Research Of Full Response Time Frequency Stability Based On Digital Linear Phase Comparison

With the continuous development of society,the time accuracy and the accuracy of time reference frequency sources are constantly increasing in the fields of satellite navigation,aerospace and other scientific research and exploration area.This requires the accuracy of frequency source analysis and measurement methods to be synchronized.In the time domain,frequency stability is used to measure the performance of frequency sources.According to the length of the measurement time,frequency stability is defined as transient,short-term,and long-term frequency stability.With the in-depth study of frequency stability,the frequency stability is characterized from Allen Variance to Modified Allen Variance and Hadamard Variance.The measurement method of the frequency source has been developed from the traditional analog method to the digital field,which has improved the issues of limited line resolution and sensitive single-point zero-crossing detection in analog mode.Currently,the most widely used in the digital field is the Dual Mixer Time Difference（DMTD）method,it uses Mixer to make measurement error doubling,which reduces the actual measurement frequency,and cannot obtain transient frequency stability below the order of milliseconds.But in the field of navigation and radar,the remote phase noise,that is,transient stability,is an important factor that affect the navigation accuracy and the ability of radar to capture details.Hence it is very urgent to develop technology with short response time and high resolution.A method is proposed for measuring frequency stability based on digital linear phase comparison.Aiming at the quantization errors caused by the finite number of analog-to-digital converters.This thesis innovatively adopts the method of moving clock sampling combined with edge effect,establishing a scheme to optimize the selection of sampling points.Control the sampling point by moving clock sampling model to ensure that the sampling point selects a certain transition within a linear interval near the positive zero crossing of the measured sinusoidal signal.In this interval,the quantization fuzzy region has approximately equal steps in unit time,which offsets some sampling errors.Using the principle of moving clock sampling to improve the measurement accuracy.By means of moving clock,the sampling is extended to the whole linear segment of the sinusoidal signal.The quantization error can be reduced by edge effect and moving clock sampling in digital measurement mode.On the basis of previous research,a measurement scheme for digital moving clock in phase comparison is established,the hardware and software design in the measurement scheme are introduced in detail.A series of experiments were carried out on the digital moving clock phase comparison measurement platform,the platform test experiments,self-calibration experiments and comparison experiments has been completed by using OCXO and rubidium atom clock.The experimental data were characterized by the Allen Variance,Modified Allen Variance and Hadamard Variance respectively for the transient,short-term and long-term frequency stability.The experiment verifies that the higher the sampling clock frequency is relative to the frequency of the measured signal,the smaller the effective linear interval obtained,and the higher measurement accuracy of the sampling point in the linear region.Experiments show that the measurement system in the self-calibration mode obtain the transient frequency stability of 10^-9/100μs,the short-term frequency stability of 10^-12/1s,and the long-term frequency stability of 10^-14/100s.The method realizes the full domain from the transient to long term frequency stability measurement.Under the help of moving clock sampling,high-precision time reference and frequency measurement can be realized.A new idea of digital direct phase comparison is provided for the field of time-frequency measurement and control.