Question
Draw and explain the block diagram of a communication system. Label and explain the function of each block.
Solution — Step by Step
Every communication system — whether it’s a landline phone, radio broadcast, or mobile network — has the same fundamental architecture:
Information Source → Transmitter → Channel → Receiver → Destination
The information source generates the message. The transmitter processes it for transmission. The channel is the medium through which the signal travels. The receiver extracts the message. The destination is the intended recipient.
1. Information Source (Message Signal): Produces the original message — voice, data, video, music. Outputs a baseband signal (the original information signal at low frequencies).
2. Transmitter: Processes the message signal so it can travel efficiently over the channel. The transmitter performs two key operations:
- Modulation: Superimposes the message signal onto a high-frequency carrier wave. This is necessary because: (a) High-frequency signals travel long distances as electromagnetic waves (b) Multiple signals can share the channel at different carrier frequencies (multiplexing)
- Amplification: Boosts the signal power for long-range transmission
3. Channel (Transmission Medium): The physical medium through which the signal travels. Types:
- Wired: Telephone cables, coaxial cables, optical fibre
- Wireless (free space): Atmosphere, vacuum (for satellite communication)
The channel introduces noise — unwanted random signals that get added to the transmitted signal.
4. Receiver: Extracts the original message from the received (noisy) signal. The receiver performs:
- Demodulation: Reverses the modulation process to recover the baseband signal
- Amplification: Compensates for signal attenuation in the channel
- Filtering: Removes noise as much as possible
5. Destination: The final recipient — a speaker (for audio), a screen (for video), a computer (for data).
Noise is shown entering the channel, degrading the signal. It’s an unwanted component that limits communication quality.
[Information] [Transmitter] [Channel] [Receiver] [Destination]
Source → (Modulator + → ───────── → (Demodulator → (User)
(Message) Amplifier) ↑ Noise ↑ + Amplifier)The block diagram should show:
- Message source
- Transmitter box (containing modulator + power amplifier)
- Channel block with a noise arrow pointing into it
- Receiver box (containing demodulator + amplifier)
- Destination
In CBSE exams, draw neat rectangular boxes connected by arrows, with the noise source shown as a separate input into the channel.
Why This Works
The block diagram captures the universal structure of communication — what changes between systems (radio vs fibre optics vs Wi-Fi) is only the implementation of each block, not the structure itself. This is why the block diagram is such a powerful teaching tool.
The reason we need modulation: most information signals (voice: 20 Hz–20 kHz, video: up to 4 MHz) cannot propagate long distances as electromagnetic waves because their wavelengths are enormous and they require impractically large antennas. By shifting to high-frequency carriers (AM radio: 530–1600 kHz; FM radio: 88–108 MHz; Wi-Fi: 2.4 GHz), we can transmit with compact antennas and reach long distances.
Alternative Method
For CBSE Class 12 exams, a clean hand-drawn version:
Boxes in sequence: Signal Source → Transmitter → (through) Channel → Receiver → Destination, with an arrowhead labelled “Noise” entering the channel from below.
Inside the transmitter, you can optionally show: [Transducer → Signal Processor → Modulator → Power Amplifier → Antenna].
Common Mistake
Students often draw the block diagram without showing noise — and lose marks for the most important element. Noise entering the channel is a fundamental part of any communication system. Always show it explicitly as a separate arrow entering the channel block. The entire purpose of receiver design is to combat this noise.
For CBSE 2-mark questions, you need: block diagram + one-line function of each block. For 3-mark questions, also explain what modulation is and why it’s needed. For 5-mark questions, include types of modulation (AM, FM) and the bandwidth required for each. Build your answer up from this core.