Wireless Communications

1
A radio transmitter is an electronic device which
converts sound or data into waves of electrical
current at the transmitters operating frequency.
These current waves are converted to
electromagnetic waves (radio waves) by a device
called an antenna. A radio receiver also has an
antenna, which converts electromagnetic waves to
current waves. The receiver then converts
current waves which occur at the frequency the
receiver is tuned to into their original form
sound, data, etc.
Radio Frequency Current Waves
Antenna
Radio Waves
Microphone
Transmitter
Audio Frequency Voltage Waves
Antenna
Receiver
Audio Frequency Voltage Waves
Radio Frequency Current Waves
Speaker
2
Weve seen how the transmitting antenna converts
an electrical waveform created by the transmitter
to radio waves, and how the receiving antenna
converts radio waves to an electrical waveform to
be processed by the receiver. Now lets look at
how the transmitter creates the electrical
waveform.
3
A radio transmitter exists in order to send
information from one point to another without
wires. This information can take many forms
Speech, music, or other sounds Data (information
in digital form), or images (television). Lets
consider speech. Speech consists of complex
waveforms which may include many frequencies
simultaneously, not just a single pitch. Bird
songs or whistling consist of a single frequency
at any one time. Speech is obviously more
complex. A speech waveform begins as pressure
waves traveling through the air, and is converted
to an electrical waveform by a microphone (a type
of transducer). The pressure waves cause a
diaphragm within the microphone to vibrate. This
vibration is a copy of the sound waves striking
the diaphragm. The motion of the vibrating
diaphragm is converted into an electrical voltage
by one of a number of means, depending on the
type of microphone.
4
In a dynamic microphone, the diaphragm is
mechanically coupled to a magnetized iron slug
which moves in and out of a voice coil as the
diaphragm vibrates. This induces a voltage
waveform across the coil, which is an electrical
copy (or analog) of the sound waveform striking
the diaphragm
Motion of vibrating Diaphragm
Microphone Terminals
Voice Coil
Motion of Magnetized Slug
Diaphragm
5
This audio-frequency (or AF) signal is amplified
(made larger) by an audio amplifier, sometimes
called an AF amplifier. The electrical AF
waveform is converted to a radio-frequency (or
RF) waveform through a process called
modulation. There are many modulation schemes,
each of which results in an RF waveform (or
signal) which represents the audio-frequency (or
baseband) signal in a different way. These
schemes may be divided into three basic
categories Amplitude modulation (AM), which
includes AM broadcast signals like WOWO or WLS,
single sideband which is used by radio amateurs
and others, and digital AM such as quadrature
amplitude modulation (QAM) used in cell phones
frequency modulation, including FM broadcasting,
and phase modulation (PM) which is used mainly in
digital radio systems.
6
Amplitude modulation starts with a carrier, an
electrical waveform at the frequency the
transmitter is tuned to. In the figure shown
below, the carrier frequency is 1 kHz, for
illustrative purposes. An actual AM carrier
would have a much higher frequency.
7
The amplitude (or size) of the carrier is varied
(modulated) by the AF waveform, so that if the
peaks of the carrier waveform were connected by
an imaginary line (shown in red), that line would
be a replica of the AF waveform. A line
connecting the valleys of the carrier would be a
mirror image of the AF waveform.
8
Together, the two lines are called the envelope.
9
The receiver converts the RF signal back to an AF
waveform, a close copy of the original AF
waveform, through the process of demodulation or
detection. An AM signal as described here is
demodulated by removing the carrier but leaving
the envelope. This is called envelope detection.
Later, well see how this is done.
10
Following the demodulator (in the receiver) the
AF signal is amplified and sent to another
transducer (a speaker or earphones) to be
converted into sound waves.
11
A speech signal (or music) consists of many
frequencies simultaneously. An AF signal which
consists of a single frequency could be
represented in the frequency domain as shown here
Energy
The frequency-domain depiction of this signal
shows that all of its energy is concentrated at
one frequency, f1
Frequency
f1
The variable which is plotted on the horizontal
axis is always called the domain, and the
variable plotted on the vertical axis is called
the range.
