RF Communication

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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.


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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


to sound, AC power grids (50

60 Hz)

Ultra low

frequencyULF 3003000 Hz 1000100 km


to sound, communication within

mines

Very low

frequencyVLF 330 kHz 10010 km


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.


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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.


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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 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


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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


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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.


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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


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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


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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.

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RF Communication