DATA COMMUNICATION PRINCIPLES Amplitude Modulation AM Amplitude Modulation

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DATA COMMUNICATION PRINCIPLES : Amplitude Modulation (AM)

DATA COMMUNICATION PRINCIPLES : Amplitude Modulation (AM)

Amplitude Modulation (AM) The AM signal is generated by varying the amplitude of the

Amplitude Modulation (AM) The AM signal is generated by varying the amplitude of the carrier signal in proportion to modulating signal m(t). This is easily achieved by multiplying the carrier amplitude with m(t). The AM modulator is sometimes called a multiplier or mixer for this reason. As seen in Figure 3 -12, the amplitude of the carrier makes an envelope that resembles the modulating signal. The receiver of this signal only needs to detect this envelope. In fact, one type of AM receiver is called an envelope detector. This is the simplest form of amplitude modulation and has its own name - double sideband with suppressed carrier (DSC-SC). The sideband refers to the each half of the frequency spectrum about the origin. Each side carries all the signal information and differs only in the sign of frequencies in them. The side band on the right side of the origin has positive frequencies while the other sideband has negative frequencies. Naturally, one may ask if only one of the sidebands can be used in AM. The answer is yes, as is done in single sideband AM systems. The reason why it is called as suppressed carrier is explained next.

The AM signal shown in Figure 3 -12 can not be exactly detected by

The AM signal shown in Figure 3 -12 can not be exactly detected by an envelope detector. This is because the envelope detector would detect only the positive values of the signal while the actual signal had both, the positive and negative values. The result of envelope detector is shown in Figure 3 -13. The envelope in fact receives the absolute value of the modulating signal.

One solution to this problem is to add extra carrier with the AM signal

One solution to this problem is to add extra carrier with the AM signal and instead of transmitting transmit This additional carrier with an amplitude of A is called the pilot carrier and it helps the receiver get both positive and negative values of the signal on the positive (or negative) side of the envelope by adding a dc bias of A to the envelop. Then by removing the dc bias, the original signal can be recovered. See Figure 3 -14 below for the final result. This is equivalent to receiving

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks •

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press