Communication Systems and Modulation
This domain tests why modulation is needed, how AM and FM are described numerically, and how analogue and digital transmission compare.
Core concepts
- Modulation shrinks antenna size (λ = c/f), allows multiplexing, and moves signals into bands where propagation and noise performance are better.
- AM varies carrier amplitude. Modulation index m = (V_max − V_min)/(V_max + V_min). Sidebands lie at f_c ± f_m, so bandwidth = 2f_m. Total power P_t = P_c(1 + m²/2). m > 1 means overmodulation and envelope distortion.
- FM varies carrier frequency with constant amplitude; Carson's rule gives bandwidth ≈ 2(Δf + f_m). FM resists amplitude noise but needs more bandwidth.
- Propagation: ground wave (LF/MF), sky wave via the ionosphere (HF), space wave line-of-sight (VHF/UHF).
- Digital: sampling theorem f_s ≥ 2f_m; PCM samples, quantises and encodes; ASK, FSK and PSK shift amplitude, frequency and phase. A superheterodyne receiver mixes to a fixed IF (455 kHz for AM).
Worked example
An AM wave has V_max = 12 V and V_min = 4 V, with f_m = 5 kHz. Then m = (12 − 4)/(12 + 4) = 0.5. Bandwidth = 2 × 5 kHz = 10 kHz. If P_c = 1 kW, total power = 1000(1 + 0.25/2) = 1125 W.
Traps
Using m = (V_max − V_min)/(V_max + V_min) incorrectly by mixing peak and peak-to-peak values; quoting AM bandwidth as f_m instead of 2f_m; sampling below the Nyquist rate, which causes aliasing.