Andreas Bergmann

Low loss recursive filters for basestation applications without spurious modes

In SAW filters spurious waveguide modes may seriously affect the performance: Undesired lobes in the upper pass- or stopband and ripples in the group delay are typical for devices suffering from waveguiding effects. In order to suppress spurious modes and to excite only the fundamental symmetric mode various approaches as overlap weighting or sawtooth shaping of the busbars in order to destroy the waveguide have been applied. Alternatively recently it was suggested to employ the first antisymmetric mode by splitting the acoustic track into two subtracks, which are longitudinally shifted by half a wavelength with respect to each other. The very reason of the excitation of higher modes is, however, the fact that the excitation profile is of different shape than the fundamental symmetric or antisymmetric mode. In most cases the excitation profile is of rectangular shape while the waveguide modes usually are of sinusoidal shape. While the approaches of prior arts aimed at changing the excitation profile to maximize excitation of the fundamental symmetric or antisymmetric mode we suggest an approach where the waveguide is changed in order to shape the fundamental symmetric mode rectangularly. The approach is applied to low loss recursive inline filters which are employed as intermediate frequency filters in GSM infrastructure systems. The method proves to be particulary successful in reducing group delay ripple and sidelobes. I. INTRODUCTION Spurious modes are a serious problem in SAW filters. They may lead to ripples in the passband, distortions of the group delay time and reduced out of band suppression.

Simulation of waveguiding in SAW devices on substrates with anisotropic slowness and excitation

The 2D P-matrix simulation method is extended to deal with SAW substrates exhibiting a strong anisotropy in slowness and excitation such as YZ-cut lithium niobate.In these substrates the parabolic approximation of the slowness curve is in general not valid. Instead, the exact form of the slowness as obtained from solutions of the basic wave equations of the piezoelectric half space needs to be employed. This has two major implications: Firstly, the imaginary branches cannot be simply determined as the complex roots of a parabolic equation. Secondly, the resulting transversal modes in general are not orthogonal as in the parabolic case. We determined the imaginary branches by analytical continuation of the exact slowness curve. The problem of non-orthonormal basis functions was dealt with by a projection technique including bounded as well as continuum modes. Besides the general anisotropy of the slowness also the anisotropy of excitation is taken into account. Thereto a convolution of the angle dependent excitation characteristic with the transversal charge distribution is determined. The method is verified at the example of very narrow resonators on YZ-cut lithium niobate, which exhibit strong diffraction effects. With respect to the parabolic approximation a significant improvement of simulation quality is observed.

Low resistance quartz resonators for automotive applications without spurious modes

The performance of one-port resonators is often affected by spurious transverse modes which, in particular around the fundamental antiresonance, are troublesome. We demonstrate that by employing the fundamental antisymmetric instead of the fundamental symmetric waveguide mode to carry the main acoustic resonance, a suppression of these spurious resonances is obtained. Since the approach requires no additional space, it is particularly useful in applications where the space on the device is very scarce. As an example, a one-port resonator on quartz supporting 8 transverse modes is investigated, The resonator exhibits undesired resonances just above the fundamental antiresonance. The recently presented 2D P-matrix simulator (Pocksteiner, N. et al., IEEE Ultrason. Symp. p.137-41, 2000) accurately describes this impedance characteristic. Employing the simulator, a suitable antisymmetric resonator configuration was derived which efficiently suppresses the spurious resonances. Insight into the role the transverse modes play in forming the resonators impedance characteristic is obtained by a decomposition of the resonator function into the contributions of the single transverse modes.

A powerful novel method for the simulation of waveguiding in SAW devices

A new method to simulate waveguiding in SAW filters is presented and applied to mobile communication and consumer electronics filters. It is based on the P-matrix method and discretizes the filter into longitudinal and transversal sections. Diffraction effects are introduced by considering free waveguide propagation within the longitudinal sections. This method is an extension of the pseudoinverse diffraction matrix method by Rooth et al. In contrast to the transmission matrix form of that scheme the presented method uses an intrinsically reciprocal 2D scattering matrix description. It is shown that the dispersion relations of the method are equivalent to those obtained by a 2D COM approach and hence both methods are equivalent. The scheme has proved to predict 2D effects in a wide range of filter types reliably. This is exemplified for recursive filters, as well as filters with apodized and fan-type IDTs built on quartz and lithium niobate. In order to better understand the filter behaviour visualizations of the wavefields are presented.

