A Bulk Acoustic Wave (BAW) resonator (20) is provided which comprises a piezoelectric layer (22); a first and second protective layer (38b,a), a first electrode (24), a second electrode (26), a bridge (also referred to as a "membrane" (28), a pair of etch windows (40a,b), an air gap (34), and a substrate (36). A portion of the piezoelectric layer (22) is positioned atop the first electrode (24), and the second electrode (26) is positioned atop the piezoelectric layer (22); thereby forming a parallel plate structure between which the piezoelectric layer (22) is allowed to resonate or vibrate. The piezoelectric layer (22) comprises, by example, zinc-oxide (ZnO), and has a thickness of 1.7 µm. The electrodes (24,26) comprise, by example, gold (Au) and have thicknesses of 0.1 µm.

The membrane (28) comprises two layers (30,32) namely a top layer (30) and a bottom layer (32). The top layer (30), which preferably has a thickness of 0.6 µm and comprises poly-silicon, has a top surface which is in contact with the first electrode (24) and portions of the piezoelectric layer (22). The top layer (30) is situated atop a portion of the bottom layer (32), which preferably has a thickness of 0.4 µm and is comprised of silicon-dioxide (SiO₂).

A portion of a bottom surface of the membrane (28) is situated adjacent to the air gap (34), which separates this portion of the membrane's bottom surface from a portion of the substrate (36). This air gap (34), which is typically filled with air but any suitable material may be used, is formed by etching a portion of the substrate (36). The air gap (34) is bounded by the etch windows (40a,b), by a portion of the first protective layer (38b), by the bottom surface portion of the membrane (28), and by inner faces (36b,c,d) of the substrate (36). The air gap (34) functions to isolate acoustic vibrations created by the piezoelectric layer (22) from the substrate (36).

Embodiments relate to an apparatus (10), a transmitter (300), a method and a computer program suitable for controlling a contribution of a first plurality of continuously oscillating nano-mechanical oscillator modules (100a; 100b) to a composite signal (50). The apparatus (10) comprises at least one interface (12) to transmit a control signal and to obtain information related to a plurality of frequency components of the composite signal. The apparatus (10) further comprises a control module (14) to determine the control signal based on the information related to the plurality of frequency components. The control signal controls the contribution of the first plurality (100a; 100b) to the composite signal (50).

The invention relates to a radio frequency, RF, signal generator (100), for providing an RF signal (RF_out), said RF signal generator (100) comprising:

- at least two coupling elements (2; 41; 51), wherein each coupling element (2; 41; 51) comprises two coupling electrodes (3a, 3b) at least one of which is capable of performing a mechanical self-oscillation with a specific resonance frequency (fl, .., fN) and/or a specific attenuation to enable an electric coupling between said coupling electrodes (3a, 3b) which depends on said specific resonance frequency (fl, .., fN) and/or specific attenuation,

- stimulating means (IG1, IG2, .., IGN) that are configured to selectively stimulate a mechanical self-oscillation of at least one of said coupling elements (2; 41; 51), and

- signal processing means (22) configured to determine at least one parameter characterizing an actual resonance frequency and/or an actual attenuation exhibited by at least one coupling element (2; 41; 51) in response to a stimulation applied by said stimulating means (IG1, IG2, .., IGN).