RC Filter Calculator
Calculate cutoff frequency, time constant, and attenuation for RC low-pass and high-pass filters. Analyze frequency response at any test frequency.
What is an RC filter?
An RC filter is a passive electronic filter made from a resistor (R) and capacitor (C). It attenuates certain frequencies while passing others. Low-pass filters pass low frequencies and block high frequencies. High-pass filters do the opposite.
How do I calculate RC filter cutoff frequency?
Cutoff frequency (fc) is calculated using fc = 1/(2πRC), where R is resistance in ohms and C is capacitance in farads. At this frequency, the signal is attenuated by -3dB (reduced to 70.7% of input voltage).
What is the -3dB point?
The -3dB point (cutoff frequency) is where output power is half the input power, or voltage is reduced to 0.707 (70.7%) of input. It marks the boundary between passband and stopband. Beyond this point, attenuation increases rapidly.
What is filter roll-off rate?
Roll-off rate describes how quickly a filter attenuates signals beyond cutoff frequency. First-order RC filters have a roll-off of 20dB/decade (or 6dB/octave). Each doubling of frequency beyond cutoff reduces signal by 6dB. Higher-order filters have steeper roll-offs.
How does a low-pass filter work?
A low-pass RC filter has the resistor before the capacitor. At low frequencies, the capacitor has high impedance (open circuit) so signal passes. At high frequencies, capacitor impedance is low (short circuit) so signal is shunted to ground, blocking it.
How does a high-pass filter work?
A high-pass RC filter has the capacitor before the resistor. At low frequencies, the capacitor blocks DC and low frequencies (high impedance). At high frequencies, the capacitor has low impedance and passes the signal through to the resistor and output.
How do I choose R and C values?
First determine desired cutoff frequency (fc), then choose convenient values where fc = 1/(2πRC). For example, for 1kHz: Use C=100nF, then R=1/(2π×1000×100×10⁻⁹)≈1.6kΩ. Use standard values like 1.5kΩ or 1.8kΩ. Typical R: 1kΩ-100kΩ, C: 1nF-10µF.
What is filter impedance and why does it matter?
Filter impedance affects loading on source and load circuits. High impedance (high R) minimizes loading but is noise-sensitive. Low impedance (low R) drives loads better but draws more current. Impedance at cutoff frequency is Z = R for first-order filters.
Can I cascade RC filters for steeper roll-off?
Yes, but simple cascading doesn't multiply roll-off rates due to loading effects. Each stage loads the previous one, changing cutoff frequency. Use buffer amplifiers (op-amps) between stages, or design proper multi-order active filters for predictable steep roll-offs.
What is the difference between passive and active filters?
Passive filters use only R, L, C components. They're simple, cheap, but attenuate signals and can't provide gain. Active filters use op-amps and can provide gain, have better roll-off rates, and sharper characteristics, but require power and are more complex.
How do I calculate attenuation at a specific frequency?
For low-pass: Attenuation (dB) = -20×log₁₀(√(1+(f/fc)²)). For high-pass: Attenuation (dB) = -20×log₁₀(√(1+(fc/f)²)). At fc, attenuation is -3dB. At 10×fc (low-pass), attenuation is approximately -20dB.
What are common applications of RC filters?
Audio tone controls, anti-aliasing filters before ADCs, noise reduction, power supply smoothing, signal conditioning, speaker crossovers, sensor signal filtering, debouncing switches, and removing high-frequency interference. They're essential in analog electronics.