Kerala Plus Two Electronics (DHSE): Communication Systems and Modulation

Communication Systems and Modulation tests why modulation is needed, AM and FM waveforms, modulation index and power, receiver architecture, and basic digital modulation. Numerical work usually centres on AM power or FM bandwidth.

Core concepts

Need for modulation: reduces antenna size, allows frequency multiplexing, moves signals to assigned bands and can improve noise performance.
AM (DSB-FC): modulation index m = Vm/Vc using peak amplitudes. Total power Pt = Pc(1 + m²/2); total sideband power = Pc m²/2; each sideband = Pc m²/4; bandwidth = 2fm. Overmodulation occurs when m > 1 and causes envelope distortion.
FM: modulation index mf = Δf/fm; Carson’s bandwidth ≈ 2(Δf + fm). FM has constant amplitude, better noise immunity but wider bandwidth; limiters and discriminators are used.
Superheterodyne receiver: RF amplifier, mixer, local oscillator, fixed IF stage (455 kHz for AM, 10.7 MHz for FM), detector, AF amplifier and AGC.
Digital modulation: ASK, FSK and PSK. PCM samples at fs ≥ 2fm, quantises and encodes; bit rate = samples per second × bits per sample.
Multiplexing: FDM shares frequency, TDM shares time.

Worked example

An AM transmitter has carrier power Pc = 5 kW, carrier peak Vc = 10 V and modulating peak Vm = 6 V. Then m = 6/10 = 0.6. Total power is 5000(1 + 0.36/2) = 5000 × 1.18 = 5900 W. Total sideband power is 5900 − 5000 = 900 W, so each sideband carries about 450 W.

Traps that cost marks

Using rms instead of peak amplitudes to calculate m.
Writing AM bandwidth as fm instead of 2fm.
Using only 2Δf for FM bandwidth and omitting the modulating frequency in Carson’s rule.
Swapping the AM and FM intermediate frequencies (455 kHz and 10.7 MHz).
Adding sideband power to carrier power incorrectly; sidebands are part of total power.
Sampling below the Nyquist rate and expecting perfect reconstruction.