Calculator KB-38
RC Filter Calculator
The simplest filter: smoothing PWM, cleaning up a sensor signal, or splitting audio.
Result
In brief: Calculate low-pass and high-pass RC filters: cutoff frequency, attenuation in decibels at a specified frequency, phase shift, and frequency response. Example: Cutoff frequency: 1.592 kHz.
Calculation example
| Type | Low-pass (passes low frequencies) |
|---|---|
| Resistance | 1 kΩ |
| Capacity | 100 nF |
| Check at frequency | 10000 Hz |
| Cutoff frequency | 1.592 kHz |
|---|---|
| Time constant | 100 µs |
| Attenuation at frequency | -16.1 dB |
| Signal remaining | 15.7 % |
| Phase shift | -81° |
How it is calculated
|H| = 1 / √(1 + (f / fc)²)
At the cutoff frequency, the signal is attenuated by 3 dB, or to 70.7% of its amplitude. Beyond that, the roll-off is 20 dB per decade. For a steeper filter, connect stages in series with a buffer between them.
What to look for
- Source and load resistance shift the cutoff frequency. The load resistance should be at least 10 times R.
- For PWM smoothing, set the cutoff frequency 10–100 times lower than the PWM frequency.
Frequently asked questions
How do I find the cutoff frequency of an RC filter?
fc = 1 / (2π · R · C). 1 kΩ and 100 nF give 1.59 kHz. At this frequency, the signal drops by 3 dB; at 10 kHz, it drops by 16 dB.
How do I filter PWM into a steady voltage?
Set the cutoff frequency 10–100 times lower than the PWM frequency. For 1 kHz, 10 kΩ and 1 µF will work: fc is about 16 Hz, and the fundamental is attenuated by about 60 times.
How does the load affect an RC filter?
It forms a divider with the resistor and shifts the cutoff frequency. The load resistance should be at least 10 times R; otherwise, use an op-amp buffer.
