Communication Systems and Modulation Techniques
This domain tests the block diagram of a communication link, the purpose of modulation, AM/FM parameters, bandwidth, noise and receiver structures. Numerical questions on modulation index, sidebands, FM deviation and IF stages are common.
Core rules
- Modulation shifts a baseband signal onto a carrier for efficient radiation and frequency reuse.
- AM: modulation index m = Vm/Vc; bandwidth = 2fm; total power Pt = Pc(1 + m²/2).
- FM: modulation index β = Δf/fm; Carson's rule bandwidth B ≈ 2(Δf + fm).
- The superheterodyne receiver mixes incoming RF with a local oscillator to a fixed intermediate frequency (IF); image frequency must be rejected by the front-end filter.
- Noise figure and SNR determine receiver sensitivity; FM offers better noise immunity at the cost of bandwidth.
Worked example
An FM signal has maximum deviation Δf = 15 kHz and modulating frequency fm = 3 kHz. Then β = 15/3 = 5. Carson's rule gives B = 2(15 + 3) = 36 kHz. A student who writes 2fm = 6 kHz has calculated an AM bandwidth, not an FM bandwidth.
Traps that cost marks
- Reporting AM bandwidth as fm instead of 2fm.
- Confusing deviation Δf with modulation index β.
- Forgetting that AM power includes the carrier plus both sidebands.
- Mixing up the local oscillator tracking in superheterodyne receivers.
- Assuming FM has infinite noise immunity; it only improves SNR above a threshold.
Exam approach
Write the standard formula, substitute in consistent units (kHz or Hz), and state the result with its unit. For block-diagram questions, trace the signal from transducer through modulator, channel, demodulator and output.