An introduction to frequency and amplitude modulation.
Especially for wireless channels, like commercial radio and
television, but also for wireline systems like cable television,an analog message signal must be
modulated : The
transmitted signal's spectrum occurs at much higher frequenciesthan those occupied by the signal.
We use analog communication techniques for analog message
signals, like music, speech, and television. Transmission andreception of analog signals using analog results in an
inherently noisy received signal (assuming the channel addsnoise, which it almost certainly does).
The key idea of modulation is to affect the amplitude, frequency
or phase of what is known as the
carrier sinusoid. Frequency modulation (FM) and less frequently used
phase modulation (PM) are not discussed here; we focus onamplitude modulation (AM). The amplitude modulated message
signal has the form
where
is the
carrier frequency and
the
carrier amplitude . Also, the signal's
amplitude is assumed to be less than one:
. From our previous exposure to amplitude modulation
(see the
Fourier Transform example ), we know that the
transmitted signal's spectrum occupies the frequency range
, assuming the signal's bandwidth is
Hz (see the
figure ). The carrier
frequency is usually much larger than the signal's highestfrequency:
, which means that the transmitter antenna and carrier
frequency are chosen jointly during the design process.
Ignoring the attenuation and noise introduced by the channel for
the moment, reception of an amplitude modulated signal is quiteeasy (see
[link] ).
The so-called
coherent receiver multiplies the
input signal by a sinusoid and lowpass-filters the result (
[link] ).
Because of our trigonometric identities, we know that
At this point, the message signal is multiplied by a constant
and a sinusoid at twice the carrier frequency. Multiplication bythe constant term returns the message signal to baseband (where
we want it to be!) while multiplication by the double-frequencyterm yields a very high frequency signal. The lowpass filter
removes this high-frequency signal, leaving only the basebandsignal. Thus, the received signal is
This derivation relies solely on the time domain; derive the
same result in the frequency domain. You won't need thetrigonometric identity with this approach.
The signal-related portion of the transmitted spectrum is
given by
.
Multiplying at the receiver by the carrier shifts thisspectrum to
and to
, and scales the result by half.
The signal components centered at twice the carrier frequency
are removed by the lowpass filter, while the baseband signal
emerges.
Because it is so easy to remove the constant term by electrical
means—we insert a capacitor in series with the receiver'soutput—we typically ignore it and concentrate on the signal
portion of the receiver's output when calculatingsignal-to-noise ratio.
Questions & Answers
I'm interested in biological psychology and cognitive psychology
Communication is effective because it allows individuals to share ideas, thoughts, and information with others.
effective communication can lead to improved outcomes in various settings, including personal relationships, business environments, and educational settings. By communicating effectively, individuals can negotiate effectively, solve problems collaboratively, and work towards common goals.
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miss
Every time someone flushes a toilet in the apartment building, the person begins to jumb back automatically after hearing the flush, before the water temperature changes. Identify the types of learning, if it is classical conditioning identify the NS, UCS, CS and CR. If it is operant conditioning, identify the type of consequence positive reinforcement, negative reinforcement or punishment
nature is an hereditary factor while nurture is an environmental factor which constitute an individual personality. so if an individual's parent has a deviant behavior and was also brought up in an deviant environment, observation of the behavior and the inborn trait we make the individual deviant.
Samuel
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