Johanna Meltaus

Millimeter-wave beam shaping using holograms

We synthesize amplitude- and phase-type computer-generated holograms (diffractive gratings) for shaping millimeter-wave fields. We design holograms using quasi-optical back-propagation and rigorous optimization methods adopted from diffractive optics. We present experimental results from a plane-wave-generating hologram and a custom-designed field shaper at 310 GHz. Holograms can be applied, e.g., in a compact antenna test range and we propose their use for alignment purposes.

Holograms for shaping radio-wave fields

Holograms—diffractive elements—are designed and fabricated for shaping millimetre-wave radio fields. Methods for the synthesis of hologram elements are discussed and several beam shapes are tested: plane waves, radio-wave vortices and Bessel beams. Here we present an overview of the methods applied and results obtained with quasi-optical hologram techniques using both amplitude and phase holograms.

Parametric study of laterally acoustically coupled bulk acoustic wave filters

Acoustically coupled thin-film bulk acoustic wave resonator filters, in which the coupling takes place mechanically in the lateral direction between closely-spaced narrow resonators, are a promising approach to passband filtering at gigahertz frequencies. In this paper, filters with interdigital electrode structures are studied. Electrode number, electrode width, and coupling gap width are varied. The resonators are solidly mounted, having an acoustic mirror isolating the resonator from a Si substrate and providing the means to engineer the acoustic dispersion properties of the resonators. The center frequency of the filters is around 2 GHz. Electrical frequency responses of the filters are measured and the strength of the lateral acoustic coupling is calculated from the measurements. The effects of device parameters on the acoustic coupling and the obtainable filter bandwidth are analyzed in detail. A bandpass filter with 4.9% bandwidth, minimum insertion loss of 2 dB and sharp transition from passband to suppression band, is presented.

Low-loss, multimode 5-IDT SAW filter

Longitudinally coupled resonator filters provide unbalanced-balanced operation with wide bandwidth, low loss, arid high suppression levels. However, reducing the insertion loss in the 1.8-2.2 GHz range remains a challenging problem because at high frequencies the resistive losses arising from the relatively wide aperture of the filter may degrade the performance. A 5-interdigital transducer (IDT) filter has six gaps at which the periodicity of the grating is broken, resulting in additional loss due to scattering into the bulk. In this paper, we show that replacing the gaps between the transducers with short transducer sections having their pitch different from that of the main transducers reduces the insertion loss of the device. We present devices with balun operation at 1842 MHz with wide bandwidth of 4.5% and -40 dB suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of -1.2 dB in the passband. The passband is quite flat, with <1 dB ripple. We also discuss the layout of the contact pads and the connections, and its effect on the device performance and balance characteristics.

SAW filter based on parallel-connected CRFs with offset frequencies

We present designs for two-track devices, based on the coupled resonator filter (CRF) concept, having either two or three transducers. The tracks are connected in parallel, imposing a frequency shift between them. Examples of this approach are demonstrated in different frequency ranges, both on quartz and - for the first time - on 42/spl deg/-LiTaO/sub 3/. We also introduce a parallel-track design with distributed gaps. The device can also operate as a single-to-balanced (BALUN) transformer.

2-D modeling of laterally acoustically coupled thin film bulk acoustic wave resonator filters

A 2-D model is developed for calculating lateral acoustical coupling between adjacent thin film BAW resonators forming an electrical N-port. The model is based on solution and superposition of lateral eigenmodes and eigenfrequencies in a structure consisting of adjacent regions with known plate wave dispersion properties. Mechanical and electrical response of the device are calculated as a superposition of eigenmodes according to voltage drive at one electrical port at a time while extracting current induced in the other ports, leading to a full Y-parameter description of the device. Exemplary cases are simulated to show the usefulness of the model in the study of the basic design rules of laterally coupled thin film BAW resonator filters. Model predictions are compared to an experimental 1.9-GHz band-pass filter based on aluminum nitride thin film technology and lateral acoustical coupling. Good agreement is obtained in prediction of passband behavior. The eigenmode-based model forms a useful tool for fast simulation of laterally coupled acoustic devices. It allows one to gain insight into basic device physics in a very intuitive fashion compared with more detailed but heavier finite element method. Shortcomings of this model and possible improvements are discussed.

An eigenmode superposition model for lateral acoustic coupling between thin film BAW resonators

A simple model is developed for calculating lateral acoustical coupling between adjacent thin film BAW resonators forming an electrical N-port. The model is based on plate wave dispersion properties of different regions of the device and solution of lateral eigenmodes and eigenfrequencies. Mechanical and electrical response of the device is calculated as a superposition of eigenmodes according to voltage drive at one electrical port at a time while extracting current induced in the other ports, leading to a full Y-parameter description of the device. Comparison of model prediction with experimental results from adjacent large square resonators and narrow finger-like resonators is performed. Large resonators exhibit comb-shaped oscillating transmission characteristics over wide frequency band, while narrow finger create a single pass-band. Fair qualitative agreement is obtained.

Modelling of 2-D lateral modes in solidly-mounted BAW resonators

Lateral resonances occurring in bulk acoustic wave resonators contribute to the device operation, e.g., by creating spurious electrical responses. Predicting lateral effects in two dimensions typically requires computationally heavy 3-D finite element modelling. Here, dispersion characteristics calculated using a 1-D transfer matrix model are used in a finite-element software to model lateral resonance modes of solidly-mounted bulk acoustic wave resonators. Based on the resulting spectrum of 2-D eigenmodes, electrical frequency response as well as mechanical displacement is then calculated using mode superposition. Resonators with elliptic and rectangular shapes are studied. Simulated results for ellipses are compared to measured electrical frequency response and mechanical displacement. Simulations are found to agree with measured data. Dependence of electrical spurious responses on resonator shape and eigenmode spectrum is studied.

Radio-wave beam shaping using holograms

Millimetre-wave radio fields are shaped using both amplitude- and phase-type computer-generated holograms (diffractive elements). Methods for hologram element synthesis are described. Holograms that produce plane waves, radio-wave vortices, and Bessel beams at 310 GHz are fabricated and tested.

Measurement of evanescent wave properties of a bulk acoustic wave resonator [Letters]

Acoustic wave fields in a thin-film bulk acoustic wave resonator are studied using a heterodyne laser interferometer. The measurement area is extended outside the active electrode region of the resonator, so that wave fields in both the active and surrounding regions can be characterized. At frequencies at which the region surrounding the resonator does not support laterally propagating acoustic waves, the analysis of the measurement data shows exponentially decaying amplitude fields outside the active resonator area, as suggested by theory. The magnitude of the imaginary wave vectors is determined by fitting an exponential function to the measured amplitude data, and thereby the experimentally determined dispersion diagram is extended into the region of imaginary wave numbers.