Wide-band acoustically coupled thin-film BAW filter

An acoustically coupled thin-film Bulk Acoustic Wave filter, comprising;

• an input-port for receiving an electrical input signal;
• an output-port for outputting an electrical output signal;
• a piezoelectric layer;
• more than ten interdigital electrodes on the surface of the piezoelectric layer, the interdigital electrodes comprising a number of first electrodes spaced from each other by a spacing and a number of second electrodes such that one second electrode is located in each spacing between two consecutive first electrodes;
• the first electrodes being connected to the input port for receiving the electrical input signal and causing a corresponding acoustic wave in the piezoelectric layer;
• the second electrodes being connected to the output port for obtaining the electrical ouput signal from the acoustic wave in the piezoelectric layer;
• wherein the filter is capable of operating in the first order thickness-extensional TE1 mode;
• wherein the mass loading by the interdigital electrodes is such that the k=0 frequency of the outside region's TS2 mode is between 93% and 99.9% of the electrode region's TE1 cutoff frequency;
• and
• wherein the width W of each interdigital electrode is between 25% and 50% of the wavelength of the lateral acoustic wave at the desired odd mode resonance wavelength λodd.

An acoustically coupled thin-film Bulk Acoustic Wave filter, comprising;

• an input-port for receiving an electrical input signal;
• an output-port for outputting an electrical output signal;
• an acoustic Bragg structure comprising layers of at least two different materials such that the first material has a higher acoustic impedance than the second material;
• a piezoelectric layer on the acoustic Bragg structure;
• more than ten interdigital electrodes on the surface of the piezoelectric layer, the interdigital electrodes comprising a number of first electrodes spaced from each other by a spacing and a number of second electrodes such that one second electrode is located in each spacing between two consecutive first electrodes;
• the first electrodes being connected to the input port for receiving the electrical input signal and causing a corresponding acoustic wave in the piezoelectric layer;
• the second electrodes being connected to the output port for obtaining the electrical ouput signal from the acoustic wave in the piezoelectric layer;
• wherein the filter is capable of operating in the first order thickness-extensional TE1 mode;
• wherein the mass loading by the interdigital electrodes is such that the k=0 frequency of the outside region's TS2 mode is between 93% and 99.9% of the electrode region's TE1 cutoff frequency;
• and
• wherein the width W of each interdigital electrode is between 25% and 50% of the wavelength of the lateral acoustic wave at the desired odd mode resonance wavelength λodd.