**5. Acknowledgment**

3G-cellular-phone processor.

This work was done in cooperation with H. Mizuno, S. Komatsu, and Y. Kondoh of the Hitachi, Ltd., and T. Irita, K. Hirose, R. Mori, and Y. Yasu of the Renesas Electronics Corporation. We thank T. Yamada and N. Irie of Hitachi Ltd., and T. Hattori, T. Takeda of Renesas Electronics Corporation, and K. Ishibashi of The University of Electro-Communications, for their support and helpful comments. We also express our gratitude to Y. Tsuchihashi, G. Tanaka, Y. Miyairi, T. Ajioka, and N. Morino of Renesas Electronics Corporation for their valuable advice and assistance.

**6** 

*China* 

**High-Precision Frequency Measurement** 

*1National Time Service Center, Chinese Academy Sciences, Xi'an, Shaanxi* 

*2Key Laboratory of Time and Frequency Primary Standard, Institute of National Time* 

High-precision frequency measurement techniques are important in any branch of science and technology such as radio astronomy, high-speed digital communications, and highprecision 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

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 highprecision 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

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

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

**1. Introduction** 

measure it (C. A. Greenhall, 2007).

measure precise oscillators.

DFSA design goal of low noise floor (Ya Liu, 2008).

**Using Digital Signal Processing** 

*Service Center Chinese Academy of Sciences, Xi'an, Shaanxi* 

Ya Liu1,2, Xiao Hui Li1 and Wen Li Wang1

#### **6. References**

