Modulation and Demodulation

Signals consist of two components – the information signal and the carrier signal. The transmission of any signal over some communication medium usually involves MODULATION of a carrier. Prior to their transmission the information signal and the carrier signal are combined and the process of combining these two signals is called MODULATION.

A Basic Example of Modulation:

I am sitting at the back of a big spacious hall. I see my friend at the front. I have information which I want to pass to my friend. Assuming for any reason I cannot get close to my friend. I write the information I want to pass to my friend on a piece of paper and wrap the paper around a pen (carrier) and throw (transmit) the two (information and carrier) to my friend. The information will reach her.

The high frequency wave which carries the information through a medium is called the CARRIER. The information is superimposed onto the ‘carrier wave' by modulation. Therefore the carrier provides a means of transferring information at high frequency. Before modulation the carrier does not contain any ‘intelligence’ and it only serves to carry information. Once the information is extracted from the carrier at the receiving end, the carrier becomes redundant The carrier is a single frequency generally represented as:

a(t) = A sin(Ft + ö)

A = the Amplitude

FT = Frequency

ö = Phase

Modulating the carrier means changing something in the equation above. Only three things can be changed as shown above: amplitude, frequency or phase reference. This leads to one class of analog modulation.

Changing A leads to Amplitude Modulation

Changing F leads to Frequency Modulation

Changing ö leads to Phase Modulation.

Before being modulated by an information signal, the carrier signal has a single FREQUENCY and a steady AMPLITUDE. After modulation the carrier will have varying amplitude or frequency. There are various types of modulation and the following three will be covered in this class:

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)

The figure on the following slide shows an audio wave (fm) and carrier (fc) applied to a modulator. The modulator output is shown drawn both in time spectrum and frequency spectrum. If we look carefully at the frequency spectrum we can see that there are two new frequencies which were not present at the beginning. These are the side frequencies we talked about when dealing with Frequency Division Multiplexing. Fc + fm is the upper sideband and fc – fm is the lower sideband. Can you get the meaning of this? (i.e. the relationship between Modulation and Frequency Division Multiplexing).