REMOTE FREQUENCY CONTROL VIA IEEE 1588

Kun-Yuan Tu, Chia-Shu Liao2, and Shinn-Yan Lin2

1

2

Dept. of CSIE, Vanung University, Chung-Li, Tao-Yuan, Taiwan

Telecommunication Laboratories, Chunghwa Telecom Co., Ltd. Chung-Li, Taiwan

1

A new scheme of remote frequency control based on the IEEE 1588 standards, a precision time synchronization protocol (PTP) is proposed in this paper. A remote oven-controlled crystal oscillator (OCXO) is steered by integrating the PTP system, fuzzy controller and D/A converter, such that its frequency can follow a primary cesium atomic clock used as the master clock of the PTP system. Experimental results show that for averaging times of one day, the frequency stability of the OCXO can be improved from a few parts in 109 to 1012.

Precise frequency sources play important roles in telecommunication, measurement, instrumentation, control systems, navigations, power systems and in numerous other applications [1][7][8][10]. These applications have a number of requirements that need to be met by a clock synchronization technology. In particular, timing accuracies are often in the sub-microsecond range. IEEE 1588 addresses the clock synchronization requirements of measurement and control systems. It is a protocol designed to synchronize real-time clocks in the nodes of a distributed system that communicate using a network [3][5][6][9]. This study implements a PTP system by using two IEEE 1588 interface cards. Within a subnet, the PTP system can achieve the synchronization accuracy in the sub-microsecond range. In addition, the performance of the remote OCXO clock can be improved more than 3 orders by integrating the PTP system, fuzzy control and D/A converter.

The functional block diagram of our system is shown in Fig. 1. It consists of the master station and the remote station. The master station contains PC with PTP interface and a cesium atomic clock, which is used as the master clock of the PTP system. The 1PPS of the cesium clock is connected to the PTP interface. The remote station includes low-cost OCXO, D/A converter and PC with PTP interface. Through the packet network, the slave clock of the

PTP system is synchronized with the master clock such that it can generate synchronized clock and trigger signal. The frequency offsets between the slave clock and the OCXO are measured by a time interval counter (TIC). In our system, the frequency offsetyτ(ti)and its change Δyτ(ti)(yτ(ti) yτ

(ti 1)) are chosen as the input variables of fuzzy controller. Through the D/A converter, the remote clock is then steered to synchronize with the primary clock.

Fig. 1 System architecture

The block diagram of the fuzzy controller applied in our system is shown in Fig. 2 [7][8]. The inference engine part in Fig. 3 contains the knowledge-base of a fuzzy controller, which is composed of two components, the data base and the fuzzy control rule base.

Fig. 2 Fuzzy control block diagram.

The input space is divided into five sets: negative big (NB), negative small (NS), zero (ZE), positive small (PS) and positive big (PB) for a frequency offset or its change.

1-4244-2399-6/08/$20.00 ©2008 IEEE