Communication Systems and Modern Physics Applications
The closing unit applies physics you already know - EM waves, semiconductors, optics - to communication and technology: the elements of a communication system, modulation, propagation of radio waves, optical fibres, satellites, and devices such as LEDs, photodiodes and lasers, together with applications of nuclear and semiconductor physics. Questions here are usually short answers or 1-mark definitions plus one numerical on antenna length or bandwidth.
Core rules
- Block diagram: information source → transducer → transmitter → channel → receiver → destination; noise enters through the channel.
- Modulation is needed because (i) antennas must be comparable to λ (a quarter-wave antenna has length λ/4), (ii) low-frequency signals cannot be radiated efficiently, and (iii) many signals must share one channel by frequency allocation.
- Amplitude modulation has bandwidth 2f(m) for a modulating frequency f(m); FM has wider bandwidth ≈ 2(Δf + f(m)) and better noise immunity.
- Propagation: ground wave below about 2 MHz, sky wave (3–30 MHz) by ionospheric reflection, space wave (above 30 MHz) line-of-sight including satellite links; TV and mobile use space waves.
- Optical fibre works by total internal reflection at the core–cladding boundary and offers very high bandwidth with low loss.
- Photodiode is reverse biased to detect light; LED is forward biased to emit light; a laser gives coherent, monochromatic, highly directional light.
Worked example
A carrier of 100 MHz is to be radiated using a quarter-wave antenna. Find its length. λ = c/f = (3×10⁸)/(100×10⁶) = 3 m. Quarter-wave length = λ/4 = 0.75 m. This is why high carrier frequencies are used: at audio frequencies near 20 kHz, λ would be 15 km and a quarter-wave antenna 3.75 km tall, which is impossible in practice. The reasoning in exam answers should always be this comparison of antenna size with wavelength.
Traps that cost marks
- Claiming sky-wave propagation serves television; it requires space waves (line of sight).
- Confusing AM and FM bandwidth, or stating that modulation reduces the information content.
- Reversing the biasing of LED (forward) and photodiode (reverse).
- Using λ = c/f with frequency in kHz or MHz without converting to hertz.