An acoustic filter based on layered structure.

Acoustic filters (AFs) are key components to control wave propagation in multi-frequency systems. We present a design which selectively achieves acoustic filtering with a stop band and passive amplification at the high- and low-frequencies, respectively. Measurement results from the prototypes closely match the design predictions. The AF suppresses the high frequency aliasing echo by 14.5 dB and amplifies the low frequency transmission by 8.0 dB, increasing an axial resolution from 416 to 86 µm in imaging. The AF design approach is proved to be effective in multi-frequency systems.

An efficient approach to computing third- order scattering of sound by sound with application to parametric arrays.

A mathematical description of third-order scattered sound fields is derived using a multi-Gaussian beam (MGB) model that describes the sound field of any arbitrary axially symmetric beam as a series of Gaussian base functions. The third-order intermodulation (IM3) frequency components are produced by considering the cascaded nonlinear secondorder effects when analyzing the interaction between the firstand second-order frequency components during the nonlinear scattering of sound by sound from two noncollinear ultrasonic baffled piston sources. The theory is extended to the modeling of the sound beams generated by parametric transducer arrays, showing that the MGB model can be efficiently used to calculate both the second- and third-order sound fields of the array. Measurements are presented for the IM3 frequency components and parametric array sound fields and comparisons of the model are made with traditional simulation results from direct numerical integration.

Third-order parametric array generated by distantly spaced primary ultrasonic tones.

Traditional parametric arrays are produced by a second-order nonlinear interaction between two primary ultrasonic tones that are close in frequency, resulting in a difference tone that is in the audio band. This article presents a parametric array produced by a third-order nonlinear interaction between two primary ultrasonic tones that are distantly spaced in frequency such that one tone is approximately the second harmonic of the other. The result is a third-order lower intermodulation (IM3) tone in the audio band with greater directivity and lower side lobe amplitude than comparable second-order fields. Measurements are presented that compare the directivity of 1-, 2-, and 4-kHz difference tones to that of 1-, 2-, and 4-kHz IM3 lower tones. Furthermore, a cascaded second-order approach for N-element transducer arrays is used to model third-order scattering with good agreement between measurement and theory.

Scaling issues in ferroelectric barium strontium titanate tunable planar capacitors.

We report on the geometric limits associated with tunability of interdigitated capacitors, specifically regarding the impact of a parasitic non-tunable component that necessarily accompanies a ferroelectric surface capacitor, and can dominate the voltage-dependent response as capacitor dimensions are reduced to achieve the small capacitance values required for impedance matching in the X band. We present a case study of simple gap capacitors prepared and characterized as a function of gap width (i.e., the distance between electrodes) and gap length (i.e., the edge-to-edge gap distance). Our series of measurements reveals that for gap widths in the micrometer range, as gap lengths are reduced to meet sub-picofarad capacitance values, the non-tunable parasitic elements limit the effective tunability. These experimental measurements are supported by a companion set of microwave models that clarify the existence of parallel parasitic elements.

The impact of metallization thickness and geometry for X-band tunable microwave filters.

The impact of dc resistance on the performance of X-band filters with ferroelectric varactors was investigated. Two series of combline bandpass filters with specific geometries to isolate sources of conductor losses were designed and synthesized. Combining the changes in filter geometry with microwave measurements and planar filter solver (Sonnet software) simulations quantitatively identified the dependency of insertion loss on overall metallization thickness and local regions of thin metallization. The optimized 8-GHz bandpass filters exhibited insertion losses of 6.8 dB. These filters required 2.5 microm of metal thickness (or 3 effective skin depths) to achieve this loss. The trend of loss with thickness indicates diminishing return with additional metal. The integration scheme requires thin regions of metal in the immediate vicinity of the varactors. It is shown through experiment and simulation that short distances (i.e., 15 microm) of thin metallization can be tolerated provided that they are located in regions where the resonant microwave current is low.