R.C. Bray

Direct Phase Noise Measurements of SAW Resonators

The most significant contribution of SAW resonators is in the frequency control of low phase noise oscillators at UHF frequencies. Techniques for measuring the phase noise of oscillators are well known, but the contribution to that phase noise of the frequency determining element is usually estimated or found by indirect methods. We present a method for directly measuring the phase noise in a SAW resonator (or any other linear circuit element) at or near its resonant frequency. The correlation between the phase noise measured in a SAW resonator by this method and that measured in an oscillator containing this device is excellent. The single-sideband noise spectrum contributed by SAW resonators is found to follow a l/fy trend away from the measurement frequency, where Y is close to 1.

High-speed optical modulator for application in instrumentation

The design and characterization of the first integrated optic modulator for a commercial instrument application are described. Alternative forms of modulation, including direct laser current modulation and optical heterodyne techniques, are reviewed for the application, which requires modulation to 20 GHz. The advantages of an integrated optic modulator fabricated using titanium in-diffused lithium niobate are discussed. Performance tradeoffs between x-cut and z-cut LiNbO/sub 3/ including bandwidth, modulation efficiency, bias point stability, and acoustic resonances are detailed. The x-cut device is found to give the best overall performance for this instrument application. Device reliability and testing, including temperature cycling, humidity, electrostatic discharge, mechanical shock, and vibration are described for complete packaged and pigtailed devices. >

Surface transverse wave resonator

A surface transverse wave resonator comprises a piezoelectric substrate having at least one surface, wherein said substrate includes a crystalline structure that permits the propagation of transverse waves. The resonator also comprises a transducing set of periodic perturbations which are positioned atop the substrate surface, wherein the transducing set converts electromagnetic energy from an external source into the transverse waves. The resonator further comprises a receiving set of periodic perturbations which are positioned atop the substrate surface remote of the transducing set of periodic perturbations, wherein the receiving set converts the transverse waves into resultant electromagnetic energy. Moreover, the resonator includes a central set of surface-trapping periodic perturbations for maintaining the transverse waves adjacent to the substrate surface, wherein the central set is positioned intermediate the transducing set and the receiving set. Further, the resonator includes at least two sets of reflecting periodic perturbations for reflecting the transverse waves, wherein each of the sets of reflecting periodic perturbations is positioned adjacent to a respective one of the transducing set and the receiving set remote of the central set.

Annealing Behavior and Phase Noise Performance of SAW Resonators

Mechanical and structural aspects of SAW device transducer metallization were investigated in a series of experiments. The center frequency of a SAW resonator on ST Quartz was found to increase by up to several hundred ppm during heat treatment. This behavior was studied as a function of transducer metal thickness and device center frequency for a range of annealing times. Alternative explanations of the phenomenon in terms of the change in the properties of the substrate during the anneal were considered. Resistivity of A1 and CuAl films were characterized as a function of thickness and annealing time. Finally, a correlation is found between SAW device phase noise and structural details of the transducer thin films.

Polyimide as an acoustic absorber for high frequency SAW applications

A novel method is described for damping unwanted acoustic energy in high-frequency SAW (surface acoustic wave) devices through the use of a polyimide layer which is patternable with high resolution on the surface of the device. The polyimide layer maintains its absorptive acoustic properties after a high-temperature bakeout at 300 degrees C and exhibits excellent reliability. The application process and process tolerances are described. Experimental results for the acoustic attenuation factor are presented as a function of SAW frequency. The attenuation of 3.0-3.5- mu m-thick polyimide films for SAWs on LiNbO/sub 2/ was measured to be in the range of 10-20 dB/100 mu m at frequencies in the range of 300 MHz to 500 MHz.<<ETX>>

The Development of a Surface Mount Hybrid if Section Using Low Loss SAW Filters

A thick film hybrid circuit incorporating two low loss group-type unidirectional transducer (GUDT) SAW filters was developed for use in a receiver IF section centered at 310.7 MHz. The two filters were cascaded with a preamp and a buffer amplifier on one hybrid to achieve IF gain and filtering. The low loss GUDT SAW filter was designed using a Hamming apodization. Performance, assembly and reliability were enhanced by taking a surface mount hybrid approach. A leadless chip carrier package for the SAW device was designed and optimized for low loss and high out of band rejection. Surface mount chip inductors were evaluated and optimized for match and insertion loss. A high temperature hermetic seal process was developed for the custom SAW package and extensive environmental tests were performed on both the packaged SAW and the fully assembled hybrid to insure a high degree of reliability. Packaged filter losses were less than 6.5 dB per filter. Rejection at the image frequency for the hybrid IF section exceeded 90 dBc. Typical peak-to-peak passband ripple was .2 dB.

Delay Dependence of Phase Noise in SAW Filters

Measurements of phase noise in a number of lithium niobate and ST-quartz delay lines have been performed. Our observations indicate that noise in these devices is produced in the transducer region as well as in the delay path. Linear and quadratic models for the delay dependence are proposed and compared to the experimental data. With the present data we are unable to differentiate between these models. The use of an acoustic absorber on the ends of the device is shown to reduce phase noise between -5 dBc/Hz and -10 dBc/Hz. We suggest that any acoustic scattering process, from the saw cut at the end of the device or from crystal imperfections in the delay path, may contribute to the total device noise.

SAW Filters Using Group-Type Unidirectional Transducers: Sources of Problems

Low-loss low ripple SAW filters using Group-type unidirectional transducers (GUDTs) on 128O-rotated Y-X LiNb03 were fabricated and tested at 308 MHz. Comparisons were made for filter configurations consisting of GUDTs and multistrip couplers. The synthesis was done using an impulse model which computed the optimum matching and phase shifting networks required for GUDTs. Both air- wound and thin-film planar coils were used and their performances compared. The performance of the filters was measured as a function of temperature. Sources of losses, such as meander line resistive and reactive losses, were analyzed. Time domain reflectometry (TDR) measurements were made to investigate the sources of internal reflections and ripples for various GUDTs using different apodization patterns.

Development of an ultra-flat SAW filter module and its application to FASS: a frequency agile signal source

The development of a fast, agile signal in the frequency range of 10 MHz to 3 GHz based on direct digital synthesis of signals from 14 to 58 MHz is described. A high-performance SAW (surface acoustic wave) filter module was designed to provide IF (intermediate frequency) filtering after the baseband signal was converted to a center frequency of 304 MHz. The 44-MHz-wide filter module showed typical passband ripple less than 0.4 dB peak-to-peak. Triple transit, electromagnetic feedthrough, and other spurious responses were rejected by at least 50 dB. The circuit, the SAW filter, and its package were designed with careful attention to parasitics in order to achieve this performance. The filtering was accomplished by two identical SAW filters on 128 degrees Y-rotated, X LiNbO/sub 3/ operated in cascade, with isolation amplifiers buffering each filter. The isolation amplifiers were designed to provide a low-impedance, balanced termination to each IDT (interdigital transducer), minimizing triple transit and EM feedthrough. The SAW filter was designed using the Remez exchange algorithm and optimized to compensate for and reduce second-order effects. An absorbing layer of polyimide was an important factor in assuring repeatably low spurious time-domain responses from the chip edges, while being compatible with 300 degrees C package sealing. An on-wafer test hardware and software system was developed to predict packaged-device ripple performance and a precision test fixture with two-port calibration standards was developed to test feedthrough and spurious time-domain responses in the same environment as the final circuit.<<ETX>>