CBSE Class 12 Physics Chapter 15 Communication Systems Notes

Elements of a Communication System

Key Terminology

Transducer

A device that converts one form of energy into another. For example, a microphone converts sound energy (message) into electrical energy.

Signal

Information converted into a suitable electrical format for transmission. It can be analog (continuous values) or digital (discrete values).

Attenuation

The loss of signal strength as it propagates through the communication channel.

Amplification

The process of increasing the amplitude (and hence strength) of a signal using an electronic circuit called an amplifier.

Bandwidth

The range of frequencies over which a communication system operates or a signal contains. It is the difference between the upper and lower frequencies.

Modulation

The process of superimposing a low-frequency message signal onto a high-frequency carrier wave.

Demodulation

The process of retrieving the original message signal from the modulated carrier wave at the receiver end. Also known as detection.

Repeater

A combination of a receiver and a transmitter used to extend the communication range by amplifying and retransmitting the signal.

Need for Modulation

Why can't we just transmit our original low-frequency audio or video signals directly through an antenna? The answer lies in the necessity of modulation. Modulation is the process of superimposing the low-frequency message signal (also called the baseband signal) onto a high-frequency wave, known as the carrier wave. There are several critical reasons for this:

1. Practical Antenna Size: For effective signal radiation, the antenna length should be comparable to the wavelength (λ) of the signal, ideally λ/4. For a typical audio signal of 15 kHz, the required antenna length would be λ/4 = (c/4f) = (3x10⁸)/(4x15x10³) ≈ 5000 meters, which is impractically large. By modulating this onto a 1 MHz carrier wave, the antenna length becomes a manageable 75 meters.

1. Effective Power Radiation: The power radiated by an antenna is proportional to (l/λ)², where 'l' is the antenna length. For a given antenna length, power radiation is very low for long-wavelength (low-frequency) signals. Shifting the signal to a higher frequency (shorter wavelength) drastically increases the radiated power.

1. Avoiding Signal Mixing: If multiple users transmitted their low-frequency baseband signals (e.g., audio) simultaneously, all the signals would mix up in the communication channel, making it impossible to distinguish between them at the receiver. Modulation allows us to assign different high-frequency carrier bands to different users (like different radio stations), preventing interference. This is the principle behind Frequency Division Multiplexing (FDM).

Must Remember Formulas & Facts

• Modulation Index (µ) for AM: µ = Vm / Vc, where Vm is the amplitude of the modulating signal and Vc is the amplitude of the carrier wave. For avoiding distortion, µ ≤ 1.
• AM Equation: c(t) = (Vc + Vm sin(ωm t)) sin(ωc t) = Vc(1 + µ sin(ωm t)) sin(ωc t).
• AM Sidebands: An AM wave has three frequency components: ωc (carrier), ωc - ωm (Lower Sideband, LSB), and ωc + ωm (Upper Sideband, USB).
• Bandwidth of AM: BW_AM = 2 × f_m (twice the maximum frequency of the message signal).
• Bandwidth of FM: BW_FM = 2(n f_m + Δf) = 2(β + 1)f_m, where β is the modulation index for FM. It is much wider than AM.
• Line-of-Sight (LOS) Communication Range: d = √(2Rh_T) + √(2Rh_R), where R is the radius of Earth, h_T is the height of the transmitting antenna, and h_R is the height of the receiving antenna.
• Maximum LOS distance from a single antenna: d_m = √(2Rh_T).
• Ground Wave Propagation: Used for low frequencies (< 2 MHz), e.g., AM radio broadcasts. Follows Earth's curvature.
• Sky Wave Propagation: Used for frequencies in the 3 MHz to 30 MHz range. Relies on reflection from the ionosphere.
• Space Wave Propagation: Used for very high frequencies (> 40 MHz), e.g., TV, FM radio, satellite. Travels in a straight line (LOS).

Amplitude Modulation (AM) vs. Frequency Modulation (FM)

Worked Example: Modulation Index

• {"name":"Calculating Modulation Index","bodyMarkdown":"Question: A sinusoidal carrier voltage of frequency 1.5 MHz and amplitude 50 V is amplitude modulated by a sinusoidal voltage of frequency 10 kHz producing 50% modulation. Calculate the amplitude of the modulating signal.\n\nSolution:\nGiven:\nCarrier Amplitude, Vc = 50 V\nModulation, m (or µ) = 50% = 0.5\n\nWe know the formula for modulation index is:\nµ = Vm / Vc\n\nRearranging for the amplitude of the modulating signal (Vm):\nVm = µ × Vc\nVm = 0.5 × 50 V\nVm = 25 V\n\nAnswer: The amplitude of the modulating signal is 25 V."}

Exam Tip: Block Diagrams & Derivations

Block diagrams are a frequent feature in questions from this chapter. Always draw them using a ruler, label every block clearly (e.g., 'Modulator', 'RF Amplifier'), and indicate the direction of signal flow with arrows. For the AM wave equation, practice deriving the sideband frequencies (ωc + ωm and ωc - ωm) from the trigonometric expansion. Marks are often awarded for showing these intermediate steps.

Practice Questions with Solutions

• What is the function of a transducer in a communication system? A transducer converts the original message (like sound or light) into a corresponding electrical signal, or vice-versa.
• Why is sky wave propagation not suitable for frequencies above 30 MHz? Frequencies above 30 MHz are not reflected by the ionosphere; they penetrate it and escape into space. Therefore, sky wave propagation is ineffective for these high frequencies.
• What is the bandwidth of a speech signal if its frequency ranges from 300 Hz to 3100 Hz? Bandwidth = Highest Frequency - Lowest Frequency = 3100 Hz - 300 Hz = 2800 Hz or 2.8 kHz.
• What does it mean if the modulation index (µ) of an AM wave is greater than 1? If µ > 1, it results in overmodulation. The carrier wave is reduced to zero for more than a complete half-cycle, leading to severe distortion and loss of information.

Frequently Asked Questions