Communication Systems and Modulation
This domain tests the need for modulation, AM and FM parameters, sidebands, bandwidth, and basic digital communication. Numerical questions often use modulation index and bandwidth formulas.
Core rules
- Modulation places information on a high-frequency carrier for efficient radiation and frequency separation.
- AM: carrier amplitude varies. Sidebands are at fc ± fm; bandwidth = 2fm. Modulation index m = Vm/Vc. m > 1 causes overmodulation and distortion.
- AM total power Pt = Pc(1 + m²/2) for a single-tone modulating signal; sideband power is Pt − Pc.
- FM: carrier frequency varies. Modulation index β = Δf/fm. Carson’s rule gives approximate bandwidth BW ≈ 2(Δf + fm).
- FM is more resistant to amplitude noise but needs more bandwidth than AM.
- Digital modulation: ASK varies amplitude, FSK varies frequency, PSK varies phase. Nyquist sampling requires fs ≥ 2fmax to avoid aliasing.
- A receiver needs selectivity, sensitivity and fidelity; superheterodyne receivers use mixing to a fixed intermediate frequency.
Worked example
An AM transmitter uses a 1 MHz carrier and a 5 kHz sine modulating signal. Upper sideband = 1000 + 5 = 1005 kHz; lower sideband = 995 kHz. Bandwidth = 2 × 5 kHz = 10 kHz. If Vm = 4 V and Vc = 10 V, m = 0.4. With Pc = 100 W, total power = 100(1 + 0.16/2) = 108 W.
Marks-losing traps
Do not use bandwidth = fm for AM; it is 2fm. Do not call m = 1 overmodulation; overmodulation begins above 1. In FM, bandwidth depends on both deviation and modulating frequency. In digital modulation, bit rate and bandwidth are not always equal. Aliasing cannot be removed by filtering after sampling.