Low Pass/High Pass Filter Calculator

Simply choose your filter type (low-pass or high-pass) and choose whether it’s a resistor-capacitor (RC) or resistor-inductor (RL) configuration. Then add two values from resistance, capacitance/inductance (based on configuration) and cut-off frequency and the calculator will work out the remaining value. Give it a go below!

Low pass and high pass filters are used in various industries to remove unwanted noise, focus on a specific range of frequency and even to protect sensitive components from frequency spikes. Low-pass filters block high frequencies and allow low frequencies to pass through whilst high-pass filters do the opposite.

Low-pass filters are generally used to remove high-frequency noise, pass bass or low-end audio signals, limit bandwidth in signal processing and to smooth out voltage or audio signals. On the other hand, high-pass filters are perfect for removing rumble, hum or DC offset, passing treble or high-end audio and removing low-frequency drift to prepare signals for digitisation.

Formulas

There are two main formulas to use when trying to determine the cutoff frequency and they depend on whether it’s for an RC or an RL configuration. Below are the formulas for both configurations.

RC formula: fc = 1 / (2πRC)

RL formula: fc = R / (2πL)

Fc = cut-off frequency

R = resistance

C = capacitance

Q: What does cut-off frequency mean?

A: Cut-off frequency refers to the point where the filter starts to reduce the strength of the signal. It’s often written as ‘-3dB cut-off frequency’ which refers to a drop in power of roughly 50%.

Q: What is the difference between RL and RC filters?

A: RC filters use a resistor and a capacitor whereas an RL filter uses a resistor and inductor. The former is used in low-frequency circuits and RL filters are more common in high-frequency applications.

Q: Do passive filters work with AC and DC signals?

A: Yes, low-pass filters can remove high-frequency AC signals whilst high-pass filters can block DC signals.