The design of surface acoustic wave bandpass filters which utilize interdigital electrode transducers is reviewed. The impulse-response description of interdigital transducers is extended to allow calculation of transducer input admittance and filter frequency response with much less effort than required by earlier equivalent-circuit model approaches. The application of the impulse model to the straightforward design of VHF and higher frequency bandpass filters is discussed and several examples of high-performance surface-wave bandpass filters are given.

Impulse Model Design of Acoustic Surface-Wave Filters

A sensitive sensor for the measurement of pressure and temperature is created by fabricating a surface-acoustic-wave (SAW) delay path on the surface of a thin diaphragm which, in turn, is in contact with the medium to be measured. The SAW delay path and associated electroacoustic transducers are utilized as the feedback circuit in a miniature electronic oscillator. Since the SAW time delay varies almost linearly with diaphragm temperature and pressure induced strain, the oscillator frequency is also a predominantly linear function of pressure and temperature. A frequency output sensor is desirable both for high-measurement resolution and for compatibility with digital data processors. Sensor configurations are discussed which can be optimized for independent measurement of either temperature or pressure.

Surface-acoustic-wave pressure and temperature sensors

A highly stable and selective SAW-chemosensor has been realized. The sensor operation is based upon the interaction of SAW fields with the simultaneous changes of mass and electrical conductivity of a chemical interface originating from changes in the gas concentra- tion. The sensor system is based upon a dual delay-line oscillator. The difference between the two oscillator frequencies is a measure of the gas concentration. The delay lines are implemented in an STX-quartz substrate. One delay line is provided with the chemical interface ob- tained by vacuum sublimation of an organic semiconductor, metal-free phthalocyanine. The oscillator is stabilized by means of automatic gain control. The oscillator operates without tuning inductors and with symmetrical driving of the transducers, aiming at a (total) monolithic integration of this sensor in a ZnO-SiOz-Si layered structure. A spe- cial feature of the sensor is that the delay lines and the electronic sys- tem are constructed in separate modules. This simplifies the inter- change of different delay lines and allows measurements to be performed when only the delay lines are at an elevated temperature (e.g., environmental monitoring and automobile emission control). The operating frequencies of the sensors are between 30 and 110 MHz. To date one sensor has a sensitivity of about 100 Hz/ppm NOZ at an op- erating frequency of 39.48 MHz with a threshold sensitivity of about 0.5 ppm.

NO 2 Gas-Concentration Measurement with a SAW-Chemosensor

Lead titanate PbTiO3 ceramics with a zero temperature coefficient of surface wave delay time are developed by addition of Nd2O3, In2O3, and MnO2. These ceramics have very small temperature coefficients of less than 1 ppm/ °C over a wide temperature range (−10–+60 °C). They have a large electromechanical coupling factor and low dielectric constant. The propagation loss is 4 dB/cm at 57 MHz. These modified PbTiO3 ceramics have high potential for use in surface wave applications, e.g., filters.

Temperature‐compensated PbTiO3 ceramics for surface acoustic wave applications

The letter discusses various aspects of the design and performance of surface-acoustic-wave oscillators, including two proved solutions to the mode-selection problem and a demonstration of the wide frequency-modulation capability

Surface-acoustic-wave oscillators: mode selection and frequency modulation

Surface acoustic wave delay lines having a vanishing first-order temperature coefficient of delay time near room temperature have been constructed. The devices were operated near 60 MHz and had about 5 µs delay. The surface waves, generated and received using interdigital electrodes, were propagated along the twofold crystallographic axis of ST-cut quartz. The phase delay change as a function of temperature has a parabolic temperature dependence with a second-order coefficient of approximately 31.5 × 10-9/° C2.

M.B. Schulz

Papers

Surface acoustic wave delay lines with small temperature coefficient