What is an RF filter

RF filter

RF filters are frequency-selective circuits that allow certain frequencies to pass while blocking others. The frequencies allowed through are referred to as the pass band, and the blocked frequencies are referred to as the stop band. Important parameters of HF filters are the frequency, the bandwidth, which is affected by the attenuation or the quality depends on the slope, as well as the insertion loss and the transmission loss.

HF filters can be constructed from discrete components, from capacitances, inductances and resistors. Since the trend towards miniaturization remains unchanged in mobile devices, filter manufacturers must offer miniaturized HF filters, ideally in chip design. These can only be realized up to a certain size with discrete components, and therefore work with different techniques. For example, with ferroelectric material like the ceramic filter, quartz-based like the quartz filter, microsystem technology like the MEMS filter, silicon-based like the Bulk Acoustic Wave (BAW) and the Film Bulk Acoustic Resonator (FBAR) or electro-acoustic -electric basis like the surface acoustic wave filter.

As far as miniaturization is concerned, the tiniest HF filters are surface acoustic wave filters in a very small chip design. An example of this are the CSSP packages developed by Epcos, which have a footprint of just 1.1 x 0.9 mm, and the even smaller DSSP packages, whose footprint is only half a square millimeter.