UPSC ESE E&T Communication Systems Syllabus

Communication Systems syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Communication Systems in UPSC ESE E&T, not a summary of it.

1. 1

   Analog Communication

   4 topics

   • Amplitude Modulation
   • Frequency Modulation
   • Phase Modulation
   • Transmitters and Receivers
2. 2

   Digital Communication

   4 topics

   • Pulse Code Modulation
   • Digital Modulation Techniques
   • Error Control Coding
   • Data Compression
3. 3

   Information Theory and Coding

   4 topics

   • Entropy
   • Channel Capacity
   • Source Coding
   • Error Detection and Correction

Communication Systems flashcards for UPSC ESE E&T

18 of 50 cards from the Communication Systems deck — real questions with worked answers.

1. What is amplitude modulation (AM)?

   A modulation scheme in which the amplitude (envelope) of a high-frequency carrier is varied in proportion to the instantaneous amplitude of the message signal, while the carrier frequency and phase remain constant.

2. Define modulation index (m) for an AM wave and its standard expression.

   The modulation index m = Am/Ac, the ratio of message amplitude to carrier amplitude. Equivalently m = (Vmax - Vmin)/(Vmax + Vmin). For distortionless AM, 0 < m ≤ 1; m > 1 causes overmodulation and envelope distortion.

3. What fraction of total transmitted power lies in the sidebands of a standard AM (DSB-FC) wave at 100% modulation?

   At m = 1, total power Pt = Pc(1 + m²/2) = 1.5Pc, so sidebands carry Pc/2 out of 1.5Pc = 1/3 (about 33.3%); the carrier carries 2/3 (66.7%) and conveys no information.

4. What is the transmission bandwidth of a standard AM (DSB) signal?

   BW = 2·fm, twice the highest message frequency, because both upper and lower sidebands are transmitted.

5. What is the power and bandwidth advantage of SSB-SC over standard AM?

   Single-sideband suppressed carrier transmits only one sideband and no carrier, so it needs half the bandwidth (BW = fm instead of 2fm) and uses far less power, since all transmitted power conveys information.

6. In DSB-SC, why is coherent (synchronous) detection required instead of envelope detection?

   Because the carrier is suppressed, the envelope no longer follows the message and reverses phase at zero crossings; recovering the message requires multiplying by a locally generated carrier of identical frequency and phase, then low-pass filtering.

7. Define frequency modulation (FM).

   An angle modulation scheme in which the instantaneous frequency of the carrier is varied linearly with the instantaneous amplitude of the message signal, while the carrier amplitude stays constant.

8. Write the expression for frequency deviation and modulation index of an FM signal.

   Frequency deviation Δf = kf·Am (kf is the frequency sensitivity). FM modulation index β = Δf/fm, the ratio of peak frequency deviation to the modulating frequency.

9. State Carson's rule for the bandwidth of an FM signal.

   BW ≈ 2(Δf + fm) = 2fm(β + 1), where Δf is peak frequency deviation, fm is the highest message frequency, and β is the modulation index.

10. Distinguish narrowband FM (NBFM) from wideband FM (WBFM).

    NBFM has β << 1 (typically β < 1), occupies a bandwidth ≈ 2fm similar to AM, and has essentially one significant sideband pair. WBFM has β >> 1, occupies a much larger bandwidth (per Carson's rule), and gives better noise immunity.

11. Why does FM provide better noise immunity than AM?

    Because information is carried in frequency, not amplitude, amplitude disturbances (noise) can be removed by a limiter before detection. FM also benefits from wider bandwidth and capture effect, improving SNR.

12. What is the FM capture effect?

    The tendency of an FM receiver to suppress the weaker of two signals at the same frequency; the stronger signal 'captures' the demodulator, so a relatively small signal-strength advantage results in the weaker signal being largely rejected.

13. What are pre-emphasis and de-emphasis in FM, and why are they used?

    Pre-emphasis boosts high-frequency components of the message before transmission; de-emphasis attenuates them by the inverse amount at the receiver. This improves the high-frequency SNR because FM noise power rises with frequency (parabolic noise spectrum).

14. Define phase modulation (PM).

    An angle modulation scheme in which the instantaneous phase of the carrier is varied linearly with the instantaneous amplitude of the message signal, while the carrier amplitude remains constant.

15. State the relationship between FM and PM.

    PM and FM are closely related forms of angle modulation: FM of a message m(t) is equivalent to PM of its integral, and PM of m(t) is equivalent to FM of its derivative. One can be generated from the other using an integrator or differentiator.

16. What is the modulation index of a PM signal?

    β_PM = kp·Am, where kp is the phase sensitivity (rad/volt) and Am is the peak message amplitude. Unlike FM, the PM index is independent of the modulating frequency fm.

17. How does the modulation index of FM versus PM vary with modulating frequency?

    In FM, β = Δf/fm decreases as fm increases (inversely proportional). In PM, β = kp·Am is independent of fm. This difference distinguishes the two schemes experimentally.

18. What is the difference between direct and indirect (Armstrong) methods of FM generation?

    Direct method varies a VCO/reactance modulator with the message (good deviation but poor carrier stability). Indirect (Armstrong) method generates NBFM via phase modulation of a crystal-stable carrier, then uses frequency multipliers to obtain WBFM (excellent stability, more complex).

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