| Frequency source is the basic signal source of a time-frequency system.Frequency stability,as an important index of frequency source,can directly reflect the quality of the whole time frequency system.In recent years,the rapid development of technology has imposed stricter requirement on the performance index of time-frequency systems,and thus more stable frequency source is required.Therefore,it becomes a very important task to explore how to improve frequency stability of frequency source.Different frequency stabilizing methods often lead to different levels of frequency source internal noises,which in turn results in different frequency stability variations.The frequency stability variations of the frequency source with frequency control is 1/（?）,and with phase control is a more ideal result of 1/τ.Due to the limitations of the traditional understanding of phase,most passive atomic clocks still use frequency control,which leads to the variation law of their frequency stability being 1/（?）,and directly restricts the passive atomic clocks’s wide applications in the field of precision time and frequency.This paper analyzes the energy level transition process of the passive rubidium atomic clock,discovers that in the frequency modulation driving process of the physical part in atomic clock,the phenomenon that the frequency of the drive signal is equal to the atomic energy level transition frequency at a certain moment will appear over and over,and at this moment atomic level transition has the highest probability.The modulation domain characteristic of the transition maximum probability point not only reflects the frequency deviation of the VCOCXO in atomic clock,but also reflects the phase information in the corresponding time.Using the generalized phase group synchronization theory and principle of virtual reconstruction based on phase,combining with the digital technology,we recovers a single phase difference information from each measured frequency deviation and gate time,and we accumulate the single phase difference information to obtain the real-time phase difference information of the controlled VCOCXO,then we correct the output signal of the VCOCXO by the phase difference information,finally we realize the high stability of digital phase processing of passive rubidium atomic clocks.Experimental results show that the passive rubidium atomic clock using digital phase processing method for frequency stabilization can give full play to the digital phase processing’s high precision in the field of time-frequency measurement and control.The change rule of its frequency stability is close to 1/τ,and the second level frequency stability is better than 4×10-12. |
