o a (hardware) clock subsystem that generates a clocking signal, but often engineers refer to it simply using the term clock. You might hear the statement, "the clock on node A is not synchronized to a reference clock," whereas the real meaning of clock in this sentence is that the clock signals are not synchronized. So, clock signal and clock are technically different terms with different meanings, but because the common usage has made one refer to the other, this chapter will also use the term clock to refer to a clock signal. 5.1.1 Oscillators An electronic oscillator is a device that "oscillates" when an electric current is applied to it, causing the device to generate a periodic and continuous waveform. This waveform can be of different wave shapes and frequencies, but for most purposes, the clock signals utilized are sine waves or square waves. Thus, oscillators are a simple form of clock signal generation device. There are a few different types of oscillators (as described in "Chapter 3. Synchronization and Tim- ing Concepts"), but in modern electronics, crystal oscillators (referred to as XO) are the most com- mon. The crystal oscillator is made up of a special type of crystal, which is piezoelectric in nature, which means that when electric current is applied to this crystal, it oscillates and emits a signal at a very specific frequency. The frequency could vary from a few tens of kHz to hundreds of MHz de- pending on the physical properties of the crystal. Quartz is one example of a piezoelectric crystal and is commonly used in many consumer devices, such as wristwatches, wall clocks, and computers. Similar devices are also used in networking devices such as switches, routers, radio base stations, and SO on. Figure 5-1 shows a typical crystal commonly utilized in such a device. Figure 5-1 16-MHz Crystal Oscillator The quartz that is being used in crystal oscillators is a naturally occurring element, although manu- facturers grow their own for purity. The natural frequency of the clock signal generated b