Abstract
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/√m , 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.
Highlights
Given the flexibility of the tracking direct digital synthesizer (TDDS), our methods find an immediate application to the measurement of the 250-MHz output of the femtosecond combs
T HIS paper is made possible by the simultaneous availability in the same place of three cryogenic sapphire six-channel tracking direct digital synthesizer (TDDS) for the Manuscript received July 27, 2018; accepted September 11, 2018
CALOSSO et al.: FREQUENCY STABILITY MEASUREMENT OF cryogenic sapphire oscillators (CSOs) are each phase-locked to one input signal, extracting the phase information from the phase-control word
Summary
T HIS paper is made possible by the simultaneous availability in the same place of three cryogenic sapphire six-channel tracking direct digital synthesizer (TDDS) for the Manuscript received July 27, 2018; accepted September 11, 2018. CALOSSO et al.: FREQUENCY STABILITY MEASUREMENT OF CSOs are each phase-locked to one input signal, extracting the phase information from the phase-control word At once, this eliminates the complexity of the dual-mixer system [12]–[14], mitigates the thermal instability by using only wideband components and filtering numerically at the output, and enables the simultaneous measurement of the six inputs at quite different frequencies scattered in a wide range (presently, 5–400 MHz). Our TDDS [15] exhibits a background noise of 1.5 × 10−14/τ (ADEV) per channel at 100 MHz. The most common approach for phase measurement with digital methods starts from sampling and digitizing the input signal [16]–[20].
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