Phase-Locked Loop (PLL) Devices

A Phase-Locked Loop (PLL) device is a closed-loop electronic circuit that controls an oscillator so that it provides an output signal that maintains a constant phase angle with respect to a reference signal, which can range from a fraction of a Hz to many GHz. It is one of the most widely used linear IC's for communications applications today, having the capability to do one or more of the following: 1) compare signal frequencies; 2) synthesize an output signal that has a frequency that's equal to that of a reference signal; 3) keep another signal equal in frequency with the reference signal.

A basic PLL circuit generally consists of a phase frequency detector, a charge pump, a loop filter, a voltage-controlled oscillator (VCO), and some form of output. The oscillator generates the periodic output signal that needs to be in phase with the reference signal. If the frequency of this oscillator-generated signal lags behind that of the reference signal, the phase detector causes the charge pump to drive current into the loop filter which changes the oscillator's control voltage in such a way that the oscillator frequency is increased. By the same token, the phase detector causes the charge pump to draw current from the loop filter system to change the control voltage and slow down the oscillator if its output signal starts leading the reference signal. The loop filter also removes jitters from the charge pump to 'smoothen' the control voltage.

The VCO output stabilizes when it has already attained the same frequency and phase as the reference signal. In effect, this system ensures that the oscillator frequency gets 'locked' into the reference signal frequency. Depending on the application, the useful output derived from the PLL system would either be the output signal of the voltage-controlled oscillator or the control voltage to the oscillator.

Figure 1. Simple PLL Block Diagram

PLL devices are used heavily in communications applications, primarily for keeping a communications signal locked on a given frequency, or for generating a signal of a given frequency. For instance, almost all transceivers utilize PLL devices to synthesize the stable, high-frequency oscillations needed for radio and wireless communications. PLL's are also used in the demodulation of both AM and FM signals. In space communications, PLL devices are employed for coherent carrier tracking and threshold extension, bit synchronization, and symbol synchronization.

PLL devices are also used in the recovery of small signals that would otherwise be lost. Clock timing information from a data stream (such as from a disk drive) may also be recovered by PLL devices. Other PLL applications include microprocessor clock multipliers, modems, and various decoding circuits.

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