**1. Introduction**

16 Will-be-set-by-IN-TECH

114 Applications of Digital Signal Processing

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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 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 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 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

High-Precision Frequency Measurement Using Digital Signal Processing 117

The DMTD setup is arguably the most precise way of measuring an ensemble of clocks all having the same nominal frequency. The usual idea thought that the noise of the common offset oscillator could be cancelled out in the overall measurement process. However, if the oscillator 1 and oscillator 2 are independent, then the beat signals of being fed into counter are not coherent. Figure 2 shows the beat signals that are fed into the time interval counter, thus, the beat signals of two test oscillators against the common offset oscillator are zero crossing at different sets of points on the time axis, such as t1 and t2. When time interval counter is used to measure the time difference of two beat signals, the time difference will be contaminated by short-term offset oscillator noise, here called common-source phase error (C. A. Greenhall, 2001, 2006). This DMTD method is inevitable common-source phase error when use counter to measure time difference. To remove the effect of common-source phase

Measurement interval Tau

To remove the effect of common offset oscillator phase noise and improve the accuracy of measuring frequency, we proposed to make use of digital signal processing method measuring frequency. A Multi-Channel Digital Frequency Stability Analyzer has been

This section will report on the Multi-Channel Digital Frequency Stability Analyzer (DFSA) based upon the reformed DMTD scheme working at 10MHz with 100Hz beat frequency. DFSA has eight parallel channels, and it can measure simultaneously seven oscillators. The

Common offset reference oscillator generates frequency signal, which has a constant frequency difference with reference oscillator. Reference oscillator and under test oscillator at the same nominal frequency are down-converted to beat signals of low frequency by mixing them with the common offset reference to beat frequency. A pair of analog-to-digital converters (ADC) simultaneously digitizes the beat signals output from the double-balance mixers. All sampling frequency of ADCs are droved by a reference oscillator to realize simultaneously sampling. The digital beat signals are fed into personal computer (PC) to

block diagram of the DFSA that only includes two channels is reported in Fig. 3.

computer the drift frequency or phase difference during measuring time interval.

error need to propose other processing method.

t2 - t1

t1

Fig. 2. Beat signals from double-balance mixers

developed in NTSC.

**3.1 System configuration** 

t2

**3. Frequency measurement using digital signal processing** 

short-term stability, low cost, high reliability measurement system. A description of a classical DMTD method is given in Section 2. Some of the tests of the cross-correlation algorithm using simulated data are discussed in Section 3.2. The design of DFSA including hardware and software is proposed in Section 3.3-3.4. In section 4 the DFSA is applied to measure NTSC's cesium signal and the results of noise floor of DFSA is given. Future possible modifications to the DFSA and conclusions are discussed in Section 4.
