Page 97 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes modulation respectively. The angular frequency is defined as the 2p times of the frequency of
carrier signal.

Figure 6.13: Two Different Sinusoids

f m f m

time time

In other words, modulation is used to superimpose a message (voice, image, data, etc.) on to a
carrier wave for transmission. The frequencies that comprise the message (base band) are
translated to a higher range of frequencies. The frequencies that comprise the message is preserved,
that is, every frequency in that message is scaled by a constant value as explained above.
Modulation is necessary for data communication because of several reasons. It allows the
simultaneous transmission of two or more base band signals by translating them to different
frequencies. It also reduces the size of antenna for higher frequencies with the greater efficiency.

Inter Modulation

Inter modulation is a special case where two (or more) sinusoids effect one another to produce
undesired products, that is, unwanted frequencies (noise). Again, this can only occur when both
waves share the same non-linear device. The non-linearity results in several even or odd
harmonics. Harmonics are the multiple of the fundamental frequency, that is, the message
frequency. The modulating index is the ratio of the peak of the modulating signal to the peak of
the carrier in case of amplitude modulation. In angular modulation, the ratio of the frequency
deviation of the modulated signal to the frequency of a sinusoidal modulating signals.

Did u know? The modulation index is numerically equal to the phase deviation in radians.

6.5.1 Amplitude Modulation

This describes the technique in which the carrier wave is multiplied by the digital signal f(t).
Mathematically, the modulated carrier signal y(t) is: y(t) = f(t) sin(2p¦ct+j) where fc is a carrier
frequency and t is instantaneous time. Figure 6.14 shows the technique of amplitude modulation.
The main advantage of this technique is that it is easy to produce such signals and also to detect
them. This technique has two major disadvantages. The first is that the speed of the changing
amplitude is limited by the bandwidth of the line. The second is that the small amplitude
changes suffer from unreliable detection. Telephone lines limit amplitude changes to some 3000
changes per second. The disadvantages of amplitude modulation causes this technique to no
longer be used by modems, however, it is used in conjunction with other techniques.
QAM (Quadrature Amplitude Modulation)
This technique is based on the basic amplitude modulation. This technique improves the
performance of the basic amplitude modulation. In this technique two carrier signals are
transmitted simultaneously. The two carrier signals are at the same frequency with a 90 degrees
phase shift.

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