K.C. Wagner

SAW devices for consumer communication applications

An overview of surface acoustic wave (SAW) filter techniques available for different applications is given. Techniques for TV IF applications are outlined, and typical structures are presented. This is followed by a discussion of applications for SAW resonators. Low-loss devices for mobile communication systems and pager applications are examined. Tapped delay lines (matched filters) and convolvers for code-division multiaccess (CDMA) systems are also covered. Although simulation procedures are not considered, for many devices the theoretical frequency response is presented along with the measurement curve.<<ETX>>

Review of models for low-loss filter design and applications

The most frequently used models for surface acoustic wave (SAW) devices are the impulse model, the equivalent circuit models, the Coupling-of-Modes model, and the matrix models. While the impulse-model is only a first order model the other models include second order effects, e.g. reflections, dispersion, and charge distribution effects. The influence of diffraction and refraction on the transfer function of a SAW filter can be described by the angular spectrum of straight-crested waves model. A survey of these different models will be given. The simulation of low-loss filters requires flexible analysis tools, which can cope with different geometries and substrates. Operating with a parameter set, which depends only on the substrate crystal and not on the specific geometry of the SAW filter, is advantageous. Due to the high insertion attenuation of conventional transversal filters the requirements on the accuracy of the analysis are focused on S21, whereas for low-loss filters all elements of the S-matrix are important. The comparison of simulations with a P-matrix model, which fulfills the above mentioned prerequisites, and measurements of different types of low-loss filters, e.g. SPUDT, DMS, and transverse-mode coupled resonator filters are presented

Optimum design of low time-bandwidth product SAW filters

For years linear optimization algorithms have been used successfully in bandpass filter design. This method has been adopted for the design of pulse compression filters with low time-bandwidth products. In the bandpass filter design, the sidelobe level in the frequency domain is optimized for a fixed impulse response length. In case of the design of a low time-bandwidth product filters, we optimize the sidelobe suppression in the time domain for a fixed overall bandwidth. We get the optimum time response for a given frequency bandwidth and width of the compressed pulse. The spurious time response is minimized. The corresponding optimum frequency response is split up onto expander and compressor, each consisting of two chirped and apodized IDTs. As an example, an expander-compressor pair at 350 MHz with a 3 dB bandwidth of approximately 11 MHz, an overall bandwidth of approximately 33 MHz, and a chirp time of 1.0 μs was designed. The sidelobe suppression in the design is better than 40 dB. The measured filters exhibit a sidelobe level close to -40 dB. Design and measurement are in excellent agreement

Advanced numerical methods for the simulation of SAW devices

The continuing improvement of the computer models for parasitics and second-order effects and their straight-forward implementation often leads to a dramatic increase in the complexity of the simulation code and a corresponding increase in the run-time of the programs, which may easily surpass the speed-up provided by the ongoing technical advance in computer hardware. When the refined computer models are to be used on a daily basis in the iterative design processes of SAW devices, special attention has to be paid to the selection of the numerical algorithms and their proper implementation in order to find a good compromise between acceptable program run time and the necessary accuracy. Several areas symptomatic for the impact of numerical methods on the simulation of SAW devices are addressed in this paper. We look at improving the numerical accuracy in the calculation of the electrostatic Greens function, and we investigate how the underlying tapped-delay-line structure of all SAW devices can be used to speed up the cascading of P-matrix unit cells and help to efficiently provide IDT parameters during diffraction simulation. We also address the efficient solution of large systems of linear equations that typically arise when equations have to be solved numerically. The high rank and/or the complicated structure may prevent a direct solution of these equations. We show how an iterative technique may be successfully used under such circumstances.

Two-track-reflector-filters for CDMA mobile telephones

IF-filters for channel selection in mobile phones based on a CDMA system must offer very steep skirts as well as excellent phase linearity and small amplitude ripple within the passband. To fulfill all these requirements a conventional transversal surface acoustic wave (SAW) filter needs a large chip size. In order to reduce the total chip length we folded the propagation path of the SAW taking advantage of the reflector filter technique. Therefore, the usable time window for weighting becomes almost twice as large. We employed weighted reflectors not only to reflect the SAW but also to shape the filter frequency response. Thus we have more flexibility to design the transducers. We can make them shorter but still have lower impedances on quartz than with an unweighted reflector design. With this technique, we built filters at a center frequency of 110 MHz with an insertion attenuation of less than 15 dB, a 5 dB bandwidth of 1.26 MHz and an attenuation of more than 30 dB outside of a 1.80 MHz band. The chip is mounted in a 15.6/spl times/6.8 mm/sup 2/ SMD package. The reflector filter fulfills the frequency response specification required for CDMA mobile telephones on a chip length reduced by about 4 mm compared with the transversal or SPUDT design technique.

