RCTiefpass

RCTiefpass, or RC Tiefpass, is a simple resistor-capacitor low-pass filter used to attenuate high-frequency signals while allowing low-frequency components to pass with minimal attenuation. In the standard configuration, a resistor is connected in series with the input and a capacitor is connected from the junction to ground. The output is taken across the capacitor.

The behavior of the RC Tiefpass is described by its transfer function. In the Laplace domain, the complex gain is H(s) = 1 / (1 + sRC), where R is the resistance and C is the capacitance. In the frequency domain, the magnitude is |H(jω)| = 1 / sqrt(1 + (ωRC)^2). The threshold between pass and stop bands is defined by the cutoff frequency fc = 1 / (2πRC); at this frequency the output power is half the input and the voltage is down by 3 dB.

Key characteristics include being a single-pole, passive filter with a maximum gain of unity (0 dB) and a slope of -20 dB per decade beyond the cutoff. The filter’s performance depends on source and load impedances, since the RC network forms a voltage divider with them. Tolerances in R and C, as well as parasitic elements, can shift fc and alter attenuation.

Variants and extensions include RC high-pass filters (output taken across the resistor in a series RC network) and active RC filters that use amplifiers to provide buffering or gain, enabling steeper roll-offs with multiple RC stages.

Applications of RC Tiefpasses include audio filtering, sensor signal conditioning, and anti-aliasing in analog-to-digital conversion, where simple, cost-effective frequency shaping is required. Practical design notes emphasize component tolerances, temperature coefficients, and potential loading effects.