Modulation
is the
process by
which
voice,
music, and
other
"intelligence"
is added
to the
radio
waves
produced
by a
transmitter.
The
different
methods of
modulating
a radio
signal are
called modes.
An
unmodulated
radio
signal is
known as a
carrier.
When you
hear
"dead
air"
between
songs or
announcements
on a radio
station,
you're
"hearing"
the carrier.
While a
carrier
contains
no
intelligence,
you can
tell it is
being
transmitted
because of
the way it
quiets the
background
noise on
your
radio.
The
different
modes of
modulation
have their
advantages
and
disadvantages.
Here is a
summary:
Continuous
Wave (CW)
CW is
the
simplest
form of
modulation.
The output
of the
transmitter
is
switched
on and
off,
typically
to form
the
characters
of the
Morse
code.
CW
transmitters
are simple
and
inexpensive,
and the
transmitted
CW signal
doesn't
occupy
much
frequency
space
(usually
less than
500 Hz).
However,
the CW
signals
will be
difficult
to hear on
a normal
receiver;
you'll
just hear
the faint
quieting
of the
background
noise as
the CW
signals
are
transmitted.
To
overcome
this
problem,
short-wave
and ham
radio
receivers
include a beat
frequency
oscillator
(BFO)
circuit.
The BFO
circuit
produces
an
internally-generated
second
carrier
that
"beats"
against
the
received
CW signal,
producing
a tone
that turns
on and off
in step
with the
received
CW signal.
This is
how Morse
code
signals
are
received
on
short-wave.
In
amplitude
modulation,
the
strength
(amplitude)
of the
carrier
from a
transmitter
is varied
according
to how a
modulating
signal
varies.
When
you speak
into the
microphone
of an AM
transmitter,
the
microphone
converts
your voice
into a
varying
voltage.
This
voltage is
amplified
and then
used to
vary the
strength
of the
transmitter's
output.
Amplitude
modulation
adds power
to the
carrier,
with the
amount
added
depending
on the
strength
of the
modulating
voltage.
Amplitude
modulation
results in
three
separate
frequencies
being
transmitted:
the
original
carrier
frequency,
a lower
sideband
(LSB)
below the
carrier
frequency,
and an
upper
sideband
(USB)
above the
carrier
frequency.
The
sidebands
are
"mirror
images"
of each
other and
contain
the same
intelligence.
When an AM
signal is
received,
these
frequencies
are
combined
to produce
the sounds
you hear.
Each
sideband
occupies
as much
frequency
space as
the
highest
audio
frequency
being
transmitted.
If the
highest
audio
frequency
being
transmitted
is 5 kHz,
then the
total
frequency
space
occupied
by an AM
signal
will be 10
kHz (the
carrier
occupies
negligible
frequency
space).
AM has
the
advantages
of being
easy to
produce in
a
transmitter
and AM
receivers
are simple
in design.
Its main
disadvantage
is its
inefficiency.
About
two-thirds
of an AM
signal's
power is
concentrated
in the
carrier,
which
contains
no
intelligence.
One-third
of the
power is
in the
sidebands,
which
contain
the
signal's
intelligence.
Since the
sidebands
contain
the same
intelligence,
however,
one is
essentially
"wasted."
Of the
total
power
output of
an AM
transmitter,
only about
one-sixth
is
actually
productive,
useful
output!
Other
disadvantages
of AM
include
the
relatively
wide
amount of
frequency
space an
AM signal
occupies
and its
susceptibility
to static
and other
forms of
electrical
noise.
Despite
this, AM
is simple
to tune on
ordinary
receivers,
and that
is why it
is used
for almost
all
short-wave
broadcasting.
Since
so much
power is
wasted in
AM, radio
engineers
devised a
method to
transmit
just one
sideband
and put
all of the
transmitter's
power into
sending
useful
intelligence.
This
method is
known as single
sideband
(SSB). In
SSB
transmitters,
the
carrier
and one
sideband
are
removed
before the
signal is
amplified.
Either the
upper
sideband (USB)
or lower
sideband
(LSB) of
the
original
AM signal
can be
transmitted.
SSB is
a much
more
efficient
mode than
AM since
all of the
transmitter's
power goes
into
transmitting
useful
intelligence.
A SSB
signal
also
occupies
only about
half the
frequency
space of a
comparable
AM signal.
