John D. Adam

Magnetostatic wave signal-to-noise-enhancer

A magnetostatic wave device having a microstrip transmission line in conjunction with a YIG thin film biased for magnetostatic wave operation at a level where relatively lower power input microwave signals are highly attenuated up to a critical point past which the relative attenuation is greatly reduced. Such devices may be utilized as signal-to-noise enhancers in microwave circuits such as frequency memory loops.

Inherent linearity in carbon nanotube field-effect transistors

The authors consider the suitability of carbon nanotubes for use in analog rf amplifiers, where the linearity of the device is critical. They show that in the limit of large electrostatic gate-channel capacitance, their theory predicts that an Ohmically contacted, ballistic carbon-nanotube-based field-effect transistor is inherently linear. While they have not achieved this limit in their experimental work, they compare the theory to experiment in the limit of small electrostatic gate-channel capacitance and find excellent agreement at virtually all bias conditions.

Carbon nanotube field-effect transistor operation at microwave frequencies

A top-gated carbon nanotube (CNT) field-effect transistor (FET) was fabricated on a quartz substrate. We used a novel measurement approach and demonstrated for the first time frequency-independent performance of a CNT FET for frequencies as high as 23GHz. This observed maximum operating frequency represents a significant breakthrough in the realization of carbon nanotube-based electronics for high frequency applications.

Thick yttrium-iron-garnet (YIG) films produced by pulsed laser deposition (PLD) for integration applications

High magnetic and dielectric quality, thick (50-100 /spl mu/m), epitaxial, yttrium-iron-garnet (YIG) films were deposited at high rate by PLD. A two-step (low temperature deposition followed by rapid thermal anneal) low thermal budget PLD process was demonstrated suitable to deposit thick polycrystalline YIG films on metallized Si and GaAs. A modified PLD apparatus is used to deposit uniform, 80-100 /spl mu/m, thick YIG films on 3-inch semiconductor wafers for integrated microwave circulator fabrication.

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. >

A broadband microwave signal to noise enhancer

A signal to noise enhancer exhibits a high attenuation to small noise level signals and a lower attenuation to larger above threshold signals. The devices described rely on magnetostatic waves as saturable absorbers of power from a microwave transmission line, typically microstrip. Experiments performed with epitaxial YIG show that instantaneous bandwidths of 900 MHz with threshold levels of less than 0 dBm can be obtained below 4.2 GHz. Enhancement of greater than 20 dB can be achieved for an increase in power level of 20 dB above threshold. Characteristics obtained with lithium ferrite in the 4-8 GHz band are comparable to those obtained with YIG in the 2-4 GHz range. Owing to the increased fmr and spin wave linewidth of the lithium ferrite, the threshold level is higher, + 6 dBm, and the frequency selectivity poorer than observed with YIG.

Magnetostatic forward volume wave propagation in YIG strips

Theory and measurements on magnetostatic forward volume wave (MFVW) propagation in YIG strips are described. Calculated cut-off frequencies and variation of delay with frequency for the MFVW width modes are in good agreement with measurements. An expression for the radiation resistance for MFVW transduction in YIG samples of finite width is derived which exhibits a pole at the low frequency cut-off of the mode. Results show that width modes are an important source of amplitude and delay ripple in MFVW delay lines.

The status of magnetostatic devices

Techniques under investigation to modify the dispersion and bandwidth of magnetostatic waves for device applications are Briefly reviewed. Four devices exhibiting useful characteristics are described in detail. These are a delay line with a delay against frequency variation which is linear within ± 5nS over a 1GHz bandwidth in X-band, a programmable 4-tap delay line which can generate and correlate Barker coded signals at 9GHz, a delay stabilized oscillator showing better frequency stability with temperature than YIG sphere oscillators and a signal to noise enhancer. This latter device is unique as it provides a low loss to coherent signals above a threshold power but increased losses to broadband noise or other signals below threshold.

Monolithic integration of an X-band circulator with GaAs MMICs

Monolithic integration of circulators with GaAs MMICs offers the potential of lower cost, reduced size and improved uniformity over the present hybrid approaches. Development of MMIC compatible ferrite film deposition techniques, device design and fabrication are described. Results on ferrite film circulators deposited on Si and GaAs substrates are presented and integration with FETs discussed.<<ETX>>

Magnetostatic Wave Devices For Microwave Signal Processing

Basic devices required for analog microwave signal processing have been demonstrated using magnetostatic wave propagation in epitaxial yttrium iron garnet films. 100 nsec delay lines with bandwidths of 400 MHz, dispersive delay lines with differential delays of 200 nsec and bandwidths of 1 GHz and a programmable tapped delay line, all operating at S-band through X-band, are described. The operation of a new type of device, a signal-to-noise enhancer or power expander, is discussed which has the opposite characteristic to the more familiar microwave limiter. All these devices share the unique features of operation directly at microwave frequencies and frequency tunability via the strength of the magnetic bias field which is required for their operation.