Research On Stable Frequency Source And Time & Frequency Transfer Technology

The development of high precise aerospace measuring and controlling system and radar system will have great demand for frequency source with high frequency and low phase noise, and will be confronted with the major challenge accurately transferring synchronized time and standard frequency. Research of high quality frequency source genatation and high precision time and frequency transferring technology based on optical fiber has important theory meaning and applied value.To meet the demands of high quality frequency source, we propose a frequency drift compensation method to improve the long-term stability of tunable optoelectronic oscillator (OEO). A low-frcqucncy sinusoidal signal is introduced into the feedback loop to measure the phase variation signal induced by the fiber loop delay variation. The delay variation signal is extracted by the phase discriminator, and then used to compensate the fiber loop delay variation by altering the wavelength of a tunable laser. The monitoring and compensation precision are greatly improved. A highly stable tunable OEO oscillating from 2.4 GHz to 10 GHz is obtained. The long-term stability of 2.4 GHz oscillating signal has reached 5×10-9 after 1000-s averaging time, which is two orders of magnitude better than free-running OEO. Less than-120 dBc/Hz at 10 kHz offset frequency from 2.4 GHz carrier in both uncompensated and compensated cases is obtained respectively. These results prove that the short-term stability of the generated microwave signal does not deteriorate while its long-term stability is improved.To solve the large compensating range and limited stable frequency access problem, we propose two fiber-based stabel frequency transfer methods. (1) Stable frequency delivery by rapid and endless post error cancellation. By round-trip transferring a radio frequency standard whose frequency is half of the received signal, the phase fluctuation can be corrected by simple mixing, without phase discrimination and tracking, which is expected to simplify the antenna without remote down-conversion and digitalization. (2) Multiple-access stable frequency delivery. Stably distributed frequency signal can be regenerated at arbitrary node along the entire 30-km fiber link. Neither phase discrimination nor dynamic tracking part is involved, which results in a rapid and robust operation.To fulfill the multipul projects control and synchronization requeirmets of future antenna array, we propose two wideband frequency transfer methods. (1) Broadband downlink stable frequency delivery. By exploiting fiber’s chromatic dispersion, a calibration signal modulated on a computer-controlled tunable laser from the center is used for link-length compensation, a broadband frequency independent stabilized fiber link is fulfilled. Radio frequency was transferred 10 km and a phase fluctuation compression factor greater than 30 is achieved both for 1205 MHz in 3000 seconds and for 2460 MHz in 5000 seconds. (2) Time and frequency transfer simultaneously. By integrating time transfer and frequency transfer, we study multiplexing method and modulation technology to simultaneously transfer time and frequency. Stable fiber-optic time transfer by active radio frequency phase locking. A stability of the time signal transfer over 50-km fiber with a time deviation of 40 ps at 1-s average and 2.3 ps at 1000-s average is demonstrated. The performance of the RF reference delivery with an Allan Deviation of 2×10-15 at 1000-s average is achieved.