However,
SSB
transmitters
and
receivers
are far
more
complicated
than those
for AM. In
fact, a
SSB signal
cannot be
received
intelligibly
on an AM
receiver;
the SSB
signal
will have
a badly
distorted
"Donald
Duck"
sound.
This is
because
the
carrier of
an AM
signal
does play
a major
role in
demodulating
(that is,
recovering
the
transmitted
audio) the
sidebands
of an AM
signal. To
successfully
demodulate
a SSB
signal,
you need a
"substitute
carrier"
.
A
substitute
carrier
can be
supplied
by the
beat
frequency
oscillator
(BFO)
circuit
used when
receiving
CW
signals.
However,
this means
that a SSB
signal
must be
carefully
tuned to
precise
"beat"
it against
the
replacement
carrier
from the
BFO. For
best
performance,
a SSB
receiver
needs more
precise
tuning and
stability
than an AM
receiver,
and it
must be
tuned more
carefully
than an AM
receiver.
Even when
precisely
tuned, the
audio
quality of
a SSB
signal is
less than
that of an
AM signal.
SSB is
used
mainly by
ham radio
operators,
military
services,
maritime
and
aeronautical
radio
services,
and other
situations
where
skilled
operators
and
quality
receiving
equipment
are
common.
There have
been a few
experiments
in using
SSB for
short-wave
broadcasting,
but AM
remains
the
preferred
mode for
broadcasting
because of
its
simplicity.
In CW,
AM, and
SSB, the
carrier of
the signal
will not
change in
a normally
operating
transmitter.
However,
it is
possible
to
modulate a
signal by
changing
its
frequency
in
accordance
with a
modulating
signal.
This is
the idea
behind frequency
modulation
(FM).
The
unmodulated
frequency
of a FM
signal is
called its
center
frequency.
When a
modulating
signal is
applied,
the FM
transmitter's
frequency
will swing
above and
below the
center
frequency
according
to the
modulating
signal.
The amount
of
"swing"
in the
transmitter's
frequency
in any
direction
above or
below the
center
frequency
is called
its deviation.
The total
frequency
space
occupied
by a FM
signal is
twice its
deviation.
As you
might
suspect,
FM signals
occupy a
great deal
of
frequency
space. The
deviation
of a FM
broadcast
station is
75 kHz,
for a
total
frequency
space of
150 kHz.
Most other
users of
FM (police
and fire
departments,
business
radio
services,
etc.) use
a
deviation
of 5 kHz,
for a
total
frequency
space
occupied
of 10 kHz.
For these
reasons,
FM is
mainly
used on
frequency
above 30
MHz, where
adequate
frequency
space is
available.
This is
why most
scanner
radios can
only
receive FM
signals,
since most
signals
found
above 30
MHz are
FM.
The big
advantage
of FM is
its audio
quality
and
immunity
to noise.
Most forms
of static
and
electrical
noise are
naturally
AM, and a
FM
receiver
will not
respond to
AM
signals.
FM
receivers
also
exhibit a
characteristic
known as
the capture
effect.
If two or
more FM
signals
are on the
same
frequency,
the FM
receiver
will
respond to
the
strongest
of the
signals
and ignore
the rest.
The audio
quality of
a FM
signal
increases
as its
deviation
increases,
which is
why FM
broadcast
stations
use such
large
deviation.
The main
disadvantage
of FM is
the amount
of
frequency
space a
signal
requires.
Frequency-Shift
Keying (FSK)
Like
FM, frequency-shift
keying
(FSK)
shifts the
carrier
frequency
of the
transmitter.
Unlike FM,
however,
FSK shifts
the
frequency
between
just two
separate,
fixed
points.
The higher
frequency
is called
the mark
frequency
while the
lower of
the two
frequencies
is called
the space
frequency.
(By
contrast,
an FM
signal can
swing to
any
frequency
within its
deviation
range.)
FSK was
originally
developed
to send
text via
radioteleprinter
devices,
like those
used by
the
Teletype
Corporation.
The
shifting
of the
carrier
between
the mark
and space
was used
to
generate
characters
in the Baudot
code,
which can
be thought
of as a
more
elaborate
version of
the Morse
code. At
the
receiver,
the Baudot
signals
were used
to produce
printed
text on
printers
and,
later,
video
screens.
