Acoustic components used for filtration - Overview of the different technologies

The concept of electrical filtering emerged with the first communication systems using very narrow spectral bands. In the 1910s, the first radio engineers envisaged the possibility of transmitting several telephone calls on the same line: to do this, they modulated the signal around a carrier frequency, different for each communication channel. The same idea still prevails today in virtually all multiplex communications (as in the case of unbundling, where telephony, television and digital transmissions coexist on the same transmission line). In this way, the signal spectrum is distributed over a frequency band centered around the carrier frequency. On reception, selection of a specific channel requires filtering of the signal to select only the useful band. This brings us to the concept of a selective filter, capable of effectively eliminating any signal outside the useful band without significantly altering the information transmitted. Initially, discrete-element linear filters of the resistance, inductance and capacitance (R, L, C ) type were used for filtering operations at frequencies ranging from 10 to 40 kHz. The use of wireless communication methods and the need to extend the useful bands to increase the amount of information conveyed quickly led to the development of other filtering principles, better suited to such needs. The need for stable, compact passive elements for filtering also led to the search for alternative solutions to waveguides and electromagnetic resonant cavities, which are generally efficient but expensive and cumbersome.

Filtering techniques were rapidly extended by the introduction of active elements, resolving, among other things, the generally high losses in the useful band associated with the use of passive elements (RLC ). Nevertheless, and despite constant progress in microelectronics, the bandwidths of active amplification circuits often remain incompatible with "telecom" applications, for which working frequencies are constantly increasing. Numerous attempts have been made to replace analog linear filtering with digital techniques. These require electronic components capable of sampling and processing the signal at equally high frequencies, and are less immune to interference and spectral overlap effects than passive components.

The focus here is on "electrical" filters, based on the use of acoustoelectric components, which are designed to be inserted into an electronic circuit in order to perform a frequency filtering function. The aim of this article is to describe and compare passive filter technologies belonging to the special category of "acoustic" filters, based on the conversion of electrical energy into acoustic energy, and vice versa. A second article