RClågpass

RClågpass is a term used in electronics to denote a first-order RC low-pass filter, formed by a resistor in series with a capacitor to ground. The name derives from combining RC with lågpass, the Scandinavian word for low-pass, and is commonly encountered in Nordic electronics literature. It describes a simple network intended to pass low frequencies while attenuating higher ones.

In the standard passive configuration, an input signal is applied to the resistor, with the capacitor connected from the node between R and C to ground. The output is taken at that same node (across the capacitor). The transfer function is H(jω) = 1/(1 + jωRC), and the cutoff frequency is fc = 1/(2πRC). The magnitude rolls off at about -20 dB per decade above fc, with a phase lag increasing toward -90 degrees at high frequencies. Component tolerances and parasitics can shift fc and affect the response near the cutoff.

Variants include multi-stage RC low-pass networks for steeper attenuation and active RC low-pass configurations that use operational amplifiers to provide buffering, gain, or higher input impedance. These adaptations broaden practical use in situations where loading effects or precise edge frequencies matter, such as data acquisition, audio smoothing, and PWM signal conditioning.

RClågpass remains a foundational concept in passive and active filter design, valued for its simplicity, predictability, and straightforward tuning via the RC time constant. See also RC filter, first-order filter, anti-aliasing.