Role of amplitude modulation:
The role of amplitude modulation in many of these applications should be obvious if one understands the frequency-shifting property of amplitude modulation.
Several other types of applications are also feasible due to the fact that power of the carrier signal can be increased somewhat arbitrarily, irrespective of the power level of the data (modulating) signal. Let us discuss, one by one, the six categories of applications mentioned above.
AC signal conditioning devices such as ac amplifiers are known to be more “stable” than their dc counterparts. In particular, drift problems are not as severe and nonlinearity effects are lower in ac signal conditioning devices. Hence, instead of conditioning a dc signal using dc hardware, we can first use the signal to modulate a high-frequency carrier signal.
Then, the resulting high-frequency modulated signal may be conditioned more effectively using ac hardware.
The frequency-shifting property of amplitude modulation can be exploited in making low-frequency signals immune to low-frequency noise.
low-frequency spectrum of the modulating signal can be shifted out into a very high frequency region, by choosing a carrier frequency ƒc that is sufficiently large.
Then, any low-frequency noise (within the band 0 to fc-fb) would not distort the spectrum of the modulated signal. Hence, this noise could be removed by a high-pass filter (with cutoff at fc-fb) so that it would not affect the data. Finally, the original data signal can be recovered using demodulation.
Since the frequency of a noise component can very well be within the bandwidth fb of the data signal, if amplitude modulation was not employed, noise could directly distort the data signal.
Transmission of ac signals is more efficient than that of dc signals.
Advantages of ac transmission include lower energy dissipation problems. As a result, a modulated signal can be transmitted
over long distances more effectively than could the original data signal alone.
Furthermore, the transmission of low-frequency (large wave-length) signals requires large antennas. Hence, when amplitude modulation is employed (with an associated reduction in signal wave length), the size of broadcast antenna can be effectively reduced.
Transmission of weak signals over long distances is not desirable because further signal weakening and corruption by noise could produce disastrous results.
