Bruce R. McAvoy

Low- and high-temperature superconducting microwave filters

Stripline and microstrip filters at X-band were designed and fabricated using low- and high-temperature superconductors in quarter-wave, parallel-coupled section configurations. Low-temperature superconducting niobium thin films, deposited on single-crystal sapphire, were used to build to six-pole stripline filters with adjacent passbands and approximately 3 dB crossovers and 1.2% bandwidth. Four- and six-pole microstrip filters were made with in situ epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films on LaAlO/sub 3/ substrates. All the YBCO filters showed 77 K passbands with clean skirts and high out-of-band rejection. The six-pole filters had adjacent passbands with -28 dB crossovers and 1.5% bandwidth. >

Superconducting stripline resonator performance

Reliable techniques for evaluating the microwave properties of superconductors are essential in providing calibrated data for exchange between laboratories and for developing practical device designs. The authors are examining the techniques which utilize microwave stripline resonators. These resonators provide for the rapid measurement of microwave parameters in a repeatable fashion with minimal constraints on processing. Sandwiched microstrip line resonators are used to compare the performance at 4.2 K of OFHC copper and superconducting films of Pb, Nb, and YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) at C-band and X-band. Typical results for the Nb resonators show a loaded Q/sub L/ of about 8*10/sup 4/ with a transmission insertion loss of 5 dB at 3 GHz. Initial results on a YBCO a-axis film used as a ground plane in the Nb resonator yield a surface resistance value of about 10/sup -3/ Omega at 2.8 GHz. Preliminary results on the phase noise performance of a Nb resonator at 2.9 GHz are presented. >

Microwave superconducting filters

Four-pole, parallel-coupled microstrip line filters were fabricated using epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films deposited by off-axis sputtering on LaAlO/sub 3/ substrates. Filters were made with and without superconducting ground planes. Because it is not yet possible to grow in-situ epitaxial films on both sides of a substrate, two separate substrates were used, one for the filter and one for the ground-plane. The devices were packaged so that the microstrip-to-ground-plane distance was the thickness of one of the substrates (0.43 mm). Passband characteristics, input power, and temperature dependence were measured, as well as the thermal cycling stability.

Towards a miniature laser-pumped cesium cell frequency standard

With the goal of minimizing the overall size of a gas cell frequency standard the authors have characterized cylindrical Cs cells with inner dimensions R=2 mm and L=18 mm. Under conditions of present interest the lifetime in the absence of a tuned microwave field tends to be dominated by Cs-Cs collisions. Wall collisions have only a marginal effect on the lifetime, provided appropriate conditions on the N/sub 2//Ar buffer gas pressure, are satisfied. Using a Te/sub 101/ mode rectangular cavity the microwave resonance lineshape signal parameters were measured over a range of optical and microwave powers at various cell temperatures and used to predict the achievable short term stability. For the optimum combination of optical and microwave powers the linewidth is dominated by power broadening, and the short term stability, sigma ( tau ), predicted from these measurements decreases from sigma ( tau ) tau /sup 1/2/=6.4*10/sup -12/ to 4.2*10/sup -12/, where tau is in seconds, as the cell cold spot temperature is raised from 50 degrees C to 65 degrees C.<<ETX>>

Magnetically tuned high overtone bulk acoustic resonator

Electroacoustic resonating apparatus capable of generating resonances of tunable frequencies. The apparatus includes two transducers coupled to a ferrimagnetic substrate at opposite ends thereof. The first transducer converts an input signal into a signal that produces bulk acoustic waves in the ferrimagnetic substrate. The second transducer receives the bulk acoustic waves and converts them into electrical signals, or alternatively a single transducer which performs both input and output functions. Variable magnetizing means located proximate to the ferromagnetic substrate induces a magnetic field throughout the substrate wherein the magnetic field alters the frequencies of the resonance of the acoustic waves. By varying the magnitude of the magnetic field, the frequencies of the resonances may be varied, thereby allowing tuning of the apparatus.

Evaluating superconducting resonator materials

Techniques for microwave evaluation of superconductors using microwave stripline resonators are examined. These resonators provide rapid measurement of microwave parameters with minimal constraints on processing. Sandwiched microstrip line resonators are used to compare the performance at 4.2 K of oxygen-free copper, lead films, niobium films and ceramic high-T/sub c/ films. Measurement at C-band (2.8 GHz) and X-band (7.6 GHz) are described. Typical results for a niobium film microstrip resonator show a loaded Q value of about 80000 with a transmission insertion loss of 5 dB at 3 GHz. Such high-Q microwave resonators have application in low-phase-noise reference oscillators. Preliminary results on the phase-noise performance of a Nb stripline resonator at 2.8 GHz are presented.<<ETX>>