# highpass filter (HPF)

A high-pass filter (HPF) is a four-pole filter with a frequency-dependent function. With this filter, all frequencies are passed that lie above the cutoff frequency. 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 high-pass filter corresponds to the series connection of a capacitance (C) with an inductance (L) or a capacitance with a resistance (R). The slope of the filter curve is much better for the LC combination than for the RC combination. It also depends on the quality factor of the components and can be increased by connecting several LC elements in series. The slope of the filter is given in dB/ octave, for example 12 dB/octave or 24 dB/octave for higher slopes.

Like the low-pass filter, the high-pass filter is a basic component for the band-pass filter and the band-stop filter.

High-pass filters allow only high-frequency signal components to pass, which means that only changes can be transmitted with such circuits. They are used in acoustics and video technology, electronics and RF technology, but also in mechanics. In acoustics and audio engineering, high-pass filters are found as bass-cut filters, rumble filters or impact filters.

Since high-pass filters only allow high-frequency signal components to pass, only the pulse edge of a pulse is transmitted, for example, but this edge changes over time. If the pulse edge is rising rapidly, a positive pulse will form which will turn negative as soon as the rate of change of the rising edge becomes less. Mathematically, this function corresponds to the differential, which also leads to the name differentiator.