Amplitude Modulation and Demodulation

By:Bikram ModakDepartment -Computer Sc ience and Engineer ingSemester -4 th

Rol l no. -11900115013Regist rat ion no. -151190110013

Introduction

• Modulation

-In the modulation process, some characteristic of a high-frequency carrier signal (bandpass), is changed according to the instantaneous amplitude of the information (baseband) signal.

-It is the simplest and earliest form of transmitters.

• Why Modulation

-Suitable for signal transmission (distance…etc).

-Multiple signals transmitted on the same channel.

-Capacitive or inductive devices require high frequency AC input (carrier) to operate.

-Stability and noise rejection.

Amplitude Modulation

• AMPLITUDE MODULATION is formally defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value linearly with a message signal m(t) keeping frequency and phase constant .

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5

0

5

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1

0

1

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1

0

1

• The AM signal

• The Modulating signal

• The Carrier signal

ttmkAts cc cos1

tm

tAtc cc cos

(cont.)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-5

-4

-3

-2

-1

0

1

2

3

4

5• The Envelope

• The AM signal

ttmkAts cc cos1

tmkAts c 1

Amplitude Sensitivity

Modulation Index - The ratio between the amplitudes between the amplitudes of the modulating signal and carrier, expressed by the equation:

c

m

E

Em =

AM-Percentage Modulation

Under modulated (<100%)

Envelope Detector Can be used

100% modulated

Envelope Detector Can be used

Over Modulated(>100%)

Envelope Detector GivesDistorted Signal

Examples

An Example of message signal m(t) Waveform for Amplitude modulation of the message signal m(t)

Amplitude Modulator

Electro-Optic Mode Converter

The Versawave 40 & 50 Gb/s Amplitude Modulators represent a revolutionary method for modulating CW laser light into data carrying optical pulse trains. By employing proprietary GaAs technology, the Versawave modulators establish new benchmarks for low drive voltage, ultra-wide bandwidth, and chirp-free operation within a small footprint.The innovative and IP protected design of Versawave’s modulators exploit the unique material properties of GaAs to provide chirp free modulation. By using a unique polarization mode converter approach, Versawave eliminates many of the intrinsic limitations of designs based on Mach-Zehnder and other electro-absorption architectures. In addition, the Versawave Amplitude Modulators are able to deliver best-in-class performance over the entire Telcordia GR-468 environmental range without the need of a thermo-electric cooler (TEC).

(Cont.)

Applications.

• Analog and digital• SONET OC-768 and SDH STM-256 transmissions• 40 & 50 Gb/s transponders• High-speed Internet routers• DWDM, high-speed Ethernet and TDM• High-speed test equipment

Features.

• High modulation bandwidth• Low drive voltage• Chirp <0.1• High extinction ratio• Small footprint• Optional PIN diode for optical power monitoring and bias control• Covers C and L Band• GaAs technology

Demodulation

Demodulation is the process of recovering the signal intelligence from a modulated carrier wave. This process, also called detection, is the reverse process of modulation.

Why Do we Need Demodulation?

-The wireless signal consists of radio frequency (high frequency) carrier wave modulated by audio frequency (low frequency).

-The diaphragm of a telephone receiver or a loud speaker cannot vibrate with high frequency. Moreover, this frequency is beyond the audible range of human ear. So, it is necessary to separate the audio frequencies from radio- frequency carrier waves.

Amplitude Demodulation Process

IMPORTANCE OF RECTIFICATION AND DEMODULATION

• The modulation enveloped on the positive and negative sides are 180o out of phase.

• If rectification is not done, then after filtering, the positive and negative envelopes would cancel each other .

• So, the original modulating signal cannot be recovered. Thus, rectification is necessary in demodulation.

Types of AM Demodulation

The major types of AM demodulator are:

• Diode AM detector: This is by far the simplest form of AM demodulator or detector, requiring just a semiconductor (or other form) of diode along with a capacitor to remove the high frequency components. It suffers from a number of disadvantages, but its performance is more than adequate for most applications including broadcast receivers where cost is a significant driver.

• Synchronous AM detector: This form of AM detector offers a higher level of performance, but at the cost of considerably the use of considerably more components. This means that it is only used in receivers where the levels of performance are paramount and can justify the additional component costs.