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Physics > Communication Systems > Detection of Amplitude Modulated Wave
Communication Systems

Detection of Amplitude Modulated Wave

We have all studied Amplitude Modulation, its applications and uses in modern life. In this discussion, we look at the stages involved in reconverting a signal received into its original form by undergoing various processes. This is what we call Detection of Amplitude Modulated Wave, also known as Demodulation.

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Introduction to Amplitude Modulation

We have already studied how a signal generated is modulated via superimposition with a carrier signal to modify the original signal to be able to be transmitted at a certain bandwidth. The signal is then transmitted over long distances using either analog or digital mediums.

At the same time, we also know that the signal is repeated time and again at various intervals of distance and time to ensure the least degradation of the quality of the signal and to prevent additional noise i.e. data which is not required. At the receiver end too, the signal received has to undergo many processes to regenerate or obtain the original signal in the required format.

We will have a look at all the processes in an order. The receiver usually consists of an arrangement of a receiving antenna coupled with an amplifier, a detector, and demodulator along with a filter and a final amplifier to regenerate the original signal as shown above.

We have seen that the signal experiences ‘attenuation’ i.e. a loss in strength and quality as it travels long distances from the transmitter to the receiver. It is therefore imperative to have the signal amplified as it is received at the antenna in a particular bandwidth. Thus an amplifier is placed right at the base of the antenna and amplification is the first process a signal undergoes at the receiver station.

What does the Amplifier do?

The amplifier not only amplifies the signal it also amplifies the noise that the signal brings along with it as it travels over large distances. But do we need the noise?

The answer is obviously, No. This noise is undesirable as it deteriorates the quality of the signal. It, therefore, needs to be separated from the signal. How to do that? To achieve this objective a filter is used which is in most cases a simple diode which allows signals only in a particular bandwidth to be transmitted.

After this, the signal needs to be converted to its original frequency. But this objective cannot be attained in a single step as the frequency conversion ratio is not always optimal. Hence firstly the higher frequency is converted into an intermediate frequency using an IF stage converter. After conversion to an IF stage, the signal undergoes what is called a ‘Detection’.

Detection

Detection is the process of retrieving the original signal from the modulated form.

Amplitude Modulation

Consider the above diagram showing ‘demodulation’ of a received signal. The original frequency of the signal is let us say ‘ωc’ corresponding to the signal ‘c’ in the diagram. This signal is modulated using a carrier signal having a frequency ‘ωc ± ωb’

This is Squaring of Frequency. It gives us a signal with a frequency bandwidth of ‘ωa’ as shown in the figure. To receive the original signal we demodulate the received signal knowing the predefined frequencies which are added to the signal for generating the current bandwidth. In this case, the frequency range is ‘±ωb’.

So the detector actually removes these frequencies from the signal using diodes for an analog signal or uses digital means to obtain the natural frequency of the signal. Thus the detector generates the original frequency of the signal.

An important point to note is that in the above process a simple RC circuit can be additionally used along with the detector to generate the original frequency of the signal. This is known as a Detector Envelope which can be used to differentiate the incoming signal from the IF stage signal.

Filtration

After the original signal has been generated it needs to be polished before it is sent as an output. Most of the times this involves a series of filtrations. After the filtrations, the signal is again amplified at the final stage. This is done to enhance the effect of the signal and improve its quality.

Once this is done the signal either an audio or audio-visual is sent at the output via either speakers or headphones or on a screen. This completes the cycle of conversion for a signal transmitted from a source after undergoing modulation to a receiver and at the receiving station, the signal is again converted back to its original form after undergoing demodulation.

The processes adopted in the entire cycle help to preserve the quality of the signal preventing its deterioration due to distance and atmosphere. Modern electronic age is highly dependent on robust systems that help a very large number of signals to undergo the entire cycle on a large scale and at all times as the world gets more and more dependent on communication with regular developments in this field improving the quality of communication.

Thus, we have overseen and discussed the science and processes involved in the conversion of a signal to its original form following the detection of an amplitude modulated wave. Similar processes are adopted for signals with frequency or phase modulations the only differences being the positions and differences in types of equipment required for the processes.

A thorough understanding of these concepts will help the reader to easily grasp the challenges and solutions existing in the modern world and help to devise improvements for the same.

Solved Question on Amplitude Modulation

Q. Amplitude modulation is used for broadcasting because

  1. more noise
  2. requires less transmitting power
  3. it has simple circuitry
  4. it has high fidelity

Answer: C. Amplitude modulation is used because of it simpler and cheaper as compared to other techniques.

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