Amplitude Modulation Principles, Slides of Communications Engineering

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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

A. AM WAVEFORM

THE AM WAVEFORM

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 By applying little trigonometry,

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) Coefficient of Modulation (m) A term used to describe the amount of amplitude changed (modulation) present in an AM waveform. Mathematically, Where: Vm= peak amplitude of the modulating signal in V Vc= peak amplitude of the carrier signal in V In terms of envelope: Where: Vmax= Vc + Vm Vmin= Vc – Vm

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) Several Cases of m: a. Undermodulation (m<1, Vm<Vc) b. 100% Modulation (m=1, Vm=Vc) c. Overmodulation (m>1, Vm>Vc)

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

where: mT= total modulation index m 1 ,m 2 ……mn = modulation indices due to several modulating signal

Amplitude

Modulation

B. SIMULTANEOUS

MODULATION

Sample Problems:

1. 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

2. 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

C. SPECTRUM OF

CONVENTIONAL AM

SIGNAL

SPECTRUM OF CONVENTIONAL AM SIGNAL

1. Frequency Spectrum **2. Voltage Spectrum

3. Power Spectrum**

Amplitude

Modulation

D. CONVENTIONAL AM

SYSTEM (DSBFC)

CONVENTIONAL AM SYSTEM (DSBFC)

**1. AM Bandwidth

2. Side Frequency (Sideband)**

Amplitude

Modulation

D. CONVENTIONAL AM

SYSTEM (DSBFC)

CONVENTIONAL AM SYSTEM (DSBFC)

Amplitude

Modulation

D. CONVENTIONAL AM

SYSTEM (DSBFC)

Sample Problems:

1. What will be the total sideband power of an AM transmitting station whose carrier power is 1200 W and a modulation of 95 %?

Amplitude

Modulation

D. CONVENTIONAL AM

SYSTEM (DSBFC)

2. Calculate the power in one sideband of an AM signal whose carrier power is 50 watts. The unmodulated current is 2 A while the modulated current is 2. 4 A.

Amplitude

Modulation

D. CONVENTIONAL AM

SYSTEM (DSBFC)

3. Calculate the total power and the power in each side frequency for a standard AM transmission that is sinusoidally modulated to a depth of 80 % if the unmodulated carrier power is 50 kW.

Amplitude

Modulation

E. SUPPRESSED-CARRIER

AM SYSTEM.

E. SUPPRESSED-CARRIER AM SYSTEM

1. Double-Sideband Suppressed-Carrier (DSB-SC)

Amplitude

Modulation

E. SUPPRESSED-CARRIER

AM SYSTEM.

Sample Problem:

1. A DSB-SC system must suppress the carrier by 50 dB from its original value of 10 W. To what value must the carrier be reduced?