# Phase-Locked Loops

Publish Date: Nov 05, 2014 | 9 Ratings | 2.22 out of 5 |  PDF

## Overview

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial covers an introduction to RF, wireless, and high-frequency signals and systems.

For the complete list of tutorials, return to the NI Measurement Fundamentals main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit www.ni.com/rf.

A phase-locked loop (PLL) is a feedback control circuit that synchronizes the phase of a generated signal with that of a reference signal. It is important to keep in mind that a PLL can generate signals that are only integer multiples of the reference signal.

### 1. Phase-Locked Loop Operation

The basic components of a phase-locked loop (PLL) are a stable reference oscillator, a phase detector, a frequency divider, a voltage-controlled oscillator (VCO), amplifiers, and filters. The following figure shows a simplified PLL block diagram.

Figure 1. PLL Block Diagram

PLLs work by comparing the generated signal phase to the reference oscillator signal phase and then using the difference to adjust the generated signal. To compare the two waves, both signals must be of the same frequency. The frequency divider is used to divide down the generated signal. Because the frequency divider can divide down only an integer number of times, the generated signal must be an integer multiple of the reference oscillator signal. The phase detector returns a voltage depending on the phase difference between the two waves. This voltage is amplified, filtered, and then fed into the VCO. The VCO adjusts the signal it outputs based on this voltage.

### 2. Conclusion

Phase-locked loops play a vital part in many modern-day circuits. They are used to demodulate amplitude and frequency modulated signals, synchronize clocks, recover small signals from noise, and they are used in devices such as dual-tone multiple frequency decoders and modems.

For the complete list of tutorials, return to the NI Measurement Fundamentals main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit  www.ni.com/rf.

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