Browsing by Subject "Radiation resistance"

Now showing 1 - 6 of 6

Open Access
Bandwidth, power and noise considerations in airborne cMUTs
(IEEE, 2009-09) Şenlik, Muhammed N.; Olcum, Selim; Köymen, Hayrettin; Atalar, Abdullah
Capacitive micromachined ultrasonic transducers (cMUTs) offer wider bandwidth in air due to their low mechanical impedances. The impedance mismatch between the air and transducer decreases with the smaller device dimensions increasing the bandwidth at the expense of the degradation in the transmit power and the receive sensitivity. In this work, the bandwidth of cMUT is optimized by increasing its radiation resistance. This is done by properly choosing the size of cMUT membranes and their placement within an array. This selection not only brings an improvement in the transmitted power when it is used as a transmitter, but also improves the noise figure when it is used as a receiver. A further improvement in the noise figure is possible when the cells are clustered and connected to separate receivers. ©2009 IEEE.
Open Access
Optimizing CMUT geometry for high power
(IEEE, 2010) Yamaner F.Y.; Olcum, Selim; Bozkurt, A.; Köymen, Hayrettin; Atalar, Abdullah
Capacitive micromachined ultrasonic transducers (CMUTs) have demonstratedvarious advantages over piezoelectric transducers. However, current CMUT designsproduce low output pressures with high harmonic distortions. Optimizing thetransducer parameters requires an iterative solution and is too time consumingusing finite element (FEM) modelling tools. In this work, we present a method ofdesigning high output pressure CMUTs with relatively low distortion. We analyzethe behavior of a membrane under high voltage continuous wave operation using anonlinear electrical circuit model. The radiation impedance of an array ofCMUTs is accurately represented using an RLC circuit in the model. The maximummembrane swing without collapse is targeted in the transmit mode. Using SPICEsimulation of the parametric circuit model, we design the CMUT cell withoptimized parameters such as the membrane radius (a), thickness (tm),insulator thickness (ti) and gap height (tg). The modelalso predicts the amount of second harmonic at the output. To verify theaccuracy of the results, we built a FEM model with the same CMUT parameters. Thedesign starts by choosing ti for the given input voltage level.First, a is selected for the maximum radiation resistance of the array at theoperating frequency. Second, tm is found for the resonance at theinput frequency. Third, tg is chosen for the maximum membrane swing.Under this condition, a frequency shift in the resonant frequency occurs. Secondand third steps are repeated until convergence. This method results in a CMUTarray with a high output power and with low distortion. © 2010 IEEE.
Open Access
Radiation impedance of an array of circular capacitive micromachined ultrasonic transducers
(IEEE, 2010) Senlik, M. N.; Olcum, S.; Köymen, Hayrettin; Atalar, Abdullah
The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications.
Open Access
Radiation impedance of an array of circular capacitive micromachined ultrasonic transducers in collapsed state
(IEEE, 2011-10) Özgürlük, Alper; Atalar, Abdullah; Köymen, Hayrettin; Olcum, Selim
Radiation impedance is one of the important parameters in designing efficient and wideband capacitive micro-machined ultrasonic transducer (CMUT) arrays. It determines how much acoustical power is generated in the surrounding medium given the membrane motion. Recently, considerable effort has been put to characterize the radiation impedance of CMUT arrays in conventional uncollapsed regime. However, the radiation impedance of an array of CMUT cells in collapsed state has not yet been investigated. To calculate the array radiation impedance in collapse mode, we first calculate the radiation impedance of a single cell CMUT. For the array case, the mutual impedances between the neighboring cells must also be taken into account.We consider an array of 7, 19, 37, and 61 cells placed in a hexagonal pattern and try to determine the radiation impedance for different degrees of collapse. We find that in the collapsed case the peak radiation resistance value is reached at higher kd values, where k is the wavenumber and d is the center to center cell spacing, compared to the uncollapsed regime.
Open Access
Radiation impedance of collapsed capacitive micromachined ultrasonic transducers
(Institute of Electrical and Electronics Engineers, 2012) Ozgurluk, A.; Atalar, Abdullah; Köymen, Hayrettin; Olçum, S.
The radiation impedance of a capacitive micromachined ultrasonic transducer (CMUT) array is a critical parameter to achieve high performance. In this paper, we present a calculation of the radiation impedance of collapsed, clamped, circular CMUTs both analytically and using finite element method (FEM) simulations. First, we model the radiation impedance of a single collapsed CMUT cell analytically by expressing its velocity profile as a linear combination of special functions for which the generated pressures are known. For an array of collapsed CMUT cells, the mutual impedance between the cells is also taken into account. The radiation impedances for arrays of 7, 19, 37, and 61 circular collapsed CMUT cells for different contact radii are calculated both analytically and by FEM simulations. The radiation resistance of an array reaches a plateau and maintains this level for a wide frequency range. The variation of radiation reactance with respect to frequency indicates an inductance-like behavior in the same frequency range. We find that the peak radiation resistance value is reached at higher kd values in the collapsed case as compared with the uncollapsed case, where k is the wavenumber and d is the center-to-center distance between two neighboring CMUT cells.
Open Access
Stagger tuned cMUT array for wideband airborne applications
(IEEE, 2006-10) Olcum, Selim; Atalar, Abdullah; Köymen Hayrtettin; Şenlik, Muhammed N.
In this study, we explore the limits of cMUTs in air-borne applications. First we investigate the ways of increasing the bandwidth of a single cMUT cell in air. The effect of array operation is also considered in order to increase the radiation resistance seen by the transducer. We calculate the bandwidth of a stagger tuned cMUT array. It is shown in this paper that more than 60% bandwidth can be obtained by three staggered frequencies. © 2006 IEEE.