R.T. Wang

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/. >

Ultra-stable performance of the superconducting cavity maser

Measurements on the superconducting cavity maser (SCM) oscillator that show frequency stability of parts in 10/sup 5/ for times from 1 s to 10000 s are reported. A phase noise of approximately -80 dB/f/sup 3/ was measured. This short- and midterm performance is believed to be better than that of any known microwave oscillator. In particular, stability at a measuring time of 1 s is ten times better than that of a hydrogen maser, and phase noise is more than 20 dB below that of the best multiplied quartz crystal oscillators. >

Cryocooled sapphire oscillator with ultrahigh stability

We present test results and design details for the first short-term frequency standard to achieve ultrahigh 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 two-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8 K to 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 s/spl les//spl tau//spl les/600 s. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as local oscillator (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.

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.

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/.

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.

Hg/sup +/ trapped ion standard with the superconducting cavity maser oscillator

The frequency stability of an atomic standard based on /sup 199/Hg/sup +/ ions confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition has been measured to be 17 mHz wide, representing a quality factor greater than 2*10/sup 12/. A 160-mHz line is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2*10/sup -15/ for 24000-s averaging times. In a separate measurement, a 37-mHz line is used to steer the output of the superconducting cavity maser oscillator to reach 1*10/sup -15/ stability at 10000 s. >

Stability and phase noise tests of two cryocooled sapphire oscillators

A cryocooled compensated sapphire oscillator (CSO), developed for the Cassini Ka-band Radio Science experiment, and operating in the 7 K-10 K temperature range was previously demonstrated to show ultra-high stability of /spl sigma//sub y/=2.5/spl times/10/sup -15/ for measuring times 200 seconds /spl les//spl tau//spl les/600 seconds using a hydrogen maser as reference. CSO-1 and CSO-3 are now both operational with new low noise receivers. We have made initial phase noise and Allan deviation measurements that show more than ten times stability improvement over the hydrogen maser for measuring times 1 second /spl les//spl tau//spl les/10 seconds, and indicate performance for the individual units of /spl sigma//sub y//spl ap/3/spl times/10/sup -15/ for measuring times from 10 to 1000 seconds. Phase noise is reduced by 20 to 28 dB over the design offset frequency range from 1 Hz to 40 Hz. Receiver design is also discussed.

Hg/sup +/ trapped ion standard performance with the superconducting cavity maser oscillator as LO

The frequency stability obtained with an ion-trap-based frequency standard where /sup 199/Hg/sup +/ ions are confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition is measured to be as narrow as 17 mHz representing a quality factor of 2.4*10/sup 12/. A hydrogen maser interrogation of a 37-mHz Hg/sup +/ resonance is used to measure 1*10/sup -15/ stability of the ion resonance at 10000-s averaging time. In another measurement the frequency of the superconducting cavity maser oscillator is steered to follow the Hg/sup +/ clock transition, again providing frequency stability of 1*10/sup -15/ at 10000-s.<<ETX>>

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

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/.<<ETX>>