An RC low pass filter is shown at the left. Low frequency signals input to the filter are transferred unchanged to the output. Higher frequencies are filtered out.

The time constant, t, of a filter is determined by

t = R · C

The cutoff frequency, f₀ is given by

f₀ = 1 / (6.28 t )

At the cutoff frequency, the amplitude of a sine wave input to an RC filter is attenuated by 3 dB (about 30%) and the phase of the signal is shifted by 45° . At lower frequencies, both the attenuation and phase shift are less.