
| RC High Pass | Above fc | Below fc | DC blocking, coupling capacitors |
Both use fc = 1/(2πRC) — only the circuit topology changes.
Worked example — 1 kHz RC low pass
Target fc = 1 kHz, choose C = 47 nF:
- R = 1 / (2π × 1000 × 47×10⁻⁹) ≈ 3.39 kΩ
- Nearest E24: 3.3 kΩ → actual fc ≈ 1.03 kHz
Common applications
- Anti-aliasing before ADC — set fc ≤ fs/2
- Power-supply filtering — e.g. 10 Ω + 100 nF → fc ≈ 159 kHz
- Switch debounce — τ = 5–10 ms (10 kΩ + 1 µF)
Related tools
- Low pass filter calculator — RC & RL low pass focused
- RC/LC filter cutoff calculator — includes RL modes
- RC time constant calculator
- Reactance calculator
Design Notes
First-order passive filters only roll off at -20dB per decade. They are great for simple noise filtering, anti-aliasing before a slow ADC, or debounce circuits, but inadequate for separating close frequencies. When designing RC filters, avoid using extremely large resistors (e.g., >1MΩ) because the input bias current of the next stage (like an Op-Amp or ADC) will cause a DC voltage error. Conversely, avoiding extremely small resistors (<100Ω) will save power and prevent overloading the driving source.
Common Mistakes
- 1
Expecting a sharp cutoff from a simple RC filter. It's a gentle curve, not a brick wall.
- 2
Not balancing the source impedance. If your signal source already has 10kΩ output impedance, and you use a 1kΩ series resistor for your filter, your effective R is 11kΩ.
- 3
Ignoring capacitor tolerances. Cheap Y5V or Z5U ceramic capacitors can vary by +20%/-80% with temperature and DC bias, completely ruining your cutoff frequency.
Engineering Handbox
1. Identify variables: R = 10,000, C = 0.0000001 2. Multiply R × C = 0.001 seconds (Time Constant τ) 3. Multiply by 2π: 0.001 × 6.283 = 0.006283 4. Divide 1 by result: 1 / 0.006283 = 159.15 Hz
Knowledge Base
Qu'est-ce que la fréquence de coupure à −3 dB ?
C'est la fréquence à laquelle la puissance de sortie est réduite de moitié (tension à 70,7 % de l'entrée). Au-delà, le filtre atténue le signal à raison de −20 dB/décade pour un premier ordre.
Quelle est la formule de coupure d'un filtre RC ?
fc = 1 / (2π × R × C). Exemple : R = 10 kΩ, C = 100 nF → fc = 159 Hz. C'est la formule de base pour les filtres passe-bas et passe-haut de premier ordre.
Quelle différence entre passe-bas et passe-haut ?
Passe-bas : laisse passer les basses fréquences, atténue les hautes (le condensateur est vers la masse). Passe-haut : laisse passer les hautes fréquences (le condensateur est en série avec le signal). Même formule de coupure dans les deux cas.
Comment cascader des filtres pour une pente plus raide ?
Deux filtres RC identiques en cascade donnent −40 dB/décade, mais attention à l'interaction entre étages. Pour des performances optimales, utilisez des topologies actives (Sallen-Key, Butterworth) avec des ampli-ops.
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