The Contrast Between Digital Direct-linear-phase Comparison And DMTD

In the field of time-frequency measurement&control,the measurement of frequency stability has always been a focus.At present,the frequency stability measurement instruments used in China are mainly 5125A&3120A which produced by Symmetricom from America.The proportion of domestic instrument is too low.In order to deal with this unfavorable situation,relevant domestic research institutions have taken some measures.Some researchers choose to imitated foreign instruments;others are also seeking innovation on the method path.For a long time,there have been two technical paths in the branch field of frequency stability measurement,named direct linear phase processing and error amplification processing.In this paper,two representative methods of the two technical paths are selected.As typical example,the Digital Direct-linear-phase Comparison method and the Dual Mixed Time Difference measurement（DMTD）method are compared in some aspects.Through theoretical analysis,the article estimates the accuracy of the two methods and the impact of various errors on the measurement system,and proposes a suppressive treatment method for some of errors which can be suppressed.A Digital Direct-linear-phase Comparator and a 3120A Phase-noise Test Probe were selected for measurement and determination of actual indicators for actual comparison.Through experimental weighted evaluation,in terms of measurement accuracy,the measurement accuracy of the existing Digital Direct-linear-phase Comparator is slightly lower than the 3120A.In the self-calibration experiment,the self-calibration weighting results of Rubidium clock,Cesium clock,and 8607 were used to prove that the Digital Direct-linear-phase Comparator used in the experiment is approximately higher than the noise floor of the3120A one order of magnitude,about 3×10^-13.At present,the DMTD method can only complete frequency stability measurement at the millisecond level and above,and the measurement range of the Digital Direct-linear-phase Comparison method can be extended down to the hundred nanosecond level,which has a full measurement capability for transient frequency stability.The transient stability corresponds to the far-end phase-noise.The ability to measure the transient stability makes this method more expandable in the communication field than the DMTD method.Based on the theoretical analysis,it is concluded that the Digital Direct-linear-phase Comparison method has better dynamic measurement capability and response speed than the DMTD method.From the perspective of the cost of the representative instrument,compared with the high selling price of more than 100,000 yuan of 3120A,the cost of the digital direct linear phase comparison instrument is limited to hardware and the cost-effective ratio is quite high.Compared with the DMTD method,which is difficult to improve the measurement accuracy,the Digital Direct-linear-phase Comparison method still has multiple methods to improve the time domain resolution and suppress the measurement noise floor.Here,according to the principle and characteristics of Digital Direct-linear-phase Comparison,this paper proposes and analyzes a new method to reduce the noise floor of the system by using the method of clock phase shift processing.Finally,in exploring complex frequency processing,an idea of measuring processing for a specific complex frequency is given,and simulated it.In general,compared with the mature&finalized DMTD method,the Digital Direct-linear-phase Comparison method has a wider scope for expansion based on the existing high-precision measurements.