G.J. Dick

Microwave frequency discriminator with a cooled sapphire resonator for ultra-low phase noise

First results are presented for an X-band frequency discriminator using a cooled sapphire microwave resonator. These results show a lower close-in (1-Hz-1-kHz offset) phase noise measurement floor than any oscillator presently available. This performance is made possible by a sapphire whispering-gallery mode resonator which shows the highest quality factor (with Qs up to 30 million) of any RF microwave, or acoustic resonator at temperatures to 77 K. Performance is increased by use of phase detection circuitry. The sapphire discriminator is used to characterize the phase noise of a single crystal quartz oscillator of the highest quality, without the use of a second similar oscillator as reference.<<ETX>>

Measurement and analysis of a microwave oscillator stabilized by a sapphire dielectric ring resonator for ultra-low noise

Phase-noise measurements are presented for a microwave oscillator whose frequency is stabilized by a whispering gallery mode sapphire ring resonator with Q of 2*10/sup 5/. The nature of the mode, which involves little metallic conduction, allows nearly full use of the very low dielectric loss in sapphire. Several mode families have been identified with good agreement with calculated frequency predictions. Waveguide coupling parameters have been characterized for the principal (lowest frequency) mode family, for n=5 to n=10 full waves around the perimeter. For a 5-cm wheel resonator in a 7.6-cm container, Q-values of above 10/sup 5/ were found at room temperature for all of the modes in this sequence. Coupling Q-values for the same modes ranged from 10/sup 4/ (n=5) to 10/sup 5/ (n=10) for a WR112 waveguide port at the center of the cylinder wall of the containing can. Phase noise measurements for a transistor oscillator locked to the n=10 (7.84-GHz) mode showed a 1/f/sup 3/ dependence for low offset frequencies, and a value of L(f)=-55 dB/Hz at an offset of 10 Hz from the carrier. The oscillator shows phase noise below the previously reported for any X-band source.<<ETX>>

Temperature-compensated sapphire resonator for ultra-stable oscillator capability at temperatures above 77 K

We report on the design and test of a whispering gallery sapphire resonator for which the dominant (WGH/sub n11/) microwave mode family shows frequency-stable, compensated operation for temperatures above 77 K. The resonator makes possible a new ultra-stable oscillator (USO) capability that promises performance improvements over the best available crystal quartz oscillators in a compact cryogenic package. A mechanical compensation mechanism, enabled by the difference between copper and sapphire expansion coefficients, tunes the resonator to cancel the temperature variation of sapphires dielectric constant. In experimental tests, the WGH/sub 811/ mode showed a frequency turnover temperature of 87 K in agreement with finite element calculations. Preliminary tests of oscillator operation show an Allan Deviation of frequency variation of 1.4-6/spl times/10/sup -12/ for measuring times 1 s /spl les//spl tau//spl les/100 s with unstabilized resonator housing temperature and a mode Q of 2/spl times/10/sup 6/. We project a frequency stability 10/sup -14/ for this resonator with stabilized housing temperature and with a mode Q of 10/sup 7/. >

Long term stability of Hg/sup +/ trapped ion frequency standards

The standards report the development of a second /sup 199/Hg/sup +/ linear ion trap (LIT) based frequency standard to provide a capability for measuring stability beyond all existing frequency standards. Increased signal using a second light collection system together with a previously demonstrated atomic line Q /spl ap/ 2 /spl times/ 10/sup 12/ yields a performance better than /spl sigma//sub y/ (/spl tau/ = 7 /spl times/ 10/sup -14///spl tau//sup 1/2/). Sensitivity to the leading perturbations are measured to identify regulation requirements to obtain a stability of 10/sup -16/.<<ETX>>

Cryo-cooled sapphire oscillator with ultra-high stability

We present test results and design details for the first short-term frequency standard to achieve ultra-high stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the compensated sapphire oscillator (10 K CSO) makes available the superior short-term stability and phase noise performance of cryogenic oscillators without periodic interruptions for cryogen replacement. Technical features of the 10 K CSO include use of a a-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8-10 K with Q=1-2/spl times/10/sup 9/. Stability of the first unit shows an Allan Deviation of /spl sigma//sub y//spl les/2.5/spl times/10/sup -15/ for measuring times of 200 seconds /spl les//spl tau//spl les/600 seconds. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as L.O. for the LITS-7 trapped mercury ion frequency standard, the CSO/LITS combination demonstrated a limiting performance of 3.0/spl times/10/sup -14///spl tau//sup 1/2/, the lowest value measured to date for a passive atomic frequency standard, and virtually identical to the value calculated from photon statistics.

Ultra-stable Hg+

Abstract We report the development of a fieldable frequency standard based on 199Hg+ ions confined in a hybrid r.f./dc linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the r.f. confining fields. A 160 mHz wide atomic resonance line for the 40·5 GHz clock transition is used to steer the output of a 5 MHz crystal oscillator to obtain a stability of 2 × 10−15 for 24 000 s averaging times. For longer averaging intervals, measurements are limited by instabilities in available hydrogen maser frequency standards. Measurements with 37 mHz linewidth for the Hg+ clock transition demonstrate that the inherent stability for this frequency standard is at least as good as 1 × 10−15.

Power dependence of distributed cavity phase-induced frequency biases in atomic fountain frequency standards

We discuss the implications of using high-power microwave tests in a fountain frequency standard to measure the frequency bias resulting from distributed cavity-phase shifts. We develop a theory which shows that the frequency bias from distributed cavity phase depends on the amplitude of the microwave field within the cavity. The dependence leads to the conclusion that the frequency bias associated with the distributed cavity phase is typically both misestimated and counted twice within the error budget of fountain frequency standards.

Improved performance of a temperature compensated LN/sub 2/ cooled sapphire oscillator

We report on improved stability in a whispering gallery sapphire resonator for which the dominant WGH/sub nll/ microwave mode family shows frequency-stable, compensated operation for temperatures above 77 K. Several modifications during the past year have led to significant improvements in performance. Current tests with improved thermal stability provide Allan Deviation of frequency of 2.6-4/spl middot/10/sup -13/ for measurement times of 1/spl les//spl tau//spl les/100 seconds. We project a frequency stability of 10/sup -14/ for this resonator with stabilized housing temperature and with a mode Q of 10/sup 7/.

Improved performance of the superconducting cavity maser at short measuring times (atomic frequency standards)

Measurements on a superconducting cavity maser oscillator are presented which show a frequency stability of parts in 10/sup 15/ for times from 1 second to 1000 seconds. A phase noise of approximately -80 dB/f/sup 3/, where f is frequency, is shown. The measured stability at a 1 second interval is 10 times better than that of a hydrogen maser, and phase noise at 8 GHz is more than 20 dB below that of a multiplied quartz crystal oscillator.<<ETX>>

Cryo-cooled sapphire oscillator for the Cassini Ka-band experiment

We present design aspects of a cryogenic sapphire oscillator which is being developed for ultra-high short term stability and low phase noise in support of the Cassini Ka-band Radio Science experiment. With cooling provided by a commercial cryocooler instead of liquid helium, this standard is designed to operate continuously for periods of a year or more. Performance targets are a stability of 3/spl times/10/sup -15/ (1 second/spl les//spl tau//spl les/100 seconds) and a phase noise of -73 dB/Hz @ 1 Hz measured at 34 GHz. Test results are reported for several subsystems; including cryocooler, vibration isolation system, and ruby compensating element.