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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
1
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.
Amplitude Shift Keying
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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:
Amplitude Shift Keying
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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:
Amplitude Shift Keying
4
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.
Amplitude Shift Keying
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TYPICAL DEPLOYMENT SYSTEM
Amplitude Shift Keying
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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.
Amplitude Shift Keying
7
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.
Amplitude Shift Keying
8
Fig: (From left to right) LM324, CD4053, NE555 and Oscilooscope.
Amplitude Shift Keying
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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
Amplitude Shift Keying
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CIRCUIT DIAGRAM
EXPECTED WAVEFORMS
Amplitude Shift Keying
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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.
Amplitude Shift Keying
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INFERENCE
The 2-ASK modulator performed as expected.
Logic Carrier
0 50KHz/1V sine
1 50KHz/2V sine
Amplitude Shift Keying
13
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