# lowpass (LP)

A low-pass filter( TP) is a four-pole filter with a frequency-dependent function. With this filter, all frequencies below the cutoff frequency are passed. This corresponds to the frequency at which the signal amplitude has decreased by 3 dB compared to the maximum signal amplitude.

The equivalent circuit of a low-pass filter corresponds to the series connection of an inductance (L) and a capacitance (C). The slope of the filter curve is much better for the LC combination than for the RC combination. It also depends on the quality of the components and can be increased by connecting several LC elements in series. Higher slopes are achieved with T-filters and Pi-filters. The latter correspond to interconnection of two low-pass filters.

The low pass and the high pass are basic components for the band pass filter and the band stop filter.

Lowpass filters only allow low frequency signal components to pass, which means that only charge changes can be transmitted with such circuits. Thus, it is not the pulse edge of a pulse that is transmitted, but the pulse roof, which charges the capacitance in the form of an E-function. Mathematically, this function corresponds to an integral, which also leads to the name integrator.

In A/D converters, the input signal to be digitized is passed through a low-pass filter before sampling so that the sampling theorem is satisfied. These low-pass filters are called antialiasing filters( AAF).