An oscillator circuit that contains superconductive and ferroelectric resonator elements is undergoing development. This circuit is designed to serve as a frequency-locked local oscillator in a communication system in which digital data are conveyed via phase modulation on a carrier signal, the frequency of which could be as high as tens of gigahertz.
The traditional practice of multiplying the frequency of a crystal-stabilized oscillator is not suitable in this application because of phase noise. The highest practical crystal frequency is a few hundred megahertz, and the phase noise is proportional to the square of the frequency-multiplication factor, N. At the large N necessary for reaching tens of gigahertz, the phase noise is large enough to contribute significantly to the bit-error rate. The developmental circuit oscillates directly at the desired frequency, obviating the phase-noise multiplication. Although a dielectric disk oscillator could operate in the desired frequency range, it also generates excessive phase noise and cannot be electronically tuned or locked in frequency. The developmental circuit can be electrically tuned and can be electronically locked in frequency, with a concomitant reduction in phase noise.
The heart of the oscillator is a pseudomorphic high-electron-mobility transistor (PHEMT) connected to a microstrip ring resonator with integral coupled lines (see figure). The coupled lines are formed from a thin film of a high-temperature superconductor (HTS) over a thin film of a ferroelectric material like BaxSr1-xTiO3(where 0 < x < 1). The combination of the HTS and the ferroelectric material is chosen to obtain the high resonance quality factor (Q) needed for low phase noise (phase noise is proportional to Q-²).
The coupled lines are designed so that most of the electromagnetic energy is confined in the odd mode of propagation, in which the electric field is concentrated in the dielectric (that is, the ferroelectric) material. Tuning is effected by applying a dc bias voltage to alter the permittivity of the ferroelectric material. To enhance tuning, the coupled lines are positioned at radio-frequency voltage maxima along the ring. To diminish loading, the dc-bias connections are made at radio-frequency voltage minima along the ring.
The output of the oscillator is fed into a hybrid ring or directional coupler to enable sampling of the output. The sampled power is fed into a diplexer, the crossover frequency of which equals the desired frequency of oscillation. The high- and low-pass outputs of the diplexer are detected by diodes, then low-pass filtered to remove radio-frequency components. The resulting dc signals are applied to the input terminals of a differential operational amplifier. The differential amplifier puts out a dc voltage that is superimposed upon a fixed offset voltage, and the resulting total voltage constitutes the dc bias applied to the resonator ring. (For the sake of clarity, the circuit is depicted in simplified form in the figure, without the components for generating the fixed offset voltage and superimposing the amplifier output.)
If the actual frequency of oscillation is greater (or less) than the crossover frequency, then the diplexer generates a high-pass (or low-pass) output, causing the differential amplifier to decrease (or increase) the dc bias. This action causes the permittivity of the ferroelectric material to increase (or decrease) thereby causing the frequency of oscillation to decrease (or increase). In other words, the dc bias is adjusted to correct for any deviation from the crossover frequency.
Alternatively, if frequency stability is not of concern, one can simply adjust the bias voltage directly to effect tuning. For example, a sawtooth or other suitable waveform could be applied to obtain a repetitive frequency sweep.
This work was done by Robert R. Romanofsky and Felix A. Miranda of Lewis Research Center . For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 114 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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Refer to LEW-16440.