As
technology
improved,
FSK was
used to
transmit
messages
in the
ASCII code
used by
computers;
this
permitted
the use of
upper and
lower case
letters
and
special
symbols.
The
introduction
of
microprocessors
made it
possible
to use FSK
to send
messages
with
automatic
error
detection
and
correction
capabilities.
This is
done by
including
error
checking
codes into
messages
and
allowing
the
receiving
station to
request a
retransmission
of a
message if
the
message
and its
error
checking
code are
in
conflict
(or if the
code is
not
received.)
Among the
most
common
such FSK
modes are amateur
teleprinting
over radio
(AMTOR)
and forward
error
correction
(FEC).
FSK is
the
fastest
way to
send text
by radio,
and the
error-correcting
modes
offer high
accuracy
and
reliability.
The
frequency
space
occupied
depends on
the amount
of
shifting,
but
typical
FSK
signals
occupy
less than
1.5 kHz of
space. The
big
disadvantage
of FSK is
the more
elaborate
receiving
gear
required.
Special
receiving
terminals
and
adapters
are
available
to let you
"see"
FSK modes.
Many of
these work
in
conjunction
with
personal
computers.
The
same
technology
that makes
it
possible
for you to
view this
Web site
is also
being used
on the
air.
Digital
modes can
organize
information
into packets
that
contain
address
fields,
information
about the
transmission
protocol
being
used,
error
detection
code, a
few
hundred
bytes of
data, and
bits to
indicate
where each
packet
begins and
ends.
Instead
of
transmitting
messages
in
continuous
streams,
packet
modes
break them
into
packets.
At the
receiving
end, the
different
packets
are
re-assembled
to form
the
original
message.
If a
packet is
missing or
received
with
errors,
the
receiving
station
can
request a
retransmission
of the
packet.
Packets
can be
received
out of
sequence
or even
from
multiple
sources
(such as
different
relaying
stations)
and still
be
assembled
into the
original
message by
the
receiving
station.
While
packet
modes have
mainly
been used
to send
text, any
information
that can
be
converted
into
digital
form---sound,
graphics,
video,
etc.---can
be
transmitted
by digital
modes.
Another
advantage
of packet
modes is
that
packets
can be
addressed
to
specific
stations
in the
address
field of
each
packet.
Other
stations
will
ignore
packets
not
addressed
to them.
The big
disadvantage
of packet
modes is
the
complexity
of the
necessary
receiving
and
transmitting
gear. The
frequency
space
occupied
is
directly
proportional
to the
speed at
which
messages
are
transmitted,
and radio
digital
modes are
very slow
compared
to their
Internet
equivalents.
The
slowest
Web
connection
via the
Internet
is 14,400
baud
(14.4K),
while the
maximum
practical
digital
mode rate
via radio
is 9600
baud
(9.6K). On
frequencies
below 30
MHz, it is
even
slower;
rates are
usually
restricted
to just
300 baud
(0.3K)! As
a result,
digital
modes via
radio
today
deliver
performance
far short
of their
potential.
Special
receiving
adapters
for packet
modes are
available,
and these
usually
work in
conjunction
with
personal
computers.
Most offer
FSK
receiving
capabilities
as well.
Another
form of
digital
modulation
is known
as spread
spectrum.
Most other
modulation
methods
pack all
of the
transmitter's
output
power into
a
bandwidth
of only a
few kHz.
(Even in
FM, the
carrier
doesn't
occupy
much
bandwidth,
although
its
frequency
may be
deviated
over a
wide
range.)
Spread
spectrum
literally
"spreads"
the
carrier
over a
frequency
range that
may be as
much as 10
kHz on
frequencies
below 30
MHz.
(Spreading
over 100
kHz or
more is
common on
the VHF
and UHF
bands.)
This
spreading
is usually
done via a
"spreading
code"
contained
in an
internal
micro
controller
chip.
When
heard on a
conventional
receiver,
spread
spectrum
sounds
like
random
noise or
"gurgling"
water. A
receiver
equipped
with a
micro
controller
having the
matching
"spreading
code"
is
necessary
to
properly
receive
the spread
spectrum
transmission.
Advantages
of spread
spectrum
include a
high
degree of
privacy
and
freedom
from
interference,
since the
spread
spectrum
receiver
will
reject any
signal not
having the
proper
spreading
code.
Almost
all users
of spread
spectrum
below 30
MHz are
various
military
and
government
services.
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