ned to an input clock signal. As shown in Figure 5-3, in its simplest form, a PLL circuit consists of three basic elements, as described in the list that follows. Output Input clock signal Phase comparator Loop filter signal Basic block diagram of PLL Figure 5-3 PLL Building Blocks Voltage-controlled oscillator (VCO): A special type of oscillator that changes frequency with changes in an input voltage (in this case from the loop filter). The frequency of the VCO with a nominal control signal applied is called the free-running frequency, indicated by the symbol Phase comparator: Compares the phase of two signals (input clock and local oscillator) and generates a voltage according to the phase difference detected between the two signals. This output voltage is fed into the loop filter. PLL loop filter: Primary function is to detect and filter out undesired phase changes passed on by the phase comparator in the form of voltage. This filtered voltage is then applied to the VCO to adjust the frequency. It is important to note that if the voltage is not filtered appropriately, it will result in a signal that exactly follows the input clock, inheriting all the variations or errors of the input clock reference. Thus, the properties of the loop filter directly affect the stability and performance of a PLL and the quality of the output signal. When a PLL is initially turned on, the VCO with a nominal control signal applied will provide its free-running frequency (fo). When fed an input signal, the phase comparator measures the phase difference compared to the VCO signal. Based on the size of the phase difference between the two signals, the phase comparator generates a correcting voltage and feeds it to the loop filter. The loop filter removes (or filters out) the noise and passes the filtered voltage to the VCO. With the new voltage applied, the VCO output frequency begins to change. Assuming the input signal and VCO frequency are not the same, the phase comparator sees this as a phase shift, and the output of the loop fi