Communication Systems
Communication Systems covers the elements of a communication link, modulation, bandwidth and wave propagation. Questions are short-answer and definition-heavy, with an AM sideband numerical.
Core rules
- Block diagram: information source → transmitter → channel → receiver → destination. The channel may be wire, cable, optical fibre or free space.
- Bandwidth is the frequency range a channel must carry, and the signal bandwidth fixes the channel bandwidth. Voice occupies roughly 300–3100 Hz; a TV signal about 6 MHz.
- Modulation is needed to reduce antenna height (λ/4 becomes practical), allow multiplexing, shift the signal into a band suited to the channel, and improve the signal-to-noise ratio.
- AM: the carrier amplitude varies with the message. For carrier f_c and modulating frequency f_m, side frequencies are f_c ± f_m and bandwidth = 2f_m. Modulation index m = A_m/A_c; m > 1 causes overmodulation and distortion.
- FM: carrier frequency varies while amplitude stays constant; wider bandwidth but better noise immunity. Carson's rule gives bandwidth ≈ 2(Δf + f_m).
- Propagation: ground wave below about 2 MHz, skywave 3–30 MHz by ionospheric reflection, space wave above 30 MHz along the line of sight, used for TV, radar and satellites.
- Antenna height can be extended by repeater towers or satellites.
Worked example
An AM broadcast uses a 1000 kHz carrier modulated by a 5 kHz audio tone. Find the sideband frequencies and the bandwidth. Upper sideband = 1000 + 5 = 1005 kHz; lower sideband = 1000 − 5 = 995 kHz. Bandwidth = 1005 − 995 = 10 kHz = 2f_m, showing that the carrier itself carries no information.
Traps
Placing AM sidebands at 2f_c ± f_m; quoting bandwidth f_m instead of 2f_m; claiming FM is noisier than AM; using skywave for TV transmission; and confusing modulation with amplification.