DIGITAL COMMUNICATIONS

PROJECT REPORT

AMPLITUDE SHIFT KEYING

SUBMITTED BY-

Aditya Narayan (08403)

Ashwin Jacob Matthews (084)

Caroline Kurian (08413)

Faheed Ahmed (084)

Jigme Rontgen (084)

KomalChawan (08433)

Mehzin Baker (08441)

Nissy Elsa John (08446)

Sonal Pinto (08449)

Sachin Suresh (08455)

Sai Kishore (08456)

VarshaGadam (08458)

Amplitude Shift Keying

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ACKNOWLEDGEMENT

We would like to take this opportunity to show our thankfulness in

specific to Dr. V. V. Mani, who so graciously aided us in making this project, all the way through her incessant guidance and undying support.

We like to thank her for making us conceptually correct. We would also like to express our gratitude to our respective, ever encouraging and forever loving families. Finally we would like to remember the wondrous

works of God the Almighty, who is a constant source of inspiration and helps us in all we do.

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INTRODUCTION

Amplitude-shift keying (ASK) is a form of modulation that represents digitaldata as variations in the amplitude of a carrier wave.

The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The

level of amplitude can be used to represent binary logic 0s and 1s. We can think of a carrier signal as an ON or OFF switch. In the modulated signal,

logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both

ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over

optical fiber. For LED transmitters, binary 1 is represented by a short pulse of light and binary 0 by the absence of light. Laser transmitters normally have a fixed "bias" current that causes the device to emit a low

light level. This low level represents binary 0, while a higher-amplitude lightwave represents binary 1.

Encoding

The simplest and most common form of ASK operates as a switch, using

the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying, and is used at radio frequencies to transmit RAYSUN code (referred to as

continuous wave operation).

More sophisticated encoding schemes have been developed which represent data in groups using additional amplitude levels. For instance, a

four-level encoding scheme can represent two bits with each shift in amplitude; an eight-level scheme can represent three bits; and so on. These forms of amplitude-shift keying require a high signal-to-noise ratio

for their recovery, as by their nature much of the signal is transmitted at reduced power.

Here is a diagram showing the ideal model for a transmission system

using an ASK modulation:

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It can be divided into three blocks. The first one represents the

transmitter, the second one is a linear model of the effects of the

channel, the third one shows the structure of the receiver. The following notation is used:

ht(f) is the carrier signal for the transmission hc(f) is the impulse response of the channel

n(t) is the noise introduced by the channel hr(f) is the filter at the receiver

L is the number of levels that are used for transmission Ts is the time between the generation of two symbols

Different symbols are represented with different voltages. If the maximum allowed value for the voltage is A, then all the possible values are in the

range [−A, A] and they are given by:

the difference between one voltage and the other is:

Considering the picture, the symbols v[n] are generated randomly by the

source S, then the impulse generator creates impulses with an area of v[n]. These impulses are sent to the filter ht to be sent through the

channel. In other words, for each symbol a different carrier wave is sent

with the relative amplitude.

Out of the transmitter, the signal s(t) can be expressed in the form:

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In the receiver, after the filtering through hr (t) the signal is:

where we use the notation:

nr(t) = n(t) * hr(f)

g(t) = ht(t) * hc(f) * hr(t)

where * indicates the convolution between two signals. After the A/D conversion the signal z[k] can be expressed in the form:

In this relationship, the second term represents the symbol to be extracted. The others are unwanted: the first one is the effect of noise,

the second one is due to the intersymbol interference.

If the filters are chosen so that g(t) will satisfy the Nyquist ISI criterion,

then there will be no intersymbol interference and the value of the sum will be zero, so:

z[k] = nr[k] + v[k]g[0]

the transmission will be affected only by noise.

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TYPICAL DEPLOYMENT SYSTEM

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ABSTRACT

Proposition 1- Digital data cannot be transmitted directly through free

space. If the digital data are to be transmitted through space using an antenna, it has to be converted into high frequency signal. For doing so,

the different modulation techniques are known as ASK (Amplitude Shift Keying), PSK (Phase Shift Keying) and FSK (Frequency Shift Keying).

Proposition 2- ASK is the digital modulation in which digital data are converted into analog signal by switching the carrier between amplitude levels.

Logic 0- Carrier Signal is at 1V

Logic 1- Carried Signal is at 2V

The ASK can be generated using an Analog Multiplexer that switches between the ASK basis signals.

The data source is an astable multivibrator using a 555 timer IC.

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Design

The core of this simple 2-ASK is the CMOS logic IC – CD4053. It is an 2x1

analog mux/demux. The mux is fed with the two ASK basis signals:

1V – Sine @ 50K (Logic – LOW)

2V – Sine @ 50K (Logic – HIGH)

A 50KHz sinusoidal at 1V amplitude is level shifted with an OpAmp Non Inverting amplifier(Gain = 2) to a 2V sinusoidal. These are fed to the

Analog Mux. These are the carrier signals.

The line selector of the Analog Mux is driven by the output of an Astable 555 timer. The Astable 555 is the data source which modulates the carrier signal.

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Fig: (From left to right) LM324, CD4053, NE555 and Oscilooscope.

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OBJECTIVES

To implement a simple 2-ASK.

To understand the use of a multiplexer as an Amplitude Shift Keying Modulator.

Ability to observe ASK digital modulation on a Oscilloscope. Ability to observe the functions of a MUX.

APPARATUS

Power Supply Function Generator

Dual Trace CRO Bread Board

COMPONENTS

LM 324 – Low Power Quad Operational Amplifier (National Semiconductor)

10kOhms x 2 CD4053 - Single 8-Channel Analog Multiplexer/Demultiplexer, Dual

4-Channel Analog Multiplexer/Demultiplexer, Triple 2-Channel

Analog Multiplexer/Demultiplexer (Fairchild Semiconductor) NE/SE 555 monolithic timing circuit (Philips Semiconductors)

40Ohms x 2

1uF 10nF

1N4007 - diode

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CIRCUIT DIAGRAM

EXPECTED WAVEFORMS

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APPLICATIONS

Amplitude-shift keying is used extensively for commercial terrestrial communications.

It is useful where satellite applications are limited. Space systems typically employ saturated power amplifiers. When an amplitude-shifted keying signal is passed through such a

nonlinear amplifier, sidelobes can grow large enough to interfere with the adjacent signals. As a result, the amount of bandwidth or power needed for signal transmission increases.

COMPARISON BETWEEN ASK AND FSK

ASK transmitters are simpler than FSK ASK transmitter current is 50% lower than FSK

SAW based ASK transmitters are more robust when exposed to extreme temperatures, vibration and shock

FSK transmission requires 1.5 times the bandwidth compared to

ASK ASK receivers are simpler than FSK

ASK receiver sensitivity is equal to or better than FSK Properly implemented, ASK receiver performance in the presence of

co-channel interference is generally better than FSK

Properly implemented, ASK receiver performance with amplitude flutter is equal to or better than FSK.

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INFERENCE

The 2-ASK modulator performed as expected.

Logic Carrier

0 50KHz/1V sine

1 50KHz/2V sine

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Bibliography

en.wikipedia.org/wiki/Amplitude-shift_keying

www.ele.uri.edu/Courses/ele436/labs/ASKnFSK.pdf www.national.com/ds/LM/LM555.pdf

www.alldatasheet.com/datasheet-pdf/pdf/8074/.../7485.html Communication Systems- Simon Haykin (4th Edition) Fundamentals of Communication Systems- John G

Proakis&MasoudSalehi