Electronic Communication and Consumer Devices
9. Electronic Communication and Consumer Devices
This domain tests the communication chain - source, modulator, transmitter, channel, receiver - and the consumer equipment built from it.
Modulation and transmission
- Modulation superimposes information on a carrier because antennas radiate efficiently only at high frequency.
- AM varies carrier amplitude; bandwidth = 2f_m; sidebands at f_c ± f_m; the carrier itself carries no information.
- FM varies carrier frequency; the amplitude stays constant, so FM is less affected by amplitude noise but needs more bandwidth (Carson's rule).
- Propagation: ground wave, sky wave (ionospheric reflection), space wave (line of sight).
Receivers and devices
- The superheterodyne receiver mixes the incoming signal with a local oscillator to a fixed intermediate frequency (455 kHz for AM), then amplifies and detects it.
- Radio, television, mobile phones, microwave ovens, remote controls and audio systems are consumer applications of these blocks.
Worked example
An AM transmitter uses a 1 MHz carrier modulated by a 5 kHz audio tone. Upper sideband = 1000 + 5 = 1005 kHz; lower sideband = 1000 − 5 = 995 kHz. Bandwidth = 2 × 5 kHz = 10 kHz, so the transmitted band is 995–1005 kHz.
Traps
- Confusing modulation with amplification - modulation changes a characteristic of the carrier, not its power level.
- Using bandwidth = f_m instead of 2f_m for AM.
- Claiming FM has constant frequency (it is the amplitude that is constant).
- Forgetting that the mixer stage produces the difference frequency, not the original carrier.