Download Wireless Signals

Wireless Signals
Profs. Chuah and Kishore
EMC 165
Spring 2005
Radio Waves

Radio waves carry music, conversations, pictures,
and data invisibly through the air over millions of
miles.

Radios can transmit and/or receive radio waves.
They’re Everywhere
All wireless technologies use radio waves to
communicate.


Some examples:




AM/FM Radios
Cell Phones
GPS Receivers
Wi-Fi

Some other examples:






Cordless Phones
Garage Door Openers
Radio-Controlled Toys
Television Broadcasts
Ham Radio
Etc.
Some Other (not-so-obvious) Examples





Radar (police, air traffic control, military applications)
Microwave ovens
Navigation systems
Airplanes (contain dozen different radio systems)
Baby monitors
Simple, Cheap Radio






Take a fresh 9V battery and a coin
Find AM radio and tune to an area of dial where there
is static
Hold battery near antenna
Quickly tap two terminals of battery using coin
Radio crackles due to connection/disconnection by
coin.
Battery/coin combo is a
radio transmitter!
Simple, Cheap Radio (Cont’d)





Battery/coin radio transmits static.
Transmits only over short distance.
Could use static to tap Morse code messages and
communicate over several inches.
May not be practical but is a simple example of a
functional radio transmitter.
Why does it work? We’ll go over this next.
How Simple Transmitters Work

Battery: connect to ends (terminals) of a battery
with a piece of wire. Result: battery sends
electricity (stream of electrons) thru the wire. There
is voltage in the wire.

When start electrons moving (create current in wire),
a magnetic field is created around the wire.

Magnetic field is strong enough to affect a compass.
Simple Transmitter (Cont’d)
Result of Simple Transmitter



Extend the experiment: take another wire, place it
parallel to the battery wire but a few centimeters
away from it.
Connect a sensitive voltmeter to this new wire.
Voltmeter will give a measure amount of electricity in
new wire.
When you connect/disconnect the battery wire, you
will read a small voltage and current in the second
wire.
Simple Transmitter (Cont’d)

Observation: by changing the magnetic field in one
wire, we can cause an change in the electric field in
the second wire.

Specifically,





Battery creates electron flow in one wire
Moving electrons create magnetic field around one wire
Magnetic field stretches out to second wire
Electrons flow in second wire whenever magnetic field in first
wire changes.
Electrons flow in second wire only when you
connect/disconnect battery.
Simple Transmitter (Cont’d)



We see then that a message can be converted to
Morse code and then tapped using first wire
(connect/disconnect).
This first wire is a simple transmitter.
The second wire is a receiver.
Simple Receiver

Voltage changes in second wire can be used to
determine Morse code taps.

Morse code message is then decoded to get the
message from the first wire.

Result: communication of message occurs
“wirelessly” (over a couple of centimeters) from the
first wire to the second wire.
Creating Simple Transmitters



When we change current in first wire in time, a
current is induced in second wire.
To create any radio transmitter, create a rapidly
changing electric current in a wire.
This can be done by connecting/disconnecting a
battery. When connected, voltage in wire is 9V.
When disconnected voltage in wire is 0V. Result:
square wave signal.
9V
0V
Time (s)
Sine Wave: Better than Square Wave

A better alternative to square wave is a continuously
varying electric current in a wire.

Simplest and smoothest continuously varying wave
is a sine wave:
A simple radio transmitter created by running a sine
wave thru a wire.
Sine Waves

By sending sine wave electric current to antenna,
you can transmit sine wave into space.

All radios today, however, transmit continuous sine
waves to transmit information (audio, video, data).

Why sine waves?
To allow many different people/devices to use radio
waves at the same time.

Sine Waves: Frequency
One cycle of a sine wave is:
Sine wave can
be written as sin(2pt/T)
T seconds
When one cycle of a sine wave lasts T seconds, we
say that the sine wave as frequency 1/T Hertz (Hz).
1 Hz = 1 cycle/second.
More on Sine Waves

If there was a way to see radio waves, we would find
there are literally thousands of different radio waves
(sine waves) traveling thru the air (TV broadcasts,
cell phone conversations, AM/FM broadcasts, etc.)

Each different radio signal uses a different sine
wave frequency.

Use of different frequencies help separate different
radio signals.
More on Frequency

When you listen to AM broacast, your radio is tuning
into sine waves oscillating at a frequency around
1,000,000 cycles per second.

For example, 880 on the AM dial corresponds to
listening to a radio (sine) wave that has frequency
880,000 Hz = 880 KHz.

FM signals operate in range of 10,000,000 Hz. So,
90.9 on FM dial corresponds to 90,900,000 Hz =
90.9 MHz.
Kilo, Mega, Giga, etc.
1 Hz
1000 Hz = 1 KHz (kilohertz)
1,000,000 Hz = 1 MHz (megahertz)
1,000,000,000 Hz = 1 GHz (gigahertz)
More on Radio Basics

Any radio setup has two parts: Transmitter and
Receiver

Transmitter takes some form of message
(someone’s voice, pictures for TV set, etc.) encodes
it into a sine wave and transmits it with radio waves.

Combination of encoded message on a radio wave
is commonly referred to as a signal.

Receiver receives radio waves and decodes
messages from the sine waves.

Both transmitter and receiver use antennas to
radiate and capture radio waves.
Transmitter Description
Radio Transmitter
Combine
Information
(voice message)
Radio Waves
Antenna
Sine
Wave
Transmitter generates its own sine wave using oscillators.
Receiver Description
Radio Transmitter
Antenna
Separate
Sine Wave
Information
(voice message)
Modulation

If you have a sine wave and a transmitter that is
transmitting the sine wave into space using an
antenna (more antennas later), you have a radio
station.

