Communication Systems and Consumer Electronics
What this domain covers
Communication Systems and Consumer Electronics covers the need for modulation, AM and FM characteristics, radio spectrum bands, transmission media and the working of everyday consumer devices.
Core rules
- Modulation transfers a baseband signal onto a carrier so it can be radiated efficiently by an antenna whose length is comparable with λ/4 or λ/2.
- AM varies carrier amplitude; the modulation index m = V_m/V_c must be ≤1 to avoid overmodulation. AM bandwidth = 2f_m; sidebands at f_c ± f_m. AM broadcast uses 535–1605 kHz.
- FM varies carrier frequency; amplitude stays constant, so FM is more noise-resistant. FM broadcast uses 88–108 MHz. Carson's rule: BW ≈ 2(Δf + f_m).
- Propagation: ground wave at low/medium frequencies, sky wave via the ionosphere at HF, line-of-sight at VHF/UHF and satellite.
- Consumer items: an IR remote modulates at about 38 kHz; a loudspeaker converts electrical to acoustic energy; Bluetooth and Wi-Fi use the 2.4 GHz ISM band.
Worked example
An AM transmitter has a 500 kHz carrier and a 5 kHz modulating tone with m = 0.6. Sidebands appear at 495 kHz and 505 kHz, the bandwidth is 2 × 5 = 10 kHz, and the total power is P_c(1 + m²/2) = P_c × 1.18.
Traps that cost marks
Saying FM bandwidth is fixed regardless of deviation; using single-sideband bandwidth for AM; allowing m > 1; and confusing carrier frequency with modulating frequency.