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Introduction to RF & Wireless Communications Systems
Publish Date: Apr 05, 2011 | 133 Ratings | out of 53.85
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
heory 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 , or for more
RF tutorials, refer to the .NI Measurement Fundamentals Main page
NI RF Fundamentals Main subpage
Table of Contents
Marconi and the First Wireless Transmissions
What is RF?
Why Operate at Higher Frequencies?
Frequency Shifting through Frequency Mixing
Looking for more RF Basics?
Relevant NI Products
Conclusions
1. Marconi and the First Wireless Transmissions
Radio Frequency (RF) and wireless have been around for over a
century with Alexander Popov and Sir Oliver Lodge laying the
groundwork for Guglielmo Marconis wireless radio developments
in
he early 20th century. In December 1901, Marconi performed his
most prominent experiment, where he successfully transmitted Morse
code from Cornwall, England, to St Johns, Canada.
2. What is RF?
RF itself has become synonymous with wireless and high-frequency
signals, describing anything from AM radio between 535 kHz and 1605
kHz to computer local area networks (LANs) at 2.4 GH
However, RF has traditionally defined frequencies from a few kHz
to roughly 1 GHz. If one considers microwave frequencies as RF,
this range extends to 300 GHz. The following two tables outline
he various nomenclatures for the frequency bands. The third
table outlines some of the applications at each of the various
frequency bands.
Table 1: Frequency Band Designations
Table 1 shows a relationship between frequency (f) and
wavelength (). A wave or sinusoid can be completely described by
either its frequency or its wavelength. They are inversely
proportional t
each other and related to the speed of light through a
particular medium. The relationship in a vacuum is shown in the
following equation:
where is the speed of light. As frequency increases, wavelength
decreases. For reference, a 1 GHz wave has a wavelength of roughly
1 foot, and a 100 MHz wave has a wavelength of roughly c
feet.
Table 2: Microwave Letter Band Designations
Table 3: Frequency Applications and Allocations in the U.S.
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RF measurement methodology can generally be divided into three
major categories: spectral analysis, vector analysis, and network
analysis. Spectrum analyzers,
which provide basic measurement capabilities, are the most
popular type of RF instrument in many general-purpose applications.
Specifically, using a spectrum
analyzer you can view power-vs-frequency information, and can
sometimes demodulate analog formats, such as amplitude modulation
(AM), frequency modulation
(FM), and phase modulation (PM).
Vector instruments include vector or real-time signal analyzers
and generators. These instruments analyze and generate
broadband waveforms, and capture time, frequency, phase, and
power information from signals of interest. These instruments
are
much more powerful than spectrum analyzers and offer excellent
modulation control and signal analysis.
Network analyzers, on the other hand, are typically used for
making S-parameter measurements and other characterization
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measurements on RF or high-frequency components. Network
analyzers are instruments that correlate both the generation
and
analysis on multiple channels but at a much higher price than
spectrum analyzers and vector signal generators/analyzers.
3. Why Operate at Higher Frequencies?
From Table 3 we notice that the frequency spectrum is quite
fragmented and dense. This encompasses one of the reasons that we
are constantly pushing applications into higher and higher
frequencies. However, some of the other reasons accounting for
this push into higher frequencies include efficiency in
propagation, immunity to some forms of noise and impairments as
well as th
size of the antenna required. The antenna size is typically
related to the wavelength of the signal and in practice is usually
wavelength.
This leads to a very interesting question. Typically, data is
structured and easily represented at low frequencies; how can we
represent it or physically translate it to these higher RF
frequencies? F
example, the human audible range is from 20 Hz to 20 kHz.
According to the Nyquist theorem, we can completely represent the
human audible range by sampling at 40 kHz or, more precisely,
at
44.1 kHz (this is where stereo audio is sampled). Cell phones,
however, operate at around 850 MHz. How does this happen?
4. Frequency Shifting through Frequency Mixing
Much of the study of RF and high-frequency measurements occurs
in the frequency domain. There is a duality between the time-domain
functions and those samefunctions represented in the
frequency-domain. Figure 1 depicts frequency shifting the human
audible range to transmit through cellular frequencies. The
most
common way to frequency shift is called mixing, which is
equivalent to multiplying your signal by a sinusoidal signal. The
following mathematical trigonometric
identity demonstrates this fact.
Therefore, by beating two sine waves against each other, you get
both sum and difference frequencies. You can shift an entire signal
to a new frequency range
(either up or down in spectrum) by selecting the appropriate
value of . In addition, any signal can be represented as the sum of
sinusoidal signals of different
frequencies. Thus, shifting a signal simply applies the
multiplication to all its sinusoidal components.
Figure 1: Frequency Shifting the Human Audible Range to the
Cellular Range
: The process of taking audible sound and transmitting over
cellular frequencies (850 MHz) includes more than simply mixing. To
make the signal more resistant to noise and otherNOTE
impairments, the process includes encoding and modulating the
data as well as perhaps using multiple stages of mixing instead of
a single stage.
5. Looking for more RF Basics?
Engineers who are new to RF or looking for a a refresher course
can attend a 3-Day National Instruments RF Fundamentals Training
Course to explore traditional
measurements, learn about digital and analog modulation, examine
modern system-level tests such as BER, MER, and EVM, and more.
Learn More about the 3-Day Course >>
6. Relevant NI Products
Customers interested in this topic were also interested in the
following NI products:
NI RF & Communications Platform
NI 5663 6.6 GHz RF Vector Signal Analyzer
NI 5673 6.6 GHz RF Vector Signal Generator
NI 5660 2.7 GHz RF Vector Signal Analyzer
NI 5671 2.7 GHz RF Vector Signal Generator
NI RF Switch Hardware
NI Spectral Measurements Toolkit Software
NI Modulation Toolkit Software
7. Conclusions
This document is meant to provide a brief overview and
introduction to RF, wireless, and high-frequency signals.
Additional information can be found in Teaching and Research
Resources for RFand Communications.
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
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