Upload
naren-kumar
View
213
Download
0
Embed Size (px)
Citation preview
7/31/2019 RF Communication
1/9
CHAPTER 10
BRIEF DESCRIPTION ABOUT RF COMMUNICATIONS
Radio Frequency (RF) and wireless have been around for over a century withAlexander Popov and Sir Oliver Lodge laying the groundwork for Guglielmo
Marconis wireless radio developments in the 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.
General physics of radio signals
RF communication works by creating electromagnetic waves at a source andbeing able to pick up those electromagnetic waves at a particular destination. These
electromagnetic waves travel through the air at near the speed of light. The
wavelength of an electromagnetic signal is inversely proportional to the frequency;
the higher the frequency, the shorter the wavelength.
Frequency is measured in Hertz (cycles per second) and radio frequencies
are measured in kilohertz (KHz or thousands of cycles per second), megahertz
(MHz or millions of cycles per second) and gigahertz (GHz or billions of cycles per
second). Higher frequencies result in shorter wavelengths. The wavelength for a 900
MHz device is longer than that of a 2.4 GHz device.
In general, signals with longer wavelengths travel a greater distance and
penetrate through, and around objects better than signals with shorter wavelengths.
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 GHz. 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.
7/31/2019 RF Communication
2/9
Radio frequency (RF) is a frequency, or rate of oscillation, of electromagnetic
radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the
frequency of alternating current electrical signals used to produce and detect radio
waves. Since most of this range is beyond the vibration rate that most mechanicalsystems can respond to, RF usually refers to oscillations in electrical circuits. The
following tables outline the various nomenclatures for the frequency bands.
Frequency Band Designations:Name Symbol Frequency Wavelength Applications
Extremely
low
frequency
ELF 330 Hz 10010 Mm
Directly audible when converted
to sound (above ~20 Hz),
communication with submarines
Super low
frequencySLF 30300 Hz 101 Mm
Directly audible when converted
to sound, AC power grids (50
60 Hz)
Ultra low
frequencyULF 3003000 Hz 1000100 km
Directly audible when converted
to sound, communication within
mines
Very low
frequencyVLF 330 kHz 10010 km
Directly audible when converted
to sound (below ~20 kHz; or
ultrasoundotherwise)
Low
frequencyLF 30300 kHz 101 km
AM broadcasting, navigational
beacons, and amateur radio.
Medium
frequencyMF 3003000 kHz 1000100 m
Navigational beacons, AM
broadcasting, amateur radio,
maritime and aviation
communication
High
frequencyHF 330 MHz 10010 m
Short wave, amateur radio,
citizens' band radio, sky wave
propagation.
7/31/2019 RF Communication
3/9
Very high
frequencyVHF 30300 MHz 101 m
FM broadcasting, amateur
radio, broadcast television,
aviation, GPR, MRI.
Ultra high
frequencyUHF
300
3000 MHz10010 cm
Broadcast television, amateur
radio, mobile telephones,
cordless telephones, wireless
networking, remote keyless
entry for automobiles,
microwave ovens, GPR
Super high
frequency SHF 330 GHz 101 cm
Wireless networking, satellite
links, amateur radio, microwave
links, satellite television, door
openers
Extremely
high
frequency
EHF 30300 GHz 101 mm
Microwave data links, radio
astronomy, amateur radio,
remote sensing, advanced
weapons systems, advanced
security scanning
The above Table 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 to 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 c 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 10 feet.
7/31/2019 RF Communication
4/9
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 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.
Why Operate at Higher Frequencies?
From the frequency spectrum we notice that it is quite fragmented and dense.
This encompasses one of the reasons that we are constantly pushing applications
7/31/2019 RF Communication
5/9
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 the size of the antenna required.
The antenna size is typically related to the wavelength of the signal and in practice isusually 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? For 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 this happens is 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 same functions 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
7/31/2019 RF Communication
6/9
frequencies. Thus, shifting a signal simply applies the multiplication to all its
sinusoidal components.
