The phase noise and frequency stability measurements of 1 GHz, 100 MHz, and 10 MHz signals are presented which have been synthesized from microwave cryogenic sapphire oscillators using ultra-low-vibration pulse-tube cryocooler technology. We present
the measured data using independent cryogenic oscillators for the 100 MHz and 10 MHz synthesized signals, whereas previously we only estimated the expected results based on residual phase noise measurements, when only one cryogenic oscillator was available. In addition we present the design of a 1 GHz synthesizer using a Crystek voltage controlled oscillator phase locked to 1 GHz output derived from a cryogenic sapphire oscillator.
The digital signal processing has greatly simplified the process of phase noise measurements, especially in oscillators, but its applications are largely confined to the frequencies below 400 MHz. We propose a novel transpose frequency technique, whi
ch extends the frequency range of digital noise measurements to the microwave domain and beyond. We discuss two basic variations of the proposed noise measurement technique, one of which enables characterization of phase fluctuations added to the passing signal by the particular microwave component, for example by an amplifier, while another one is intended for measurements of phase fluctuations of the incoming signal itself.
Two nominally identical ultra-stable cryogenic microwave oscillators are compared. Each incorporates a dielectric-sapphire resonator cooled to near 6 K in an ultra-low vibration cryostat using a low-vibration pulse-tube cryocooler. The phase noise fo
r a single oscillator is measured at -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier. The oscillator fractional frequency stability is characterized in terms of Allan deviation by 5.3 x 10^-16 tau^-1/2 + 9 x 10^-17 for integration times 0.1 s < tau < 1000 s and is limited by a flicker frequency noise floor below 1 x 10^-16. This result is better than any other microwave source even those generated from an optical comb phase-locked to a room temperature ultra-stable optical cavity.
The Hubble law, determined from the distance modulii and redshifts of galaxies, for the past 80 years, has been used as strong evidence for an expanding universe. This claim is reviewed in light of the claimed lack of necessary evidence for time dila
tion in quasar and gamma-ray burst luminosity variations and other lines of evidence. It is concluded that the observations could be used to describe either a static universe (where the Hubble law results from some as-yet-unknown mechanism) or an expanding universe described by the standard Lambda cold dark matter model. In the latter case, size evolution of galaxies is necessary for agreement with observations. Yet the simple non-expanding Euclidean universe fits most data with the least number of assumptions. From this review it is apparent that there are still many unanswered questions in cosmology and the title question of this paper is still far from being answered.
Here we present the design and implementation of a novel frequency synthesizer based on low phase noise digital dividers and a direct digital synthesizer. The synthesis produces two low noise accurate and tunable signals at 10 MHz and 100 MHz. We rep
ort on the measured residual phase noise and frequency stability of the synthesizer, and estimate the total frequency stability, which can be expected from the synthesizer seeded with a signal near 11.2 GHz from an ultra-stable cryocooled sapphire oscillator. The synthesizer residual single sideband phase noise, at 1 Hz offset, on 10 MHz and 100 MHz signals, respectively, were measured to be -135 dBc/Hz and -130 dBc/Hz. Their intrinsic frequency stability contributions, on the 10 MHz and 100 MHz signals, respectively, were measured as sigma_y = 9 x 10^-15 and sigma_y = 2.2 x 10^-15, at 1 s integration time. The Allan Deviation of the total fractional frequency noise on the 10 MHz and 100 MHz signals derived from the synthesizer with the cryocooled sapphire oscillator, may be estimated as sigma_y = 5.2 x 10^-15 tau ^-1 + 3.6 x 10^-15 tau ^-1/2 + 4 x 10^-16 and sigma_y = 2 x 10^-15 tau ^-1/2 + 3 x 10^-16, respectively, for 1 s < tau < 10^4 s. We also calculate the coherence function, (a figure of merit in VLBI) for observation frequencies of 100 GHz, 230 GHz and 345 GHz, when using the cryocooled sapphire oscillator and an hydrogen maser. The results show that the cryocooled sapphire oscillator offers a significant advantage at frequencies above 100 GHz.
We report on extremely sensitive measurements of changes in the microwave properties of high purity non-intentionally-doped single-crystal semiconductor samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when illuminated with light
of different wavelengths at cryogenic temperatures. Whispering gallery modes were excited in the semiconductors whilst they were cooled on the coldfinger of a single-stage cryocooler and their frequencies and Q-factors measured under light and dark conditions. With these materials, the whispering gallery mode technique is able to resolve changes of a few parts per million in the permittivity and the microwave losses as compared with those measured in darkness. A phenomenological model is proposed to explain the observed changes, which result not from direct valence to conduction band transitions but from detrapping and retrapping of carriers from impurity/defect sites with ionization energies that lay in the semiconductor band gap. Detrapping and retrapping relaxation times have been evaluated from comparison with measured data.
A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the worlds most stable microwave oscillator employing a cryocooler
. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is sigma_y = 1 x 10^-15 tau^-1/2 for integration times 1 < tau < 10 s with a minimum sigma_y = 3.9 x 10^-16 at tau = 20 s. The long term frequency drift is less than 5 x 10^-14/day. From the measured power spectral density of phase fluctuations the single side band phase noise can be represented by L_phi(f) = 10^-14.0/f^4+10^-11.6/f^3+10^-10.0/f^2+10^-10.2/f+ 10^-11.0 for Fourier frequencies 10^-3<f<10^3 Hz in the single oscillator. As a result L_phi approx -97.5 dBc/Hz at 1 Hz offset from the carrier.
We report on the evaluation of microwave frequency synthesis using two cryogenic sapphire oscillators developed at the University of Western Australia. A down converter is used to make comparisons between microwave clocks at different frequencies, wh
ere the synthesized signal has a stability not significantly different from the reference oscillator. By combining the CSO with a H-maser, a reference source of arbitrary frequency at X-band can be synthesized with a fractional frequency stability of sub-$4 times 10^{-15}$ for integration times between 1 s and 10,000 s.
A Cryogenic Sapphire Oscillator has been implemented at 11.2 GHz using a low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art liquid helium cooled CSO in the same laboratory, the square root Allan variance of their combined fr
actional frequency instability is $sigma_y = 1.4 times 10^{-15}tau^{-1/2}$ for integration times $1 < tau < 10$ s, dominated by white frequency noise. The minimum $sigma_y = 5.3 times 10^{-16}$ for the two oscillators was reached at $tau = 20$ s. Assuming equal contributions from both CSOs, the single oscillator phase noise $S_{phi} approx -96 ; dB ; rad^2/Hz$ at 1 Hz offset from the carrier.
We have re-analyzed the stability of pulse arrival times from pulsars and white dwarfs using several analysis tools for measuring the noise characteristics of sampled time and frequency data. We show that the best terrestrial artificial clocks substa
ntially exceed the performance of astronomical sources as time-keepers in terms of accuracy (as defined by cesium primary frequency standards) and stability. This superiority in stability can be directly demonstrated over time periods up to two years, where there is high quality data for both. Beyond 2 years there is a deficiency of data for clock/clock comparisons and both terrestrial and astronomical clocks show equal performance being equally limited by the quality of the reference timescales used to make the comparisons. Nonetheless, we show that detailed accuracy evaluations of modern terrestrial clocks imply that these new clocks are likely to have a stability better than any astronomical source up to comparison times of at least hundreds of years. This article is intended to provide a correct appreciation of the relative merits of natural and artificial clocks. The use of natural clocks as tests of physics under the most extreme conditions is entirely appropriate; however, the contention that these natural clocks, particularly white dwarfs, can compete as timekeepers against devices constructed by mankind is shown to be doubtful.
