RC Filter Calculator
Design Low Pass and High Pass RC filters.
What is an RC Filter?
An RC Filter is the simplest way to manipulate signals. By combining a Resistor (R) and Capacitor (C), you can create circuits that block specific frequencies.
- Low Pass Filter: Blocks high frequencies (treble), lets low frequencies (bass) pass. Used for subwoofer outputs or smoothing PWM.
- High Pass Filter: Blocks low frequencies (DC/bass), lets high frequencies pass. Used for blocking DC offset or protecting tweeters.
Formulas and Math
The core characteristic of any RC filter is its Cutoff Frequency (fc). This is the point where the signal power drops by half (-3 dB).
fc = 1 / ( 2 × π × R × C )
Time Constant (τ) = R × C
The signal attenuates at a rate of -6 dB per octave (or -20 dB per decade) beyond the cutoff frequency.
Practical Applications
- Debouncing Switches: A mechanical switch "bounces" rapidly when pressed. A simple low-pass RC filter smoothes this noise into a clean logic transition.
- PWM to Analog: To convert a digital PWM signal (0V/5V pulses) from an Arduino into a true analog voltage (0-5V), you use an RC Low Pass filter to average the pulses.
- Audio Tone Control: Simple guitar tone knobs are just adjustable RC Low Pass filters (using a potentiometer for R).
FAQ
What happens if I cascade two filters?
If you put two RC filters in series, you get a 2nd Order Filter. The roll-off becomes twice as steep (-12 dB/octave). However, the first stage is "loaded" by the second, shifting the cutoff frequency unless you use an Op-Amp buffer in between.
Why -3dB?
-3 decibels represents half power (or 70.7% of voltage). It is the standard convention for defining the "bandwidth" of a filter. At fc, the phase shift is exactly 45°.
How do I choose R and C values?
There are infinite combinations for the same frequency. Generally, choose R between 1kΩ and 100kΩ. If R is too low, it draws too much current. If R is too high (megaohms), the circuit becomes sensitive to noise and PCB leakage.