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Aliasing

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Accurate signal sampling and reconstruction are crucial in various signal-processing applications. A time-domain signal's spectrum can be revealed using its Fourier transform. When this signal is sampled at a specific frequency, it results in multiple scaled replicas of the original spectrum in the frequency domain. The spacing of these replicas is determined by the sampling frequency.
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Continuous-Wave Propagation Channel-Sounding Measurement System - Testing, Verification, and Measurements

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Precise and continuous time and frequency synchronisation at the 5×10⁻¹⁹ accuracy level.

B Wang¹, C Gao, W L Chen

¹Joint Institute for Measurement Science, Tsinghua University, Beijing 100084, China.

Scientific Reports

|August 8, 2012

Summary

Accurate time and frequency dissemination was achieved over an 80 km fiber link. This method offers a reliable alternative to satellite links for precise synchronization, crucial for scientific applications.

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Area of Science:

Metrology
Optical Communications
Geophysics

Background:

Precise time and frequency synchronization between remote locations is essential for numerous scientific and technological applications.
Traditional methods often rely on satellite links, which can be susceptible to environmental factors and signal degradation.
Communication fiber networks present a promising alternative for robust, long-distance time and frequency dissemination.

Purpose of the Study:

To demonstrate accurate frequency transfer and time synchronization using a standard urban fiber optic link.
To evaluate the stability and precision of time and frequency dissemination over an 80 km fiber path.
To explore the potential for scaling this technology to longer distances (1000 km) and its application in fields like radio astronomy.

Main Methods:

Utilized a 9.1 GHz microwave modulation and a timing signal.
Employed two continuous-wave lasers for signal transfer across an 80 km urban fiber link between Tsinghua University (THU) and the National Institute of Metrology of China (NIM).
Monitored system performance over several months for reliability assessment.

Main Results:

Achieved frequency transfer stability at the 5×10⁻¹⁹/day level.
Demonstrated time synchronization with a precision of 50 picoseconds (ps).
The system operated reliably and continuously for an extended period.

Conclusions:

The study successfully demonstrated high-precision time and frequency transfer over an 80 km fiber link.
This fiber-based approach offers a reliable and accurate alternative to satellite-based dissemination.
The technology shows potential for long-distance applications, including enhancing long-baseline radio astronomy.