## 2.3 PHASE-LOCKED LOOP

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.

Figure 2-7

### VCO

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.

Figure 2-8

Expressed mathematically,

 (2.15)

where K is the...

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IC Phase-locked Loops (PLL)
IC phase-locked loops (PLL) are closed-loop frequency controls that are based on the phase difference between the input signal and the output signal of a controlled oscillator.
Frequency Synthesizers
Frequency synthesizers are electronic devices that generate frequencies by using a combination of other frequencies.
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Oscillators are devices that are used to generate repetitive signals. They produce output signals without an input signal. There are two major types of electronic oscillators: harmonic oscillators and relaxation oscillators. Harmonic oscillators produce sine wave outputs. Relaxation oscillators produce non-sine wave outputs such as square wave, rectangular wave, and sawtooth outputs.
Charge Pump Regulators
Charge pump regulators are circuits that generate higher voltages from low voltage inputs by using capacitors as storage elements. They are used in notebook computers and mobile phones.
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Power supplies are electrical devices that deliver electric power to one or several loads. They generate the output power by converting an input signal into an output signal.

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