Problem with plain old sine wave: does not contain
information.

Sine wave has to modulated in some way so that it
contains information, e.g., voice message.
3 Basic Modulation Methods

Pulse Modulation (PM): turn sine wave on and off.
Easy way to send Morse code.
3 Basic Modulation Methods (Cont’d)

Amplitude Modulation (AM): Amplitude (peak-topeak voltage) of sine wave is changed so as to
contain information.

AM radio stations and picture part of TV signals use
amplitude modulation to encode information signal.
Example of AM
carrier = sine wave with a given frequency
3 Basic Modulation Methods (Cont’d)

Frequency Modulation (FM): Radio transmitter
changes frequency of sine wave according to
information signal.

Frequency modulation is most popular. Used by FM
radio stations, sound part of TV signal, cellular
phones, cordless phones, etc.
Frequency of Signal after Modulation

Radio wave transmitted after modulating sine wave
with information signal is not just a sine wave with
frequency f.

For example, in FM, the frequency varies around
this frequency f. For example, it may increase up to
f+Df and be as small as f-Df.

After modulating information signal, the radio wave
has some range of frequency, called the frequency
band, e.g., 2Df.

The bandwidth, width of frequency band, depends
on the information signal (voice, data bit rate, etc.)
Summary of Modulation

By modulating a sine wave at a transmitter,
information can be encoded into the radio wave.

The resulting radio wave occupies a band of
frequency, centered on the frequency of the sine
wave.

Receiver needs to demodulate the radio wave to
extract the information signal.
AM Modulation Example




Car radio is tuned to radio station, say 880 AM.
Transmitter’s sine wave is transmitting at 880,000
Hz (sine wave repeats 880,000 times per second).
DJ’s voice is modulated onto sine wave, i.e.,
amplitude of sine wave is varied as DJ’s voice
varies.
A power amplifier magnifies power of modulated sin
wave, e.g., to 50,000 Watts for a large AM station.
Antenna then sends radio waves into space. High
power amplification helps waves travel large
distances.
How do we receive AM signals?

Unless you sit right next to the transmitter, you need
an antenna to pick out the radio waves from the air.

An AM antenna is just a wire or a metal stick that
increases the amount of metal the transmitter’s
waves interact with.

Radio receiver also needs a tuner. Antenna will
receive thousands of sine waves; tuner separates
out the radio wave that the listener desires, e.g., the
radio wave transmitted at 880 KHz.
AM Reception (Cont’d)

Tuners operate using a principle called resonance.
That is, tuners resonate at and amplify one
particular frequency and ignore all other frequencies
in the air.

After tuning in, radio receiver has to extract the DJ’s
voice signal from the sine waves.

This is done using a demodulator (aka detector).
AM Reception (Cont’d)



One type of a AM detector is something called an
envelope detector. Simply, it determines the
magnitude (amplitude) of the sine wave.
An amplify magnifies this amplitude signal and then
the receiver sends the output to the car radio
speakers.
What we hear is the DJ’s voice.
What about FM?

FM reception is very similar.

Difference: FM detector outputs changes in the sine
wave frequency as opposed to amplitude.

Specifically, FM detector converts changes in sine
wave frequency into sound.

Antenna, tuner, amplifier are largely the same in FM
as in AM.
What about antennas?

Almost every radio you see (cell phones, car radio,
etc.) has an antenna.

Antennas come in all shapes and sizes. Shapes
and sizes depend on the frequency the antenna is
trying to receive.
Ranges from long stiff wire (as in car radios) to large
satellite dishes (as used by NASA).


For satellites that are millions of miles away NASA
uses antenna dishes that 200 feet wide.
More on Antennas



Often radio stations use extremely tall antenna
towers to transmit their signals.
Antenna at radio transmitter: launch radio signals
into space.
Antenna at radio receiver: pick up as
much of the transmitter’s power as
possible and feed it to the tuner.
Antennas (Cont’d)

Size of optimum radio antenna is related to
frequency of the signal antenna is trying to transmit
and/or receive.

Reason for this: speed of light and the distance
electrons can travel as a result.

Speed of light is 186,000 miles/sec (300,000
meters/sec).
Determining Antenna Size




Say you are building an antenna tower for radio
station 680 AM.
It is transmitting sine wave with frequency of
680,000 Hz.
In one cycle of sine wave, transmitter is going to
move electrons in the antenna in one direction,
switch and pull them back, switch push them out,
and switch and pull them back.
That is electrons change direction four times during
one cycle of the sine wave.
time
Antenna Size (Cont’d)

When operating at 680,000 Hz, each cycle
completes in 1/680,000 = 0.00000147 seconds.

One quarter of the cycle is 0.0000003675 seconds.

At the speed of light, electrons can travel 0.0684
miles (361 feet) in 0.0000003675 seconds.

Cell phones operate using 900,000,000 Hz; this
means that it needs antennas that are about 3
inches high.
Antenna Size (Cont’d)




Question: why aren’t car radio antennas 300 feet
high?
It would be impractical for one.
If you made car radio antenna higher, reception
would be better.
AM radio stations transmit at high powers to
compensate for the suboptimal receive antenna
heights.
Some Questions




Why do radio waves transmit away from antenna
into space at speed of light?
How can radio waves transmit millions of miles?
Doesn’t antenna only create magnetic field in its
vicinity?
How can the magnetic field variation be registered
millions of miles away?
Answer

When current enters antenna, it creates a magnetic
field around the antenna. This magnetic field
creates an electric field (voltage and current) in
another wire placed close to the antenna.

In space, magnetic field created by antenna induces
electric field in space.
This electric field induces another magnetic field in
space, which induces another electric field, …
These electric and magnetic fields (electromagnetic
fields) induce each other in space at the speed of
light in a direction away from the antenna.