12
A time-domain depiction of the same signal is
simply the waveform plotted with time on the
horizontal axis, like this In this example, f1
100 Hz.
13
From a mathematical viewpoint, its often useful
to treat a waveform as if it includes a negative
frequency component, which is equal to the
positive frequency component. You wont have to
worry about the mathematical details until your
third year.
Energy
Frequency
f1
-f1
0 Hz.
14
Lets add a second signal, at 110 Hz. The two
single-frequency signals look like this in the
time domain The 100 Hz. signal is shown as a
dashed blue line, for reference
15
The sound you would actually hear would be the
sum of the two single-frequency signals. The
value of the sum at any instant in time is the
sum of the 100 Hz. Signal at that instant plus
the 110 Hz. Signal at the same instant.
16
and like this in the frequency domain.
Energy
f1
f2
Frequency
-f2
-f1
0 Hz.
17
It was stated earlier that a complex waveform
like speech or music consists of many frequencies
simultaneously. Its actually more accurate to
say that the signal energy is spread over a
range, or band, of frequencies. The width of
this band is called the signals bandwidth.
The bandwidth of a typical speech waveform is
about 2700 Hz, and its frequency domain
representation (or spectrum) might look something
like this.
The energy of a speech waveform is nearly all
concentrated in the band between 300 Hz and 3000
Hz (plus -300 to -3000 Hz)
Energy
300 Hz
Frequency
-300 Hz
-3000 Hz
3000 Hz
0 Hz.
18
In full-carrier amplitude modulation (the type of
AM used for AM broadcasting and for civilian
aircraft communication) the carrier waveform is
the sum of an AC waveform at the carrier
frequency and a DC component (or DC offset
voltage) at zero frequency. The DC offset is
large enough that the carrier waveform never
becomes negative. The time-domain view looks
like this
19
And the frequency-domain view looks like this.
The carrier is modulated by multiplying its
instantaneous value (the carrier voltage at a
particular moment in time) with the instantaneous
value of the baseband waveform (at the same
moment) When this is done for all values of
time, the AM waveform we saw before is the
result.
Energy
DC Offset
-fcarrier
0 Hz.
fcarrier
20
If the modulating signal looks like this (note
the change of scale)
Energy
0 Hz.
21
then the resulting AM signal looks like
this. Tne modulation process copies the baseband
spectrum, both positive and negative frequencies,
centered on the carrier frequency. These copies
are called sidebands The sideband which is above
the carrier frequency (on the positive frequency
side) is called the upper sideband (USB), while
the sideband below the positive carrier frequency
is the lower sideband (LSB).
Energy
-fcarrier
0 Hz.
fcarrier
22
Notice that if the baseband speech signal is in
the band from 300 to 3000 Hz., the RF signal
(including both sidebands and the carrier)
extends from 3000 Hz. below the carrier to 3000
Hz above the carrier. The bandwidth of the RF
signal is more than double the bandwidth of the
speech signal. Also, the carrier consumes a
portion of the transmitters output power, but
carries no information. The information is all
contained in the sidebands.
Energy
-fcarrier
0 Hz.
fcarrier
23
It turns out that ½ of the transmitters
available output power goes into the carrier,
which is a waste of power. The efficiency of the
transmitter can be doubled by eliminating, or
suppressing, the carrier.
Energy
-fcarrier
0 Hz.
fcarrier
24
Carrier suppression is actually pretty simple.
If the carriers DC component is set to zero,
resulting in this carrier waveform (in the time
domain)
25
only the baseband waveform is copied, centered
at the frequency of the suppressed carrier. The
carrier is not transmitted, only the
sidebands. This variant of amplitude modulation
is called double-sideband, suppressed carrier or
DSB-SC. DSB-SC modulation is accomplished by a
balanced modulator, which eliminates the
carriers DC offset. The portion of the
transmitters available output power (50 ) which
would have been wasted in the carrier now goes
into the sidebands.
