What is SAW Filter ?

LTE, RF Basics

A Surface Acoustic Wave filter, or SAW filter is a filter whereby the electrical input signal is converted to a acoustic wave by so-called interdigital transducers (IDTs) on a piezoelectric substrate such as quartz. The IDTs consist of interleaved metal electrodes which are used to launch and receive the waves, so that an electrical signal is converted to an acoustic wave and then back to an electrical signal.

SAW filters are widely used in cell phone and base station applications for filtering. Here they provide considerable advantages in terms of cost and size, in an environment where these two aspects are of considerable importance. SAW filters combine low insertion loss with good rejection, can achieve broad bandwidths and are a tiny fraction of the size of traditional cavity and even ceramic filters. Because SAW filters are fabricated on wafers, they can be created in large volumes at low cost. SAW technology also allows filters and duplexers for different bands to be integrated on a single chip with little or no additional fabrication steps.

SAW filter response looks like as below:

SAW filters, however, have limitations. Above about 1 GHz, their selectivity declines, and at about 2.5 GHz their use is limited to applications that have modest performance requirements.