Communication Systems and Modern Applications
Communication Systems and Modern Applications covers modulation, bandwidth, antenna length, propagation, optical fibre, satellites, lasers and selected modern devices. The exam tests why modulation is needed and simple bandwidth or wavelength calculations.
Core rules
- A baseband signal cannot be radiated efficiently by a short antenna; modulation shifts it to a high-frequency carrier. Amplitude modulation varies carrier amplitude; frequency modulation varies carrier frequency.
- AM bandwidth = 2f_m, where f_m is the highest modulating frequency. FM bandwidth is wider and gives better noise immunity.
- Antenna length is comparable to λ/4 or λ/2. For a given frequency, λ = c/f.
- Ground-wave propagation suits low frequencies; sky-wave uses ionospheric reflection; space-wave/line-of-sight needs high towers or satellites. Optical fibre uses total internal reflection and has low loss and high bandwidth.
- A laser produces coherent, monochromatic, highly directional light. Satellites use microwave links because microwaves pass through the ionosphere.
Worked example
A carrier of 100 MHz is used. Its free-space wavelength is λ = c/f = (3×10⁸)/(100×10⁶) = 3.0 m. A quarter-wave antenna should be about 3.0/4 = 0.75 m. If the modulating signal has maximum frequency 5 kHz, the AM bandwidth is 2(5) = 10 kHz.
Traps
Using c = f/λ instead of c = fλ; forgetting the factor 2 in AM bandwidth; assuming ionospheric sky-wave works for all frequencies; and confusing modulation with amplification.