Working of RF communication system
Imagine an RF transmitter wiggling an electron in one location. This wiggling
electron causes a ripple effect, somewhat a kind of dropping a pebble in a pond. The
effect is an electromagnetic (EM) wave that travels out from the initial location
resulting in electrons wiggling in remote locations. An RF receiver can detect this
remote electron wiggling.
The RF communication system then utilizes this phenomenon by wigglingelectrons in a specific pattern to represent information. The receiver can make this
same information available at a remote location; communicating with no wires.
In most wireless systems, a designer has two overriding constraints: it must
operate over a certain distance (range) and transfer a certain amount of information
within a time frame (data rate). Then the economics of the system must work out
(price) along with acquiring government agency approvals (regulations and
licensing).
RANGE
In order to accurately compute range it is essential to understand a few
terms: dB - Decibels
Decibels are logarithmic units that are often used to represent RF power. Toconvert from watts to dB: Power in dB = 10* (log x) where x is the power in watts.
Another unit of measure that is encountered often is dBm (dB mill watts). The
conversion formula for it is Power in dBm = 10* (log x) where x is the power in mill
watts.
7/31/2019 RF Communication
7/9
Line-of-site (LOS)
Line-of-site when speaking of RF means more than just being able to see the
receiving antenna from the transmitting antenna. In, order to have true line-of-site no
objects (including trees, houses or the ground) can be in the Fresnel zone. The
Fresnel zone is the area around the visual line-of-sight that radio waves spread out
into after they leave the antenna. This area must be clear or else signal strength will
weaken. There are essentially two parameters to look at when trying to determine
range.
1) Transmit Power
Transmit power refers to the amount of RF power that comes out of the
antenna port of the radio. Transmit power is usually measured in Watts, mill watts or
dBm.
2) Receiver sensitivity
Receiver sensitivity refers to the minimum level signal the radio can
demodulate. It is convenient to use an example with sound waves; Transmit power
is how loud someone is yelling and receive sensitivity would be how soft a voice
someone can hear. Transmit power and receive sensitivity together constitute what
is know as link budget. The link budget is the total amount of signal attenuation you
can have between the transmitter and receiver and still have communication occur.
Example:
Maxstream 9XStream TX Power: 20dBm
Maxstream 9XStream RX Sensitivity: -110dBmTotal Link budget: 130dBm.
For line-of-site situations, a mathematical formula can be used to figure out
the approximate range for a given link budget. For non line-of-site applications range
7/31/2019 RF Communication
8/9
calculations are more complex because of the various ways the signal can be
attenuated.
RF communications and data rate
Data rates are usually dictated by the system - how much data must be
transferred and how often does the transfer need to take place. Lower data rates,
allow the radio module to have better receive sensitivity and thus more range. In the
XStream modules the 9600-baud module has 3dB more sensitivity than the 19200-
baud module. This means about 30% more distance in line-of-sight conditions.
Higher data rates allow the communication to take place in less time, potentially
using less power to transmit.
Radio communication
In order to receive radio signals, for instance from AM/FM radio stations, a
radio antenna must be used. However, since the antenna will pick up thousands of
sine waves at a time, a radio tuner is necessary as well to tune in to a particular
frequency (or frequency range). This is typically done via a resonator (in its simplestform, a circuit with a capacitor and an inductor). The resonator is configured to
resonate at a particular frequency (or frequency band), thus amplifying sine waves
at that radio frequency, while ignoring other sine waves. Usually, either the inductor
or the capacitor of the resonator is adjustable, allowing the user to change the
frequency it resonates at.
Special properties of RF electrical signals
Electrical currents that oscillate at RF have special properties not shared by
direct current signals. One such property is the ease with which they can ionize air
to create a conductive path through air. High frequency units used in electric arc
welding, although strictly speaking these machines do not typically employ
7/31/2019 RF Communication
9/9
frequencies within the HF band, exploit this property. Another special property is an
electromagnetic force that drives the RF current to the surface of conductors, known
as the skin effect. Another property is the ability to appear to flow through paths that
contain insulating material, like the dielectric insulator of a capacitor. The degree ofeffect of these properties depends on the frequency of the signals.