High selectivity SAW duplexer for W-CDMA Band VIII

Special requirements for Band VIII Wideband CDMA (W-CDMA) duplexers are the relatively large bandwidths of 3.9% for transmitting (TX) and 3.7% for receiving (RX) filters as well as the low duplex distance of 1.1%. Moreover, compared to systems at higher frequencies, for 1 GHz systems increased focus is put on miniaturization. In order to achieve good RX suppression within the TX band, the RX filter is comprised of one port resonators as well as a DMS track whereas the TX filter is a pure ladder type filter. The center frequencies of these building blocks show different dependencies with respect to metallization ratio and layer thickness. The consideration of this relationship within the design process was necessary to achieve a very low shift of the right skirt of the TX filter relative to the left skirt of the RX filter. Our duplexer is realized in a 3.0x2.5 mm2 package on LTCC ceramic and offers a suppression of the W-CDMA RX-signal of 50 dB within the TX band. Typical values for attenuation of the W-CDMA signal in the transmission band are 1.7 dB for the TX path and 2.0 dB for the RX path. The comparison between simulation and measurement shows the applicability of the design concept which is the most appropriate one for duplexers with small duplex distance and good RX suppression within the TX band.

6B-3 Low Loss IF Filters for Multimedia Applications - New Limits in Recursive Filter Technology

Presently IF filters for multimedia applications are mainly high loss transversal filters. For upcoming mobile applications a reduction of insertion loss will be highly welcome. We present a new class of recursive filters with an overall performance comparable to conventional transversal filters but much lower insertion attenuation. To substitute conventional transversal filters we had to realize recursive filters with a bandwidth significantly larger than known up to now. The very low center frequencies and thus large wavelengths usual for multimedia IF filters lead to a low relative filter length. SPUDT cells with very high reflectivity were required to suppress triple transit signal and a much larger set of different cells than usual was necessary to reduce discretization errors in withdrawal weighting. As an example we present a 36 MHz filter with a length of only 69 wavelengths, a bandwidth of 8 MHz and 40 dB selectivity. The insertion attenuation is about 16 dB lower than that of a comparable conventional transversal filter. Group delay ripple is below 80 ns and passband ripple below 0.8 dB

High selectivity IF SAW filters for CDMA mobile phones

The Z-path structure is a good choice for intermediate frequency SAW filters in highly miniaturized mobile phones. In order to achieve the dramatic size reduction of up to 70% compared to inline structures, the Z-path concept has been combined with the recursive filter design technique resulting in a recursive Z-path filter. One of the advantages of this structure is the multiple mechanisms which can be exploited to shape the passband and to achieve high selectivity. However, at frequencies above 100 MHz the selectivity of a Z-path filter on a chip of minimum size is affected by inductive as well as capacitive crosstalk between the input and the output transducer. To meet the rejection characteristics required in modern telecommunication systems, a novel method to improve the stopband performance of Z-path filters was developed. It allows for an independent cancellation of both the inductive and the capacitive crosstalk by balancing the net current flow to a transducer and by equalizing the effective coupling capacitance from the input to the output transducer. The finger-electrodes themselves remain unchanged and thus the electro-acoustical behaviour is not affected. CDMA-filters having a selectivity of more than 65 dB are presented. The new technique is applied to filters for the CDMA-800 system at 128.1 MHz on quartz in a 13/spl times/6.5 mm/sup 2/ ceramic package. A CDMA-1900 filter at 210.38 MHz in a 5/spl times/5 mm/sup 2/ ceramic package is also presented.

6D-5 A 2D P-Matrix Model for the Simulation of Waveguiding and Diffraction in SAW Components (Invited)

The 2D P-matrix model has proven to predict 2D propagation effects in a wide spectrum of SAW components for different applications reliably. With reasonable computational effort, a very accurate simulation of the electrical characteristics of SAW devices is achievable. After an overview of different possible approaches proposed in the literature for the inclusion of 2D effects into device simulation, the method implemented and used by Epcos is described in some detail. Comparison to other methods, especially the 2D COM model is made. The modeling of the coupling of forward and backward propagating modes in reflective gratings, which is crucial for obtaining high quality results, is highlighted. A wide variety of different application examples (inline filters using SPUDTs, resonators, filters with slanted finger transducers) will demonstrate that the 2D P-matrix model not only gives reliable predictions of electrical characteristics, but lends detailed insight into the physics of a device under consideration. Thus, it sets up a basis allowing systematic approaches for the suppression of transversal modes