Why AC Signals Are Used in Communication Systems

In the realm of communication systems, AC signals are the preferred choice over DC signals when transmitting information. This choice is fundamentally rooted in the nature of information transmission and the characteristics of these signals.

DC vs. AC Signals

A DC (Direct Current) signal is constant and does not carry any information because it lacks variation or change over time. For communication systems, which are designed to convey information, a static DC signal is insufficient and unsuitable. Hence, the need arises to utilize signals that can change over time to transport information effectively.

Fourier Theory and AC Signals

In the realm of signal processing, Fourier theory states that any periodic signal can be decomposed into a series of sines and cosines, known as AC signals. This means that a DC value that varies over time, such as rectangular pulses with different heights, can be seen as a superposition of AC signals. This is why AC signals are necessary for carrying information in communication systems.

Advantages of AC Signals in Radio Wave Propagation

At frequencies ranging from around 60kHz to tens of GHz, radio waves enable communication even on a global scale. These higher frequencies are capable of radio wave propagation, unlike DC signals which are primarily used for power supply. AC signals, when converted to radio waves, can travel long distances and even around the globe, making them ideal for communication channels.

No Information at DC

According to basic communication theory, no information can be conveyed through a DC signal. This is because a DC signal remains constant and does not vary, thus providing no means to transmit information.

Fundamentals of Signal Change

The fundamental concept in communication systems is that events on both ends of the link—transmission and reception—must change for communication to occur. Therefore, something must vary for the signal to convey information. Whether a DC signal or a sinusoidal signal is used does not matter as long as something changes.

Amplitude, Frequency, and Phase

A pure sinusoid, which is a special case of a sinusoidal signal with a frequency of 0 Hz, is inadequate for communication because it does not change. Thus, an AC signal in the conventional sense is not used. Instead, modulated sinusoids, which change in amplitude, frequency, or phase (or combinations of these), are used to convey information.

Modulated Sinusoids

For effective information transmission, a signal should not be a static sinusoid but a time-changing sinusoid that changes based on the information being sent. This modulated sinusoid can vary in its amplitude, frequency, or phase, or any combination thereof, to represent different data. For instance, if a fixed frequency and phase are used, the amplitude can vary to represent binary data—5 volts for a '1' and -5 volts for a '0'. Similarly, the frequency can be changed to represent different data.

The important point is that the signal must change according to the information being sent. This ensures that the signal carries the necessary information and is not a static, non-transmitting DC.

Conclusion

In summary, AC signals are essential in communication systems due to their ability to change and carry information, whereas DC signals are primarily used for power supply. Understanding the nature of these signals and their applications is crucial for effective communication system design and implementation.