The AM modulation
is a kind of modulation technique which is in use since the very early
days of wireless data transmission. In a radio transmission system there
is a relation between the ranges of frequencies which can be
transmitted wirelessly with the length of the transmitting antenna. The
relation is inversely proportional to one another, means as the
frequency of the signal to be transmitted increases the length of the
antenna can be reduced and as the frequency of the signal to be
transmitted decreases the length of the transmitting antenna should be
increased accordingly.
Using
an antenna of few meters the frequencies in the range of Mhz can be
easily transmitted to a distance. The basic purpose of the wireless
transmitting system in early days was to transmit the audio signals, but
to transmit audio signals which fall in the range of few Khz an antenna
of more than a kilometer height would have been required. Since it was
practically impossible to construct such a long antenna, the high
frequency signals are transmitted after they are modulated with the low
frequency audio signals.
The
amplitude modulation is the simplest modulation technique among the
wide verity of modulation techniques in use. The amplitude modulation of
a high frequency signal is easy to achieve and the demodulation is also
simple compared to other techniques. The high frequency signal which is
modulated to carry the low frequency audio signals are called ‘carrier
frequency’ and the audio signals used for modulation is called
‘modulating signal’ or ‘message signal’ or ‘base band signal’. This
article demonstrates how to generate an Amplitude Modulation (AM) using
the simplest possible circuit.
DESCRIPTION:
To
demonstrate the AM modulation of a carrier signal with a message
signal, both the carrier signal and message signal generating circuits
are also made and the details of them will be discussed in the
subsequent section. The carrier signal and message signal used in this
project are pure sine waves. Hence the entire circuit can be divided
into three blocks:
1. Carrier frequency generator (High frequency sine wave)
2. Message frequency generator (Low frequency sine wave)
3. AM Modulator.
For
both the carrier signal of high frequency and the message signal of low
frequency, exactly same sine wave generating circuits are designed but
the frequencies are set to high and low respectively with the help of
their variable components. Hence this project has two similar variable
frequency sine wave generator circuits and an AM modulator circuit.
The
sine wave generation circuit used in this project is the Wien bridge
oscillator circuit. This is the only circuit which can generate the pure
sine wave without any distortion. The amplifier component used in the
Wien bridge circuit is an op-amp with dual-power supply. Both the
circuits are built around the versatile op-amp IC, 741. The circuit of
the sine wave generator is shown in the following figure.
The
frequency of the above circuit can be varied by simply varying the
potentiometer R2 and the amplitude of the wave form can be adjusted by
varying the potentiometer R. The frequency of the sine wave generated by
the above circuit depends on the components R1, R2, C1 and C2 and the
equation for the frequency is given below;
The image of the circuit made for generating the carries signal is shown below;
The
circuit is then adjusted to generate a high frequency sine wave and
which can be observed in a CRO as shown in the following image;
An
exactly similar circuit is wired again in the bread board and this time
the circuit is adjusted to generate a low frequency sine wave. The two
sine wave generating circuits wired in a bread board is shown in the
following image;
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