Communication Systems
What this domain tests
Need for modulation, AM and FM generation and detection, bandwidth and power calculations, and propagation and transmission media. Modulation index and power-distribution numericals appear regularly.
Core rules
- Modulation is needed to radiate efficiently with practical antennas (λ/4 monopole), to multiplex many signals and to move the signal away from low-frequency noise.
- AM produces carrier plus upper and lower sidebands. Modulation index m = Vm/Vc; bandwidth = 2fm; total power Pt = Pc(1 + m²/2); sideband power = Pc·m²/2.
- AM efficiency = m²/(2 + m²), which peaks at 33.3 % when m = 1. Overmodulation (m > 1) causes envelope distortion and must be avoided.
- FM keeps amplitude constant and varies frequency; modulation index mf = Δf/fm; Carson bandwidth ≈ 2(Δf + fm). FM has better noise immunity at the cost of wider bandwidth.
- Propagation: ground wave below about 2 MHz, sky wave 3–30 MHz by ionospheric refraction, space wave above 30 MHz line of sight. Optical fibre works by total internal reflection.
Worked example
An AM transmitter radiates 100 W carrier with m = 0.5. Sideband power = Pc·m²/2 = 100 × 0.125 = 12.5 W, so Pt = 112.5 W. Efficiency = 12.5/112.5 = 11.1 %, and the bandwidth for a 5 kHz tone is 2 × 5 = 10 kHz. Raising m to 1 would give efficiency 33.3 % and Pt = 150 W.
Traps that cost marks
- Using bandwidth = fm instead of 2fm for AM.
- Believing AM efficiency can approach 100 %.
- Forgetting that sideband power depends on m², not on m.
- Stating that FM carries information in amplitude changes.