Energy distribution in a quartz resonator

We present a theoretical analysis of the acoustical energy distribution in a surface acoustic wave (SAW) test resonator on the 34/spl deg/ cut of quartz. To calculate the energy density at each point for a known voltage, we chose a combination of three different methods: a P-Matrix algorithm for the longitudinal direction, a Stack-Matrix formalism across the aperture of the resonator and a Partial Wave Method normal to the substrate surface. We determined the distribution of energy in all three dimensions and located the peak stress at the metallization-vacuum interface.

MSK SAW tapped delay lines on LiTaO/sub 3/ with moderate processing gain for CDMA indoor and mobile radio applications

We report on the design and performance of two SAM MSK Tapped Delay Lines (TDLs) using PN code sequences of length 128 chips, As a substrate, LiTaO/sub 3/ has been used due to the system requirements of a given CDMA/TDMA system. Center frequency, bandwidth of the major lobe, and integration time were 266.66 MHz, 65.7 MHz, and 3 /spl mu/s, respectively. TDL 1 employed two non-weighted transducers, whereas TDL 2 incorporated one cosine-apodized transducer. Split-fingers with a metalization height of 150 nm (aluminum) were used. The experimental data of both design approaches were very similar with an unmatched insertion loss of 37 dB which is improved by more than 18 dB by proper matching. Amplitude ripple was less than 1 and 2 db, respectively, and the close-in selectivity was 23 dB.

Analysis of obliquely propagating SAWs in periodic arrays of metal strips

In many surface acoustic wave (SAW) filters, the diffraction of the surface wave occurring in the interdigital transducers (IDTs) is as important as the diffraction on the free surface. The accurate simulation of diffraction effects requires knowledge of the slowness curves for waves propagating in the IDTs. The transducer is modeled as a periodic array of massless, infinitely long metal strips. The task is the determination of the phase velocity of the straight crested wave, which propagates with an arbitrary propagation angle in this array. The method of solution employed is valid for all frequencies, including the stopbands, and allows the determination of stopband frequencies for obliquely propagating waves. The substrate material is characterized by the numerically computed effective permittivity. Examples of slowness diagrams as well as dispersion diagrams are presented. The results of SAW device simulations using models for the SAW slowness curves are compared with each other and with measurements.<<ETX>>

Bulk wave radiation by group-type-unidirectional-transducers

In the design of broadband low-loss group-type unidirectional transducers (GUDTs) the effect of bulk wave radiation must be considered. It influences the accurate design of the phasing network for the GUDT and increases the insertion loss. In addition, the zeros of the frequency response become less pronounced. The influence of the electrode structure of the GUDT on the cutoff frequency of bulk wave radiation is studied. The ratio of bulk wave radiation to surface wave radiation is calculated for a uniform interdigital transducer (IDT), as well as for an electrode structure of a GUDT on LiNbO/sub 3/-128rotYX, LiNbO/sub 3/-YZ, and quartz-STX. The effect of bulk wave radiation on the zeros of the frequency response is shown for several adjustments of the phasing network.<<ETX>>

Imaging of the wave field of surface acoustic wave devices

An optical measurement technique for the phase preserving imaging of elastical wave fields of surface acoustic wave devices was developed. This technique uses short pulses from a mode-locked picosecond laser, harmonic mixing, and coherent detection to achieve a measurement bandwidth beyond 1 GHz. It is therefore well suited to perform measurements on surface acoustic wave devices of even the highest frequencies where modeling becomes increasingly difficult. The detection capability for surface displacements of 1 pm/ square root Hz allows a minimum dynamic range of about 40 dB for the determination of amplitude profiles of surface acoustic wave devices. As an example of a practical application, results of measurements on a bandpass filter are presented. The generated wave profiles have been determined at several different distances from the transducer both in the passband and in the stopband of the device. The measurements are in excellent agreement with computer simulations obtained with an angular spectrum of waves model.<<ETX>>