4.1 Basic Mechanisms in Phase Synchronization

The characteristics of complete PLLs depend on their elements. The basic blocks are a phase comparator, a filter, and a controlled oscillator. Even complex PLLs share this configuration. Moreover, since the PLL output is synchronized to the input signal, the characteristics of the input signal also influence PLL performance.

Figure 4.1 shows the basic configuration of the PLL. The phase difference, measured with a phase comparator, is fed back to a controlled oscillator. The output of this oscillator is altered in order to hold the phase difference below a defined value. A filter is added between the phase comparator and the controlled oscillator (compare with Figure 2.3, which demonstrates a basic phase synchronization system). Figure 4.2 shows an example of the phase synchronization process. Here, the output frequency of the controlled oscillator is initially lower than that of the input signal, and the phase difference changes over time, as shown in area (a) of Figure 4.2. The output frequency is driven higher than the input frequency, and the phase difference changes as shown in area (b) of Figure 4.2. If this control action is performed well, the output frequency of the controlled oscillator coincides with the frequency of the input signal. This holds the phase difference within the required range. Phase synchronization is achieved in area (c) of Figure 4.2. Figure 4.2 depicts digital control. The oscillator frequency is discretely controlled. In analog control, the phase difference and the output frequency smoothly change as...