1.1 Clocks in Digital Systems

Clocks mean different things to different people. The authors of this book consider a clock to be a digital signal that consists of periodic pulses, or an electronic device or circuit used to create such signals. It is not, however, merely a question of a separate signal or an isolated piece of equipment. From a communication systems point of view, a clock carries a vital nutrient characteristic called timing. This clock timing is used to control digital telecommunications, further relying on subsystems that realize various logical operations. In addition, the functions of different digital subsystems can only be coordinated with each other if they share the same clock. Information can be processed reliably in the form of digital data when the associated clock performance and timing transfer are adequate. For us the clock is an indispensable element in digital communication systems [1].

The fundamental electronic clock signal is typically produced by an oscillator, which relies on the performance of a suitable resonator as a stabilizing element [2]. Most portable devices and a number of fixed systems utilize various types of quartz oscillators [3], and their output signal normally has just the same frequency as marked on the crystal device. When better intrinsic stability or smaller absolute frequency errors are needed, we may use atomic resonators [4]. Rubidium and cesium clocks show better aging characteristics than simple crystal oscillators but may be quite expensive. Additional technologies include H-masers and optical frequency standards, but their use is...