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High-Precision Frequency Measurement
Using Digital Signal Processing
Ya Liu 1,2 , Xiao Hui Li 1 and Wen Li Wang 1
1 National Time Service Center, Chinese Academy Sciences, Xi'an, Shaanxi
2 Key Laboratory of Time and Frequency Primary Standard, Institute of National Time
Service Center Chinese Academy of Sciences, Xi'an, Shaanxi
China
1. Introduction
High-precision frequency measurement techniques are important in any branch of science
and technology such as radio astronomy, high-speed digital communications, and high-
precision time synchronization. At present, the frequency stability of some of atomic
oscillators is approximately 1E-16 at 1 second and there is no sufficient instrument to
measure it (C. A. Greenhall, 2007).
Kinds of oscillator having been developed, some of them have excellent long-term stability
when the others are extremely stable frequency sources in the short term. Since direct
frequency measurement methods is far away from the requirement of measurement high-
precision oscillator, so the research of indirect frequency measurement methods are widely
developed. Presently, common methods of measuring frequency include Dual-Mixer Time
Difference (DMTD), Frequency Difference Multiplication (FDM), and Beat-Frequency (BF).
DMTD is arguably one of the most precise ways of measuring an ensemble of clocks all
having the same nominal frequency, because it can cancel out common error in the overall
measurement process (D. A. Howe & DAVID A & D.B.Sulliivan, 1981). FDM is one of the
methods of high-precision measurement by multiplying frequency difference to
intermediate frequency. Comparing with forenamed methods, the BF has an advantage that
there is the simplest structure, and then it leads to the lowest device noise. However, the
lowest device noise doesn't means the highest accuracy, because it sacrifices accuracy to
acquire simple configuration. Therefore, the BF method wasn't paid enough attention to
measure precise oscillators.
With studying the BF methods of measuring frequency, we conclude that the abilities of
measuring frequency rest with accuracy of counter and noise floor of beat-frequency device.
So designing a scheme that it can reduce circuit noise of beat-frequency device is mainly
mission as the model of counter has been determined. As all well known, reducing circuit
noise need higher techniques to realize, and it is hardly and slowly, therefore, we need to
look for another solution to improve the accuracy of BF method. In view of this reason, we
design a set of algorithm to smooth circuit noise of beat-frequency device and realize the
DFSA design goal of low noise floor (Ya Liu, 2008).
This paper describes a study undertaken at the National Time Service Center (NTSC) of
combining dual-mixer and digital cross-correlation methods. The aim is to acquire high
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