TABLE OF CONTENTS A phase-locked loop (PLL) is basically an electronic feedback loop which consists of a phase detector (also called a phase comparator), a low-pass filter, a dc amplifier, and a voltage-controlled oscillator (VCO). The basic block diagram of a PLL is shown in Fig. 2-7. The functional blocks between the input and output are considered to be in the forward path of the loop, whereas the single connection between the VCO and phase detector is the feedback path. Before examining the overall operation of a PLL, let us examine each block separately.
A voltage-controlled oscillator is a free-running oscillator whose frequency of operation is controlled by an external dc bias voltage. In essence therefore, the input to a VCO is a dc voltage and the output is a frequency. Figure 2-8 shows the transfer curve for a typical VCO, which is a graph of the VCO's output frequency versus its dc input voltage. The output frequency with a dc input of 0 V is called the VCO's natural frequency and is designated f N. Any change in the output frequency caused by a change in the dc input voltage is called frequency deviation and is designated ? f. The transfer function or conversion gain K of the VCO can be expressed as the amount of frequency deviation ? f per unit change ? v in dc input voltage.
Expressed mathematically,
| (2.15) |  |
where K is the...
