Communication Systems and Modulation
What this domain tests
This section tests analog and digital communication: modulation, bandwidth, sidebands, noise, sampling, multiplexing, and basic receiver/transmitter concepts. Numerical items on modulation index and bandwidth are common.
Core concepts
- AM varies carrier amplitude. With peak carrier Vc and peak modulating Vm, modulation index m = Vm/Vc. Each sideband has amplitude mVc/2, and AM bandwidth is 2fm.
- Overmodulation occurs when m > 1; it causes envelope distortion and splatter. AM is inefficient because the carrier carries no information.
- FM varies carrier frequency. FM has constant amplitude, so it is more resistant to amplitude noise. Carson's rule estimates bandwidth as BW ≈ 2(Δf + fm).
- Digital modulation: ASK varies amplitude, FSK varies frequency, PSK varies phase. QPSK sends two bits per symbol.
- The Nyquist sampling theorem requires fs ≥ 2fm for a baseband signal to avoid aliasing. PCM converts samples into quantised, encoded digital words.
Worked example
An AM signal has Vc = 10 V peak and Vm = 6 V peak, with fm = 5 kHz. m = 6/10 = 0.6, or 60%. Each sideband amplitude is mVc/2 = 3 V. Bandwidth = 2fm = 10 kHz.
Traps
FM bandwidth is not simply 2fm; use Carson's rule when deviation is significant. Do not confuse modulation index with percentage modulation. Sampling below Nyquist causes aliasing, which cannot be removed by ordinary filtering after sampling.
Key points
- AM: m = Vm/Vc; BW = 2fm.
- Sideband amplitude = mVc/2.
- FM has constant amplitude and wider bandwidth.
- Nyquist: fs ≥ 2fm.
- QPSK carries two bits per symbol.