Radio waves can
carry audio, video, and digital information over great distances by using
changes in a carrier wave’s amplitude, frequency, or phase to represent the
information being transmitted.
Information can be sent from a transmitter to a receiver by
means of modulation and demodulation, respectively, whether those signals are
light waves moving through optical cables, radio waves through metallic cables,
or radio waves propagating through the air. The electromagnetic (EM) waves that
transport the information are referred to as carrier signals, while the
information they carry may be in the form of audio, video, or data.
By changing the
amplitude, frequency, or phase, or a combination of the three signal
characteristics, information can be added as modulation to a signal. Due to the
increased amount of information for transmission and reception,
signal-modulation techniques have advanced in their capabilities to handle more
data for a given amount of occupied bandwidth, although they have also grown
more complex in the process.
Modulation of a radio wave can be performed by varying one or more of its
signal components—amplitude, frequency, or phase—while keeping its other
signal components constant. (Pulse modulation is yet another form of
modulation, without a carrier, in which pulses with precisely known
characteristics are transmitted and details can be learned about a target by
receiving the reflected pulses from the target.)
AM & FM
The simplest form of
carrier modulation, amplitude modulation (AM), has long been the basis for
sending audio information to listeners with radios operating at carrier
frequencies from about 535 to 1,605 kHz in the commercial broadcast band. AM is
also used for maritime communications and navigation, as well as aircraft
navigation, at carrier frequencies from 30 to 535 kHz.
In AM radio broadcasts, the amplitudes of the lower and
upper sidebands of the center frequency of a broadcast channel are modulated
with the audio content from a radio station, to be demodulated at the receiver
of a listener. The lower and upper sidebands extend out from the carrier
frequency, usually occupying a total bandwidth of about 25% around the carrier
frequency. The audio content from a received AM radio wave can be recovered or
demodulated by using a diode to rectify the signals and extract the audio
content, or else via filtering to separate the high-frequency carrier signal
from the audio content.
In frequency modulation
(FM), the frequency of the carrier signal is varied as a function of the
message or information. As with AM, audio content is the most commonly
transmitted information using FM, such as in commercial FM broadcast radios
operating on channels from 88 to 108 MHz. FM can be created by applying message
signals directly to a voltage-controlled oscillator (VCO), so that the VCO’s
output will be a function of the input signal.
Phase-modulation and
-demodulation techniques are more complex than modulation and demodulation
based on amplitude and frequency. However, they provide the benefit of higher
data rates for the amount of bandwidth consumed. Phase modulation is the basis
for many digital modulation formats, in which a modulated signal is divided
into in-phase (0 deg.) and quadrature (90 deg.) signal components. In contrast
to sending video or audio information, digitized information can be easily
transmitted by means of digital modulation formats since the modulated
information need not be sent continuously in time, but can be sent in bursts or
staggered with time and reconstructed at the receiver and demodulator.
Keying in on Digital Modulation
Digital modulation relies
on digital signal processing, such as digital-to-analog converters (DACs) at a
receiver and analog-to-digital converters (ADCs) at a transmitter to transform
analog information (e.g., audio or video) into a digital form that can then be
represented by varying the characteristics of a carrier wave. The three
fundamental types of digital modulation—amplitude-shift-keying (ASK),
frequency-shift-keying (FSK), and phase-shift-keying (PSK) modulation—use
changes in amplitude, frequency, and phase to represent digital bits “0” and
“1.”
In
ASK, the signal amplitude is varied as a function of the information to be
transmitted, and all other parameters of the signal remain constant. When
sending digital information, one amplitude represents a 0 digital bit while a
higher or lower amplitude represents the 1 bit. Waveforms with ASK have the
rapidly changing amplitude levels representing a digital bit stream.
In FSK, two different frequencies are used to represent the
digital 0 and 1 values. The shift in frequencies in FSK is implemented in
different ways, notably in a noncoherent or coherent format. In noncoherent
FSK, discontinuities exist between the frequencies that represent the digital
bits. Termed “mark” and “space” frequencies, they are used as kinds of
frequency gaps to separate the bit-representing frequencies. In coherent FSK,
the changes in bit-representing frequencies are instantaneous, without phase
discontinuities between the frequencies.
In PSK, the phase of the
carrier is discretely changed to denote the different digital bits. The phase
can be changed in relation to a reference phase, such as using 0 deg. for a 0
digital bit and 180 deg. to represent the digital 1 bit, or if a difference of
180 deg. is used to denote different digital bits, one of the bits may be
represented by a relative phase of –90 deg. and the other by +90 deg. In such a
simple, biphase modulation format, the two phase angles of the carrier
represent two digital bits of information, so that the modulation rate is equal
to the bit rate. But if a greater number of phase angles is used, the bit rate
can be increased in parallel with an increasing number of phase angles.
In a quadrature phase-modulation format such as quadrature
phase shift keying (QPSK), where four phase angles are used to represent the
digital bits, two bits of digital information are able to be carried with each
phase angle, so that the information can be represented as 00, 01, 10, and 11.
Similarly, if eight phase angles are employed in the phase-modulation scheme,
then three digital bits can be represented by each of eight possible phase
angles. In turn, the bit rate will increase as the number of phase angles grows
in the phase-modulation scheme.
As a result, many
digital-modulation formats based on changes in phase attempt to represent the
greatest number of digital bits possible by variations in phase, so as to
support the highest bit rates possible. This performance parameter of a
modulated waveform, spectral efficiency, refers to the number of bits that can
be transmitted during a given period of time and for a given portion of
bandwidth, usually measured as b/s/Hz.
ASK can be affected by
nonlinearities in a system—for example, any form of nonlinear distortion like
nonlinear amplification—so it is essential that components with extremely
linear performance be used to preserve the amplitude characteristics of a
transmitted and received signal. FSK, on the other hand, requires high
frequency stability in a system’s signal sources, such as VCOs used for local
oscillators (LOs) in receiver and transmitters. To maintain high frequency
stability, oscillators in FSK systems are typically stabilized by means of phase-locked
loops (PLLs) to synchronize the frequency and phase of the system’s frequency
sources to a common reference source. In addition, PSK depends on tight phase
tolerances in a system, such as the lengths of transmission lines, where
variations can mean increasing phase errors with increasing transmission
frequencies.
Editor’s Note: This is part one of a two-part article on the basics of modulation and
demodulation. The next installment will examine some of the more complex forms
of digital modulation, and explain the use of the time domain and pulsed
signals in systems employing pulse modulation (e.g., military radars and
automotive collision-avoidance systems). Part 2 will also review the types of
hardware needed for each type of modulation/demodulation format, and which
modulator/demodulator performance parameters are most critical to achieving
good communications-systems performance with high spectral efficiency.
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Source: mwrf.com
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