20-07-2013, 02:39 PM
AMPLITUDE SHIFT KEYING (GENERATION AND DETECTION)
AIM:
To design and study the working of ASK modulation and demodulation system with the help of suitable circuit.
COMPONENTS REQUIRED:
2 Signal generators, resisters 39K- 2, 3.3K -2, SL 100 transistor-1, CRO & probes.
THEORY:
Introduction
The transmission of digital signals is increasing at a rapid rate. Low-frequency analog signals are often
converted to digital format (PAM) before transmission. The source signals are generally referred to as
baseband signals. We can send analog and digital signals directly over a medium. From electro-magnetic
theory, for efficient radiation of electrical energy from an antenna it must be at least in the order of magnitude
of a wavelength in size; c = fλ, where c is the velocity of light, f is the signal frequency and λ is the
wavelength. For a 1kHz audio signal, the wavelength is 300 km. An antenna of this size is not practical for
efficient transmission. The low-frequency signal is often frequency-translated to a higher frequency range for
efficient transmission. The process is called modulation. The use of a higher frequency range reduces
antenna size.
ASK
Amplitude shift keying - ASK - in the context of digital communications is a modulation process, which
imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels
adopted by the digital message.
For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated
waveform consists of bursts of a sinusoid.
A binary ASK (BASK) wave is obtained by multiplying the message signal with the carrier. The B-ASK
signal has two levels „1‟ and „0‟ representing the presence and absence of the sinusoid respectively. This can
be shown in the waveform below. The message signal must be represented in NZR uni polar format only.
Binary ASK system has the largest probability of bit error when compared to FSK and PSK systems.
There are sharp discontinuities shown at the transition points. These result in the signal having an
unnecessarily wide bandwidth. Band limiting is generally introduced before transmission, in which case
these discontinuities would be „rounded off‟. The band limiting may be applied to the digital message, or
the modulated signal itself.
PROCEDURE:
1) The circuit connections are made as per the circuit diagram.
2) A message signal with frequency about _______ Hz and amplitude of about _______ volts is fed to the
transistor and carrier is fed to the collector (nearly ______ volts).
3) ASK output is now drawn at the collector.
4) This ASK output is fed to the demodulator circuit and the message signal at the output is obtained.
5) The modulated and the modulating signal are drawn on a graph.