Energy
-fcarrier
26
The two sidebands are mirror images of each
other. This means theyre redundant, carrying
exactly the same information. Furthermore, each
of the sidebands of a DSB-SC signal consumes ½ of
the available transmitter power. This means that
50 of the available power output of a DSB-SC
transmitter is wasted.
Energy
-fcarrier
fcarrier
27
If one sideband is suppressed, the result is
called single-sideband modulation or SSB. The
half of the transmitter power which is wasted in
the redundant sideband of a DSB-SC signal now
goes into the transmitted sideband, doubling the
transmitter efficiency. A DSB-SC transmitter is
twice as efficient as a full-carrier AM
transmitter, so SSB modulation is four times as
efficient as full-carrier AM.
Energy
-fcarrier
fcarrier
28
Architecturally, an AM (full-carrier) transmitter
looks like this
AF Amplifier
Microphone
To Antenna
Carrier Oscillator
RF Power Amplifier (PA)
Modulator
29
A DSB-SC transmitter looks like this
AF Amplifier
Microphone
DSB-SC
To Antenna
Carrier Oscillator
RF Power Amplifier (PA)
Balanced Modulator
30
There are several ways to construct an SSB
transmitter. The most common is called the
filter method, in which a narrow bandpass filter
allows the desired sideband (which lies within
the filters passband, or range of frequencies
which it does not block) to pass through. The
unwanted sideband lies in the filters stopband
and is blocked.
AF Amplifier
Microphone
DSB-SC
SSB
To Antenna
Carrier Oscillator
Bandpass Filter
RF Power Amplifier (PA)
Balanced Modulator
31
The bandwidth of the desired sideband, and
therfore of the bandpass filter, is usually a
very small percentage of the (suppressed) carrier
frequency. The filter has a very small percentage
bandwidth, which makes it somewhat challenging.
Crystal or mechanical filters, which can be
rather expensive, are almost always used.
AF Amplifier
Microphone
To Antenna
Carrier Oscillator
Bandpass Filter
RF Power Amplifier (PA)
Balanced Modulator
32
If the simple architecture shown below were used,
the filters passband would have to be adjustable
if the transmitter were to be operated over a
range of carrier frequencies. It is possible to
generate an SSB signal at a fixed intermediate
frequency (or IF), allowing the used of a fixed
filter. The IF SSB signal is then translated to
the desired operating frequency by a second
oscillator (called a local oscillator, or LO).
This is how SSB transmitters are nearly always
constructed
AF Amplifier
SSB (IF)
LO
DSB-SC
SSB (RF)
Microphone
To Antenna
Carrier Oscillator
RF PA
IF Bandpass Filter
Balanced Modulator
Balanced Modulator
33
In this type of SSB transmitter, the frequency of
the transmitted signal is changed by merely
changing the frequency of the LO. The
intermediate frequency is not affected. The
balanced modulators are often called mixers There
is one more detail The IF filter is usually
preceded and/or followed by an IF amplifier. The
purpose of the IF amplifier(s) is not to amplify
the IF signal, but to isolate the filter from the
first and second mixers and prevent ay variations
in the mixer operating conditions from adversely
affecting the filter. the
AF Amplifier
LO
IF Amplifier
Microphone
To Antenna
Carrier Oscillator
RF PA
IF Bandpass Filter
1st mixer
2nd mixer
34
This is a greatly simplified representation of a
real SSB transmitter. Each section consists of
many electronic parts or components, such as
resistors, capacitors, transistors, diodes,
integrated circuits, transformers, inductors,
etc. This representation is called a block
diagram. A detailed depiction showing all
components is called a schematic diagram, or
simply a schematic.
AF Amplifier
LO
IF Amplifier
Microphone
To Antenna
Carrier Oscillator
RF PA
IF Bandpass Filter
1st mixer
2nd mixer