Principles of
Communications
AMPLITUDE MODULATION
BSECE 2nd year
Engr. Christopher V. Maglaque, MEM
Assistant Professor IV
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Amplitude Modulation Principles, Slides of Communications Engineering
A comprehensive overview of amplitude modulation (am), a method used to transmit low-frequency signals over high-frequency carrier waves. It covers the process of modulation, demodulation, waveform analysis, coefficient of modulation, and various cases of modulation index. The document also includes sample problems and discussions on simultaneous modulation, spectrum of conventional am signals, and suppressed-carrier am systems.
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2022/2023
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Principles of
Communications
AMPLITUDE MODULATION BSECE 2 nd year Engr. Christopher V. Maglaque, MEM Assistant Professor IV
Amplitude
Modulation
Modulation It is the process of impressing or imparting a low frequency source information (voice, audio, video, or data) onto a high- frequency bandpass signal with a carrier frequency fc by the introduction of amplitude, frequency or phase perturbation. Demodulation It is the reverse process where the received signals are transformed back to their original form. Amplitude Modulation It is an analog modulation scheme in which the amplitude of a relatively high-frequency carrier signal is varied in accordance with the instantaneous amplitude of an information signal.
Amplitude
Modulation
AM Waveform
- Instantaneous Amplitude of the Modulated Wave Instantaneous Amplitude of the Modulated Wave
Amplitude
Modulation
AM Waveform
- Instantaneous Amplitude of the Modulated Wave Instantaneous Amplitude of the Modulated Wave The modulating signal is more often an arbitrary waveform, such as audio signal. However, an analysis of sine wave modulation is very useful, since Fourier analysis often allows complex signals to be express as a series of sinusoids. Hence,
Amplitude
Modulation
AM Waveform
- Instantaneous Amplitude of the Modulated Wave
Amplitude
Modulation
AM Waveform
- Instantaneous Amplitude of the Modulated Wave
NOTE:
First, note that the amplitude of the carrier after modulation is the same as it was before modulation. Second, the amplitude of the upper and lower side frequencies is a function of both the carrier amplitude and the modulation index. Third, the carrier component is a +sine function, the upper side frequency a – cosine function, and the lower side frequency a +cosine function or the carrier is 90 ° out of phase with both the upper and lower side frequencies, and the upper and lower side frequency are 180 ° out of phase with each other.
Amplitude
Modulation
AM Waveform
- Coefficient of Modulation (m) Sample Problems: 1. An AM signal has the following characteristics: carrier frequency = 150 MHz; modulating frequency = 3 MHz; peak carrier voltage = 40 volts; and peak modulating voltage is 30 volts. Calculate the peak voltage of the lower sideband frequency.
Amplitude
Modulation
AM Waveform
- Coefficient of Modulation (m) 2. Calculate the modulation index for a standard AM transmission, if the maximum peak voltage of the modulated wave is 150 V and the modulating signal voltage is 50 V peak.
Amplitude
Modulation
AM Waveform
- Coefficient of Modulation (m)
NOTE:
OVERMODULATION creates side frequencies (harmonics) further from the carrier known as SPLATTER, BUCKSHOT, or SPURIOUS EMISSIONS which create interference to other radio services and since spectrum space is tightly controlled by law, overmodulation is illegal.
Amplitude
Modulation
AM Waveform
- Coefficient of Modulation (m) Sample Problem: 1. A modulating signal consists of a symmetrical triangular wave having zero dc component and peak-to-peak voltage of 11 V. Calculate the value of modulation index if it used to amplitude modulate a carrier of peak voltage 10 V.
Amplitude
Modulation
B. SIMULTANEOUS
MODULATION
Sample Problems:
- An AM transmitter is modulated by two sine waves at 1.5 kHz and 2.5 kHz, with modulation of 20 percent and 80 percent respectively. Calculate the effective modulation index.
Amplitude
Modulation
B. SIMULTANEOUS
MODULATION
- Calculate the modulating voltage of an audio signal necessary to provide 100 % modulation of a 100 - V carrier that is simultaneously modulated by 2 audio waves with m 1 and m 2 equal to 75 % and 45 % respectively.
Amplitude
Modulation
D. CONVENTIONAL AM
SYSTEM (DSBFC)
CONVENTIONAL AM SYSTEM (DSBFC)
**1. AM Bandwidth
- Side Frequency (Sideband)**