About this previous year paper
Principles of Communication (NEC-502)
Section-Wise Important Questions & Study Notes
SECTION–A
(Attempt all | 2 × 10 = 20)
This section checks basic definitions, diagrams, and short comparisons.
Write 2–3 lines only, draw small neat diagrams wherever asked.
Baseband Signal
A baseband signal is the original low-frequency information signal (speech, music, data) before modulation.
Analog Communication System (Block Diagram)
Consists of information source → transmitter → channel → receiver → destination. Draw a simple block diagram.
VSB Signal (Vestigial Sideband)
VSB is a form of AM where one sideband is fully transmitted and the other partially, reducing bandwidth.
Used in TV transmission.
Phasor Diagram of AM
Shows carrier and sidebands as rotating vectors. Application: AM radio broadcasting.
Modulation & Its Need
Modulation is the process of varying a carrier with the message signal.
Required for efficient radiation, reduced antenna size, and noise reduction.
Need of Line Coding
Line coding converts digital data into a form suitable for transmission with less DC content and better synchronization.
Quantizer
A device that converts continuous amplitude samples into discrete levels.
It introduces quantization noise in digital transmission.
Narrowband FM vs Wideband FM (Any Two Points)
NBFM: small bandwidth, low noise immunity WBFM: large bandwidth, high noise immunity
White Noise
Noise with constant power spectral density over all frequencies.
Draw flat spectral density curve.
Noise Figure & Sources of Noise
Noise figure measures degradation of SNR by a system. Sources: thermal noise, shot noise, flicker noise.
SECTION–B
(Attempt any THREE | 10 × 3 = 30)
Answers must be descriptive, with equations and diagrams.
1. Power Saving in SSB
Calculate percentage saving when carrier and one sideband are suppressed for:
100% modulation 75% modulation Remember: SSB is highly power-efficient.
2. Phase Shift Method of SSB Generation
Explain: Balanced modulator
Phase shifter Adder
Draw block diagram and explain working.
3. PWM Modulation & Demodulation
Explain principle of Pulse Width Modulation with waveforms.
Mention applications in control systems.
4. Noise & Its Effects
Explain: Types of noise
Effect of filters on noise Slope overload distortion
Granular (idle) noise
5. Differential PCM (DPCM)
Explain working with proper block diagram.
Mention advantage: reduced bit rate compared to PCM.
SECTION–C
(Attempt any ONE | 10 × 1 = 10)
Numerical + derivation based.
3(a) Efficiency of AM
Given:
η=PsPt\eta = \frac{P_s}{P_t}η=PtPs
i) Find efficiency for m = 0.5 ii) Prove maximum efficiency = 33% at m = 1
Very important derivation.
OR
3(b) SNR of AM with Envelope Detection
Derive:
(SN)=m22+m2γ\left(\frac{S}{N}\right) = \frac{m^2}{2+m^2}\gamma(NS)=2+m2m2γ
Explain terms clearly.