Design And Implementation Of Frequeney Synthesis Module For Frequency Transfer Over Optical Fiber

In recent years, the technology of atomic clock develops rapidly, now the commercial cesium atomic clock achieve the stability of 10-16 per day. The traditional satellite-based frequency transfer technique can only reach the stability of 10-15 per day which can not satisfy the need of high stability comparison of frequency signals. Transfer over the optical fiber has the adventages of high bandpass, low power loss and so on. It has the potential to realize the high stability frequency transmission and has attracted widespread attention at home and abroad.The frequency of high quality source signal is generally between 10 MHz and 100 MHz, whose phase noise is better than the noise floor of optical fiber links. In order to overcome the problem and improve the detection sensitivity of the compensation system, we need to up-convert the frequency of the original signal and down-convert it to the initial frequency at the remote end. So it can be send to the users and use it to evaluate the performance of the optical compensation system.In this paper, we use the PLL circuit to achieve up-conversion of the standard signal from 10 MHz to 1GHz and design the FPGA control circuit to realize the automatic programing. At the remote end, we use two stages of frequency division to realize changing the frequency from 1GHz to 10 MHz. The result of the experiment shows that, after the up-conversion and down-conversion, the phase noise of the signal of at the offset frequency of 1Hz is only worse 3dBc/Hz than the initial signal and at the offset frequency of 100 KHz it is better about 2dBc/Hz. Compared to frequency conversion effect of frequency synthesizer of SMF100 A it increases nearly 20dBc/Hz at the offset frequency of 1Hz which is an important index of the phase noise for the compensation system. The up-convert and down-convert frequency modules are added to the frequency transmission system of 20 km optical fiber link and compensated by the optical phase noise compensation system of our labtoary, finally it achieves the stability of 4 ×10-13 per second and 1.6× 10-15 per 1000 seconds.Firstly, this paper briefly introduces all kinds of noise and the basic components of the optical fiber link, then analyzes the effects of the phase noise of the laser and the change of environment temperature on the frequency stability of optical fiber link.Secondly, we design of the up-conversion module and simulate the result. In order to overcome the problem that the registers cannot save data when it is powered off, we design the FPGA control circuit to write the data to the PLL circuit automatically. At the same time, we use two orders of frequency division to realize the 100 frequency division.Finally, we test the frequency and the phase noise of output signal, and access the frequency stability of the optical frequency transmission system with and without compensation. At last we transfer the time and frequency signal over 100 km optical fiber link at the same time and analyse the result of the experiment.