Svetlana Malocha

Electrical parasitic modeling in SAW RF filters

Parasitic phenomena affecting the frequency response of SAW RF filters are modeled with a variety of techniques. Inter-IDT electrode capacitance is modeled with a two-dimensional charge distribution analysis. Capacitance among arbitrary die structures, including bus bars and bond pads, is modeled with an electrostatic boundary element formulation. Inductance and resistance of these structures are modeled using a filament mesh analysis technique under the magnetoquasistatic approximation.

A zero TCF band 13 SAW duplexer

RF spectrum is becoming a very sparse resource and several systems have to coexist while separated by small frequency gaps. For example, public safety band NS07 is 2.5 MHz below the LTE band 13 TX band. Standard SAW and typical Temperature Compensated SAW (TC-SAW) filters do not provide enough rejection in this band, especially within the required operating temperature range. The only way, when not using an NS07 compliant duplexer, to avoid noise radiation into the public safety band is to reduce their transmitted power when the public safety network is activated. To be compliant with NS07, the band 13 TX filter needs to have a very steep transition. Therefore, high quality factors as well as low temperature coefficient of frequency (TCF) are required. To reduce the TCF, the current standard approach is to bury the electrodes inside a silicon oxide overlay. Since silicon oxide has a positive TCF while piezoelectric substrates have negative TCFs, compensation is possible. Usually, TC-SAW filters on lithium niobate substrates have a TCF in the range of -20 to -25 ppm/°C. This corresponds to a frequency shift of 2 MHz at 800 MHz for a temperature range of 100°C. This is too large for applications requiring steep transitions bands. A band 13 duplexer meeting the NS07 rejection requirement is presented. The substrate utilized is lithium niobate with an orientation close to Y+128°. The oxide thickness is increased enough to obtain full temperature compensation. Piston mode transducers are used in order to suppress the transverse modes and to reduce the losses. By increasing the silicon oxide thickness, it is possible to reduce TCF toward some targeted ppm/degC value. The drawback is a reduction by about 40% of the coupling factor and thus, of the possible relative bandwidth. However, band 13 is narrow enough to be compatible with this approach. The resulting temperature compensation produces a frequency shift smaller than 200 kHz p.p. within the 100°C temperature range. Quality factors in the 1500 range are obtained. Typical results are a rejection of 25dB in the NS07 band within the temperature range with an insertion loss better than 3dB. It is understood that this duplexer is the first band 13 SAW duplexer meeting the NS07 rejection specification.

COM Model Characterization for RF SAW Devices

The coupling-of-modes (COM) model was applied to the analysis of surface acoustic wave (SAW) devices in the 1980s. At that time, the COM parameters were calculated using perturbation techniques or extracted from experimental data. With the improvements in computational resources it became possible to characterize the COM models parameters using this robust harmonic admittance using FEM/BEM. The topic of this paper is on a method to accomplish the COM parameter characterization for RF type SAW, leaky SAW (LSAW), and buried interdigital transducer (BIDT) SAW devices.

String weighted tapered SAW transducer analysis

A COM-based analysis approach for the devices with string weighted tapered transducers is presented. The analysis takes into account specific electrical connections inside transducers and the resulting charge distribution on their electrodes. The details of calculating COM parameters are discussed. A good agreement between experimental and calculated results for several devices built on lithium niobate is shown.

COM analysis for LSAW filters

This paper presents a modified COM - based analysis for LSAW ladder and coupled resonator filters. The acoustic sources between the adjacent elements (transducers and gratings) are taken into account by using a gap-type cell instead of a traditional electrode-type cell. Mutual capacitances between the elements are calculated from an electrostatic analysis. The Y-matrix of the device is obtained by a combination of P-matrix cascading and the circuit analysis. The presented analysis is applied to the devices with chirped and apodized elements. A good agreement between calculated and experimental results is demonstrated.

Automated COM parameter extraction for SiO 2 /LiNbO 3 and SiO 2 /LiTaO 3 substrates

In this paper automated COM parameter extraction approach is described. COM parameters (velocity, reflectivity, coupling, propagation loss) are extracted from measurements of a set of test structures. Measurement data are validated prior to the parameter extraction. Frequency dependence of the velocity and propagation loss is taken into account. The method is applied to both SAW and LSAW substrates. Good agreement between measured device responses, and responses simulated using extracted COM parameters is shown for several examples of the devices built on SiO2/LiNbO3 and SiO2/LiTaO3 substrates.

Wide band tapered SAW filters with improved shape factor and phase response

The design technique of IF filters based on tapered SPUDT technology is widely used for new picocell base stations market. The performance of these SAW filters (insertion loss, bandwidth and transition band) depends greatly on the coupling of the substrate, SPUDT cell parameters, and on the tapered transducer design (transduction and reflectivity weighting, weighting implementation and transducer periodicity as a function of position across the aperture). The electrode widths in these structures vary significantly both across the aperture and in wave propagation direction. This poses an additional challenge to the fabrication process. The problem is magnified by increasingly stringent specifications for base station filters, including wider bandwidths and higher selectivity. In the present work the effects of the fabrication process used in manufacturing of these devices was considered. A discrepancy was found between simulated and measured device responses due to the deviation of fabricated electrode width from that specified by the transducer design. The value of this deviation was found to be a function of the electrode width and local periodicity. By correcting this deviation in the layout, the agreement between the simulation and measurements was improved. Results of a 400 MHz with 17% bandwidth tapered SPUDT SAW filter with and without the layout correction are presented in this paper.

Improved analysis for string and block weighted tapered SAW transducers

An improved COM analysis for string and block weighted tapered transducers is presented. The shortcomings of the previously reported analysis [1] are discussed and the remedies are proposed. The improvement is made in COM parameter calculation. In particular, the charge distribution on transducer electrodes is calculated more accurately due to the changes made in the electrostatic problem formulation and solution. As a result, a better accuracy of the transduction and capacitance calculation is achieved. It is shown that the assumptions made in [1] when calculating potential on the electrodes connected to floating busbars can increase the discrepancy between simulated and measured responses. The details of the electrostatic problem formulation and solution are presented. Experimental data are shown for the devices built on LiNbO3, which demonstrate improved agreement between calculated and measured responses.

Effect of surface-bulk hybrid mode on strip admittance in 42/spl deg/YX and 48/spl deg/YX cuts of LiTaO/sub 3/ with Al grating

We report on a study of the effect produced by the surface-bulk hybrid (SBH) mode on the performance of resonators built on 42/spl deg/YX and 48/spl deg/YX cuts of LiTaO/sub 3/(LT). An approximation was suggested for SBH mode effect on harmonic admittance, and the main parameters of this mode were extracted from numerically found admittance, as functions of electrode thickness, metalization ratio and detuning factor. In both orientations, effective electromechanical coupling of SBH mode reduces with electrode thickness and increases with metalization ratio. Maximum coupling occurs when the detuning from synchronous resonance is about (0.4-0.6)%. In 48/spl deg/YX-LT, SBH mode is much stronger attenuated than in 42/spl deg/YX-LT and its effect on strip admittance is less apparent. The simulated behavior of SBH mode shows good agreement with experimental data.