Synchronization of Digital Telecommunications Networks

7

TIME AND FREQUENCY MEASUREMENT TECHNIQUES IN TELECOMMUNICATIONS

Several experimental set-ups have been contrived in the last few decades to measure the accuracy and stability of oscillators in both time and frequency domains [7.1]–[7.10]. in the past, measurement techniques in the frequency domain have been the most common, being based on very low-noise analogue electronics. Nowadays, the availability of high-resolution time counters has made digital time-domain measurements to be more widely used. In particular, this became the standard choice in telecommunications.

The goal of this chapter is to describe experimental methods suitable for time and frequency measurements in telecommunications. To begin, several basic concepts are introduced: for example, how to estimate frequency offset and drift of a clock, to assess the statistical confidence of Allan variance estimates and to separate the noise of the Clock Under Test (CUT) from that of the Reference Clock (RC). Moreover, the impact of the measurement configuration and of the TE sampling period on the measured quantities is discussed. A brief survey on some of the most common devices and instruments used for time and frequency measurements is also provided.

Then, the main techniques for measuring time and frequency stability are overviewed, from direct digital measurement to heterodyne and homodyne methods for sensitivity enhancement. Several test set-ups are described.

Finally, clock stability measurement in telecommunications ...