Analog Modulation | Communication System - Electronics and Communication Engineering (ECE) PDF Download

Introduction

Modulation refers to the properties of a periodic waveform (that repeats itself over a certain period), named as the carrier signal and the modulating signal. The Carrier signal is a high-frequency signal as compared to the message signal. The modulated signal is the original message signal that contains the information transmitted from the transmitter to the receiver through the communication channel. 
For Example:
1. Audio signal

• It represents the sound signal that is transmitted from the transmitter to the receiver. 
• In digital communication, it can be a digital signal represented in the bit streams of binary digits 0 and 1. The information present in the message signal needs to be carried from one place to another. The modulation helps in increasing the efficiency and frequency of the message signal. The high frequency carrier is selected to increase the frequency of the input message signal when the output requirement is high as compared to the input.
• The other purpose of modulation is FDM (Frequency Division Multiplexing), which transmits multiple signals on the same communication channel.

2. Television

• Various signals of different frequencies are transmitted on the same communication channel. The separate frequency of each channel does not allow any interference between them. At the destination, multiple signals are demodulated that appear to the customers as the output on their television screen.
• AM and FM (Frequency Modulation) were the earliest invented modulation techniques. In AM (Amplitude Modulation), the message signal is superimposed on the high-frequency carrier signal, increasing the signal's strength and frequency. 
• The recent invention is the digital modulation that superimposes the digital signal on the sequence of bitstreams. The amplitude or the voltage levels in digital modulation switches between the two levels 0 and 1 and the carrier clock frequency. The modulation that incorporates multiple signal transmission is OFDM (Orthogonal Frequency Division Multiplexing). The example includes digital radio systems, telemetry, and Wi-Fi.

Modulator

• A modulator is a device that performs the modulation process. Similarly, a demodulator is a device that performs demodulation. A device that can perform both processes (modulation and demodulation) is known as a modem. 
• It is also known as the bidirectional communication device. A modulated signal occupies a frequency band called baseband, and the carrier signal occupies a higher frequency band called Passband. It is due to the high frequency of the carrier signal.

Angle Modulation Methods

1.  Amplitude Modulation
2. Pulse Modulation
3. Angle Modulation

Amplitude Modulation

In AM, the amplitude of the carrier varies with the amplitude of the message signal. Amplitude Modulation techniques are used to improve the transmission medium's bandwidth utilization and total transmission power. 

Modulation in AM
Let's discuss some methods that perform AM modulation:

• Balance Modulator: The simplest modulation technique uses two amplitude modulators and an adder. The function of modulators is to pass the signal from one part to the other effectively. The two modulators pass the message and the carrier signal resulting in the formation of Double Sidebands. To produce a single sideband, the output of the adder further passes through the filters. The separation of the two sidebands depends on the filter's cut-off frequency. It passes a certain range of frequencies and rejects the other. In VSB (Vestigial Sideband), a sideband filter is used additionally with the circuit. It adds vestige to the sidebands.
• Phase Shift method: The phase shift method is popular in SSB transmission. It uses a 90-degree phase shifter that introduces a phase shift of 90 degrees in the input signal. It also uses two balance modulators that perform the same function as amplitude modulators. The additional and subtraction of the two outputs from balance modulators form the lower and upper sidebands.

Types of AM
AM is further categorized as:

• Single Sideband Modulation (SSB): It produces sidebands (frequency bands) on either side of the carrier wave.
• Double Sideband Modulation (DSB): It produces sidebands on both sides of the carrier wave.
• Vestigial Sideband Modulation (VSB): A section of the signal is modulated by adding a vestige to the produced sidebands.
• Quadrature Amplitude Modulation (QAM): It sends two message signals over the same spectrum unlike other modulation techniques that sends only one.

Pulse Modulation

In Pulse Modulation, the pulse carries the information. The amplitude, width, and position of the pulse vary with the message signal.

Modulation: Natural Sampling and Flat top sampling are the modulation techniques used in PAM (Pulse Amplitude Modulation). In such types of modulation, the input signal is samples at regular intervals. In PWM (Pulse Width Modulation), a comparator is used to generate a PWM signal. A PPM signal is generated by using a PWM output signal as the input signal.

Types of Pulse Modulation
Pulse Modulation is further categorized as:

• Pulse Amplitude Modulation: The amplitude of the pulse varies with the sample value of message signal.
• Pulse Width Modulation: The width of the pulse varies with the sample value of message signal.
• Pulse Position Modulation: The position of the pulse varies with the sample value of message signal.

Angle Modulation

Angle Modulation is the combination of frequency and phase modulation. Here, the frequency or phase of the carrier varies with the instantaneous value of the message signal. 

Types of Angle Modulation
Angle Modulation is further categorized as:

• Frequency Modulation: When the frequency of the carrier varies with the instantaneous value of the message signal, it is termed as Frequency Modulation.
• Phase Modulation: When the phase of the carrier varies with the instantaneous value of the message signal, it is termed as Frequency Modulation.

Modulation

The modulation process of PM (Phase Modulation) and FM is the same except the integrator that is an additional requirement to generate the output signal in FM (Frequency Modulation). Parameter variation and Armstrong Indirect methods are the modulation methods used in FM. The function of the parameter variation method depends on the VCO (Voltage Controlled Oscillator) to generate the output signal. The Armstrong Indirect method combines carrier signal source, balanced modulator, 90-degree phase shifter, and an adder.