NI-Tutorial-4805 - Document Type: Tutorial NI Supported:...

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1/5 1. 2. 3. 4. 5. : Document Type Tutorial : Yes NI Supported : Apr 5, 2011 Publish Date What is I/Q Data? 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 a brief overview and introduction to I/Q data as it relates to RF and wireless systems. For the complete list of tutorials, return to the , or for more RF tutorials, refer to the . NI Measurement Fundamentals main page NI RF Fundamentals Main subpage Put in its simplest form, I/Q data shows the changes in magnitude (or amplitude) and phase of a sine wave. If amplitude and phase changes are made in an orderly, predetermined fashion, one can use these amplitude and phase changes to encode information upon a sine wave; a process known as modulation. Modulation is the process of changing a higher frequency carrier signal in proportion to a lower frequency message, or information, signal. I/Q data is highly prevalent in RF communications systems, and more generally in signal modulation, because it is a convenient way to modulate signals. This discussion covers the theoretical background of I/Q data as well as practical considerations which make the use of I/Q data in communication so desirable. Table of Contents Background on Signals I/Q Data in Communication Systems So Why Use I/Q Data? Related NI Hardware Conclusions Background on Signals Signal modulation involves changes made to sine waves in order to encode information. The mathematical equation representing a sine wave is as follows: Figure 1: Equation of a Sine Wave If we think about possible sine wave parameters that we can manipulate, the equation above makes it clear we are limited to making changes to the amplitude, frequency, and phase of a sine wave to encode information. Frequency is simply the rate of change of phase of a sine wave (frequency is the first derivative of phase), so these two components of the sine wave equation can be collectively referred to as the phase angle. Therefore, we can represent the instantaneous state of a sine wave with a vector in the complex plane containing amplitude (magnitude) and phase coordinates in a polar coordinate system. Figure 2. Polar Representation of a Sine Wave In the graphic above, the distance from the origin to the black point represents the amplitude (magnitude) of the sine wave, and the angle from the horizontal axis represents the phase. Thus, the distance from the origin to the point will remain fixed as long as the amplitude of the sine wave is not changing (modulating). The phase of the point will change according to the current state of the sine wave. For example, a sine wave with a frequency of 1 Hz (2π radians/second) rotates counter-clockwise around the origin at a rate of one revolution per second. If the amplitude doesn't change during one revolution, the dot maps out a circle around the origin with radius equal to the amplitude along which the point will travel at a rate of one cycle per second. Because phase is a relative measurement, imagine that the phase reference used is a sine wave of frequency equal to the sine wave that is being represented by the amplitude and phase points. If
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This note was uploaded on 07/12/2011 for the course RF 121 taught by Professor Harsha during the Spring '11 term at uvt.

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NI-Tutorial-4805 - Document Type: Tutorial NI Supported:...

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