RESUMO
The three-cornered hat/Groslambert Covariance (GCov) methods are widely used to estimate the stability of each individual clock in a set of three, but no method gives reliable confidence intervals for large integration times. We propose a new KLTS (Karhunen-Loève Tansform using Sufficient statistics) method which uses these estimators to consider the statistics of all the measurements between the pairs of clocks in a Bayesian way. The resulting cumulative density function (CDF) yields confidence intervals for each clock Allan variance (AVAR). This CDF provides also a stability estimator that is always positive. Checked by massive Monte Carlo simulations, KLTS proves to be perfectly reliable even for one degree of freedom. An example of experimental measurement is given.
RESUMO
This paper shows the first measurement of three 100-MHz signals exhibiting fluctuations from 2×10-16 to parts in 10-15 for an integration time τ between 1 s and 1 day. Such stable signals are provided by three cryogenic sapphire oscillators (CSOs) operating at about 10 GHz, also delivering the 100-MHz output via a dedicated synthesizer. The measurement is made possible by a six-channel tracking direct digital synthesizer (TDDS) and the two-sample covariance tool, used to estimate the Allan variance. The use of two TDDS channels per CSO enables high rejection of the instrument background noise. The covariance outperforms the three-cornered hat (TCH) method in that the background converges to zero "out of the box," with no need of the hypothesis that the instrument channels are equally noisy, nor of more sophisticated techniques to estimate the background noise of each channel. Thanks to correlation and averaging, the instrument background (AVAR) rolls off with a slope 1/âm , the number of measurements, down to 10-18 at τ = 104 s. For consistency check, we compare the results to the traditional TCH method beating the 10-GHz outputs down to the megahertz region. Given the flexibility of the TDDS, our methods find an immediate application to the measurement of the 250-MHz output of the femtosecond combs.
RESUMO
We present the characterization of commercial tunnel diode low-level microwave power detectors at room and cryogenic temperatures. The sensitivity as well as the output voltage noise of the tunnel diodes is measured as functions of the applied microwave power. We highlight strong variations of the diode characteristics when the applied microwave power is higher than a few microwatts. For a diode operating at 4 K, the differential gain increases from 1000 V/W to about 4500 V/W when the power passes from -30 dBm to -20 dBm. The diode white noise floor is equivalent to a Noise Equivalent Power of 0.8 pW/Hz and 8 pW/Hz at 4 K and 300 K, respectively. Its flicker noise is equivalent to a relative amplitude noise power spectral density Sα(1 Hz) = - 120 dB/Hz at 4 K. Flicker noise is 10 dB higher at room temperature.
RESUMO
We present the characterization of three cryogenic sapphire oscillators (CSOs) using the three-cornered-hat method. Easily implemented with commercial components and instruments, this method reveals itself very useful to analyze the fractional frequency stability limitations of these state-of-the-art ultrastable oscillators. The best unit presents a fractional frequency stability better than 5 ×10(-16) at 1 s and below 2 ×10(-16) for [Formula: see text].
RESUMO
We present a detailed characterization of two atomic clock interrogation systems based on two different cryogenic sapphire oscillators operated simultaneously. We use them as references for two accurate fountain clock frequency standards participating in international atomic time and operating both at the quantum projection noise frequency limit. The two fountain comparison down to a few 10ô16 over 28 days demonstrates the potential of a cryocooled oscillator to replace a He refilled cryogenic oscillator.
