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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.
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.
The block diagram of the AM modulation used in this project is shown in the following diagram;
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.
Variable frequency sine wave generator
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;