Communication Systems and Applied Physics
What the exam tests
Questions cover the communication block diagram, need for modulation, AM and FM, bandwidth, sidebands, modulation index, propagation modes, ionosphere, optical fibre and basic applied physics applications.
Core rules
- A communication system has an information source, transmitter, channel, receiver and destination. Modulation makes radiation efficient, reduces antenna height and allows multiplexing.
- AM varies carrier amplitude. Sidebands are at fc ± fm, so bandwidth is 2fm. Modulation index μ = Am/Ac; μ ≤ 1 avoids overmodulation.
- FM varies carrier frequency. Its bandwidth is approximately 2(Δf + fm), and it is more noise-resistant than AM.
- Ground wave follows the Earth surface, sky wave reflects from the ionosphere, and space wave is line-of-sight. Satellite communication uses microwaves.
- Optical fibre uses total internal reflection and has low loss and high bandwidth.
Worked example
An AM carrier of 1 MHz is modulated by a 5 kHz audio signal. The sidebands are at 1000 − 5 = 995 kHz and 1000 + 5 = 1005 kHz. The bandwidth is 2×5 = 10 kHz. If the carrier amplitude is 6 V and the modulating amplitude is 3 V, the modulation index is μ = 3/6 = 0.5, or 50%.
Traps
AM bandwidth is twice the modulating frequency, not equal to it. Modulation index above 1 causes distortion. FM bandwidth is not simply 2fm when deviation is large. Sky waves depend on the ionosphere and are not used for very high frequencies. Optical fibre works by total internal reflection, not by ordinary reflection at the cladding.