| To measure the frequency stability of an atomic oscillator having high-frequency stability, a technique using Dual-Mixer Time-Difference (DMTD) is widely used. The common oscillator compared to the phase of oscillators 1 and 2, when the beat signal crosses zero, producing a pulse from the zero-crossing detector and starting the time-interval counter. On the other hand, the errors could be brought when the zero-crossing points are contained by phase noise, and the noise-sensitive property of the DMTD lead to improve measurement accuracy hardly. Therefore, the Symmetricom announced that the 5110A using DMTD technique had reached the end of its technology product lifecycle and the manufacturing of this product was being discontinued. In this paper, the authors propose a digital frequency measurement method that combines double balanced mixer with digital cross-correlation phase measurement to solve above problems. And when the beat signals are digitized the virtual instrument technique can be used to realize friendlier functions of instrument than traditional devices.This paper focuses on studying the methods of high-precision frequency measurement and noise optimization. And a prototype multi-channel digital frequency stability analyzer (DFSA) is developed based on digital frequency measurement method and obtains an Allan deviation of Sigma=4.7e-14 at Tau=1 second and Sigma=1.04e-15 at Tau=1000 seconds as a result of measuring noise floor at a frequency of 10MHz.The main research and originality of this paper can be summarized as below:Firstly, analyzes several types of high-precision frequency measurement instruments and points out the challenge that reduces noise floor of these systems with the frequency multiplication and DMTD method. Those methods measure frequency stability by detecting the single zero-crossing point what is noise-sensitive. If the zero-crossing points are added random noises what might come from mixer, zero-crossing detector and other devices in the system, it will lead to misjudgment of zero-crossing points and cause measurement error.Secondly, proposes a multi-channel digital frequency measurement method that resolves the above-mentioned problems. Although the method uses double balanced mixer (DBM) and an offset frequency generator in a similar manner as the DMTD method, it is not affected by the noise of zero-crossing points. The source under test, reference source and offset frequency generator are each connected to DBM. The DBM sends a pair of sinusoidal beat signals to analog-to-converters (ADC) to be digitized according to the sampling frequency that is 10 times more than beat frequency. Then the frequency is numerically calculated. The method not only detects single zero-crossing point, but all sampling points are detected for frequency measurement. So the effect of one of sampling points being added random noise can be ignored.Thirdly, discusses the error reasons of the proposed method and then take firm steps to remove or reduce them. It is particularly important to realize high-precision frequency measurement. The error reasons of concern that include all the noise sources as well as non linear effects and deterministic effects, such as phase noise in the offset frequency generator, sinusoidal beat signal distortion, quantization noise, are discussed in detail. Moreover, the calibration methods of these errors are proposed. Finally, a large number of experiments are carried out to verify validity of calibration methods.Based on the foregoing methods, design a multi-channel digital frequency stability analyzer implement scheme, the key technologies include:a) The technology of high-precision digital frequency measurement;b) The technology of sinusoidal beat signal generating;c) Calibrating technology of the system error;d) The technology of real-time frequency stability measurement and acquiring data integrity;e) Optimization of the DFSA.Finally, a prototype multi-channel digital frequency stability analyzer is developed based on digital frequency measurement method. To evaluate the measurement precision and other performances of the DFSA, a test platform of noise floor is designed and built. The test platform provide two no time error sources to be used to measure noise floor of DFSA. The residual noise of the test platform itself has been evaluated before measuring the DFSA. The comparsion experiments are carried out among the DFSA, Symmetricom's MMS, and the DMTD that was developed by National Time Service Center early, and the results show that the DFSA including noise floor, measurement accuracy and measurement resolution are excellent. |
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