Module-2
Signal Transmission and
Reception
Topics
Modulation
Modulation is defined as the process of changing one or more characteristics of the
high-frequency carrier signal in proportion with the instantaneous value of the analog
information (or digital data) signal.
-The low-frequency information signal is superimposed over a high-frequency carrier signal
in the process of modulation.
-Modulation translates a low-frequency signal to the pass band of communication channel.
Modulating Signal, Carrier Signal, Modulated Signal
Analog Modulation
When the carrier signal is continuous in nature such as fixed
frequency sinusoidal signal, the process of modulation for analog
information signal is known as analog modulation, also known as
Continuous Wave (CW) modulation.
Analog Modulation
Let the time-varying carrier signal be a high-frequency analog sinusoidal signal of the form:
If the amplitude (Vc) of the carrier signal is varied in proportion to the instantaneous
value of the analog information signal, the process is known as Amplitude
Modulation (AM).
• If the frequency (fc) of the carrier signal is varied in proportion to the
instantaneous value of the analog information signal, the process is known as
Frequency Modulation (FM).
• If the phase angle of the carrier signal is varied in proportion to the instantaneous
value of the analog information signal, the process is known as Phase Modulation
(PM).
Principles of Amplitude Modulation
Amplitude Modulation (AM) is the process of varying the amplitude of a relatively
high frequency carrier signal in proportion with the instantaneous value of the
information (modulating) signal.
Let the modulating signal be a
low-frequency analog sinusoidal
signal which can be expressed as
Let the analog carrier signal be
expressed as
The instantaneous value of the
amplitude modulated (AM)
signal can be expressed as
Time domain Analysis
Modulation Index
The ratio of the maximum amplitude of the modulating signal (Vm) and the maximum amplitude of
the carrier signal (Vc) is known as the modulation index for AM wave.
● Also known as depth of modulation, coefficient of modulation, degree of
modulation, modulation factor, or amplitude sensitivity.
● It describes the amount of change in amplitude (that is, modulation) present in
an amplitude-modulated waveform.
When the amplitude
modulation index, ma is
greater than one, the
AM signal is said to be
overmodulated.
Modulation index from AM Signal
Frequency Domain analysis of AM Signal
Frequency Domain analysis of AM Signal
Frequency Spectrum of AM Wave
The expression of AM signal in the frequency-domain contains three distinct terms:
• The first term is the original unmodulated carrier signal.
• The second term is below the carrier frequency signal by the modulating frequency, that is, (f c – fm).
This is called the lower-sideband frequency.
• The third term is above the carrier frequency signal by the modulating frequency, that is, (f c + fm).
This is called the upper-sideband frequency.
AM Frequency- Voltage Spectrum
From the expression of AM signal in the frequency-domain, it is observed that
• The peak amplitude of the carrier frequency term in AM wave is same as that of original carrier
signal, i.e. Vc
• The relative peak amplitude of each of two sideband frequency terms is proportional to the
modulation index ma. That is,
where Vlsb and Vusb are the peak voltage amplitudes of lower sideband and upper sideband
respectively.
For ma = 1, the peak amplitude of each sideband is half of the peak carrier signal voltage, i.e.
Vc/2.
AM Signal Bandwidth
The AM frequency spectrum extends from lower sideband frequency (fc – fm) to upper-sideband
frequency (fc + fm), where fc is the carrier signal frequency, and fm is the maximum modulating
signal frequency.
-The band of frequencies between (fc – fm) and fc is called the lower-sideband (LSB or flsb).
That is, flsb = fc – fm
-Similarly, the band of frequencies between fc and (fc + fm) is called the upper-sideband (USB or
fusb). That is, fusb = fc + fm
AM Power distribution
Assuming that the AM signal appear across a load resistance RL
Calculate
● Carrier Power
● USB & LSB Power
● Total Power
