For comparison of arbitrary frequency signals, the paper proposed two levels of length vernier based on the time-space relationship are used in three levels of phase coincidence detecting circuits to extract the phase coincidence information by proper logic calculation. The length/phase of each vernier is respectively corresponding to the accuracy and the resolution of detecting circuit. The time-space relationship is based on high-stability, high-accuracy, and high-speed of signal transmission. The method is effective to reduce the fuzzy region in the phase coincidence information and reach a higher measuring precision.

Along with the accuracy enhancement of different kinds of frequency standards, more demands are made on the measurement precision. In time and frequency measurement instruments, both digital and analog processing techniques and circuits are used widely. The paper focuses primarily on the limitations of the technology and the principle in the use of the analog and the digital measurement approaches. Based on the principle of the conventional method measuring the period of frequency difference between the reference and the measured frequency standards, the analog and the digital measurement techniques are compared in actual realization, configuration, precision and error sources etc. A number of experiments indicate that the precision of the analog method is as high as 10-13/tau and the precision of the digital method about 10-12/tau or lower. Furthermore, the digital scheme in TTL circuits has great influence of trigger error and quantified error on measurement result; while in ECL circuits, trigger error is held down and the precision is greatly enhanced. Moreover, this paper proposes some improvement step. One had better appropriately combine ultra-high speed devices with the digital method to obtain a high precision frequency measurement device.

This paper presents a new method to improve the long-term frequency stability of an oven-controlled crystal oscillator (OCXO) without an external reference source. The frequency drift of the crystal oscillator can be obtained in real time by measuring the phase shift between the pins of the crystal resonator. In this paper, according to an equivalent circuit of a crystal oscillator, a linear equivalent mathematical model based on the phase shift of the crystal resonator and the output frequency is established. The experiments were conducted to observe the relation between the phase shift and frequency drift of the OCXO. At the same time, a crystal oscillator self-calibration frequency control system is established to improve the frequency drift of the OCXO. The results show that this method can effectively improve the drift of OCXOs. The OCXO drift rate was significantly improved from 1.53 10⁻¹⁰/day to 9.8 10⁻¹²/day, and after three days, it settled at 5.24 10⁻¹¹. The long-term frequency stability also underwent remarkable improvement, from 1.16 10⁻¹¹/1000 and 3.24 10⁻¹¹/10,000 to 3.47 10⁻¹²/1000 and 1.05 10⁻¹¹/10,000.

The aging of crystal oscillators is an inherent and irreversible process. There is often thought to be no need for special processing if the aging rate varies within an allowable range and the crystal oscillator works properly. However, the impact of aging can be aggravated over the course of lengthy operation. This paper proposes an intelligent compensation scheme for the aging of batch OCXOs (oven-controlled crystal oscillators) based on a statistical distribution, analyzes the aging data of OCXOs in the same batch, selects the typical samples, acquires the best aging prediction curves after assessing different models, and efficiently compensates for every qualified product for aging using this model. The results of experiments exhibited good compensation effects, with an improvement of one to two orders of magnitude in the aging indicators of the compensated OCXOs compared with the original without compensation.