17-01-2013, 04:21 PM
Analog Communication Techniques
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INTRODUCTION
Modulation is the process of encoding information into a signal that can be transmitted (or
recorded) over a channel of interest. In analog modulation, a baseband message signal, such
as speech, audio or video, is directly transformed into a signal that can be transmitted over
a designated channel, typically a passband radio frequency (RF) channel. Digital modulation
differs from this only in the following additional step: bits are encoded into baseband message
signals, which are then transformed into passband signals to be transmitted. Thus, despite
the relentless transition from digital to analog modulation, many of the techniques developed for
analog communication systems remain important for the digital communication systems designer,
and our goal in this chapter is to study an important subset of these techniques, using legacy
analog communication systems as examples to reinforce concepts.
From Chapter 2, we know that passband signals carry information in their complex envelope,
and that the complex envelope can be represented either in terms of I and Q components, or in
terms of envelope and phase. We study two broad classes of techniques: amplitude modulation,
in which the analog message signal appears directly in the I and/or Q components; and
angle modulation, in which the analog message signal appears directly in the phase or in the
instantaneous frequency (i.e., in the derivative of the phase), of the transmitted signal. Examples
of analog communication in space include AM radio, FM radio, and broadcast television, as well
as a variety of specialized radios. Examples of analog communication in time (i.e., for storage)
include audiocassettes and VHS videotapes.
The analog-centric techniques covered in this chapter include envelope detection, superheterodyne
reception, limiter discriminators, and phase locked loops. At a high level, these techniques
tell us how to go from baseband message signals to passband transmitted signals, and back
from passband received signals to baseband message signals. For analog communication, this
is enough, since we consider continuous time message signals which are directly transformed to
passband through amplitude or angle modulation. For digital communication, we need to also
figure out how to decode the encoded bits from the received passband signal, typically after downconversion
to baseband; this is a subject discussed in later chapters. However, between encoding
at the transmitter and decoding at the receiver, a number of analog communication techniques
are relevant: for example, we need to decide between direct and superheterodyne architectures
for upconversion and downconversion, and tailor our frequency planning appropriately; we may
use a PLL to synthesize the local oscillator frequencies at the transmitter and receiver.
Amplitude Modulation
We now discuss a number of variants of amplitude modulation, in which the baseband message
signal modulates the amplitude of a sinusoidal carrier whose frequency falls in the passband over
which we wish to communicate.
FM Spectrum
We first consider a naive but useful estimate of FM bandwidth termed Carson’s rule. We
then show that the spectral properties of FM are actually quite complicated, even for a simple
sinusoidal message, and outline methods of obtaining more detailed bandwidth estimates.
Consider an angle modulated signal, up(t) = Ac cos (2fct + (t)), where (t) contains the message
information. For a baseband message m(t) of bandwidth B, the phase (t) for PM is also
a baseband signal with the same bandwidth. The phase (t) for FM is the integral of the message.
Since integration smooths out the time domain signal, or equivalently, attenuates higher
frequencies, (t) is a baseband signal with bandwidth at most B. We therefore loosely think of
(t) as having a bandwidth equal to B, the message bandwidth.
The Superheterodyne Receiver
The receiver in a radio communication system must downconvert the passband received signal
down to baseband in order to recover the message. At the turn of the twentieth century, it
was difficult to produce amplification at frequencies beyond a few MHz) using the vacuum tube
technology of that time. However, higher carrier frequencies are desirable because of the larger
available bandwidths and the smaller antennas required. The invention of the superheterodyne,
or superhet, receiver was motivated by these considerations. Basically, the idea is to use sloppy
design for front end filtering of the received radio frequency (RF) signal, and for translating it to a
lower intermediate frequency (IF). The IF signal is then processed using carefully designed filters
and amplifiers. Subsequently, the IF signal can be converted to baseband in a number of different
ways: for example, an envelope detector for AM radio, a phase locked loop or discriminator
for FM radio, and a coherent quadrature demodulator for digital cellular telephone receivers.
While the original motivation for the superheterodyne receiver is no longer strictly applicable
(modern analog electronics are capable of providing amplification at the carrier frequencies in
commercial use), it is still true that gain is easier to provide at lower frequencies than at higher
frequencies. Furthermore, it becomes possible to closely optimize the processing at a fixed IF
(in terms of amplifier and filter design), while permitting a tunable RF front end with more
relaxed specifications, which is important for the design of radios that operate over a wide
range of carrier frequencies. For example, the superhet architecture is commonly employed for
AM and FM broadcast radio receivers, where the RF front end tunes to the desired station,
translating the received signal to a fixed IF. Radio receivers built with discrete components often
take advantage of the widespread availability of inexpensive filters at certain commonly used
IF frequencies, such as 455 KHz (used for AM radio) and 10.7 MHz (used for FM radio).