Communication Systems and Consumer Electronics
What the exam tests
Communication Systems and Consumer Electronics tests modulation, bandwidth, receivers and the block-level operation of everyday devices. Questions often ask you to identify sidebands, compare AM and FM, or trace a signal through a superheterodyne receiver.
Core rules
- AM: carrier amplitude varies; modulation index m=Vm/Vc; sidebands at fc±fm; bandwidth=2fm. Overmodulation (m>1) causes distortion.
- FM: carrier frequency varies; deviation Δf proportional to modulating amplitude; Carson's rule gives bandwidth≈2(Δf+fm). FM has better noise immunity but wider bandwidth.
- Superheterodyne receiver: RF amplifier, mixer, local oscillator, IF amplifier, detector, audio amplifier. The mixer produces the difference frequency, the IF (455 kHz for AM, 10.7 MHz for FM).
- Antenna length for a quarter-wave vertical is λ/4, with λ=c/f.
- Consumer electronics: SMPS chops at high frequency for efficiency; infrared remotes use modulated 38 kHz bursts; TV audio and video use separate carriers in older analogue systems.
Worked example
An AM broadcast station has a 1 MHz carrier modulated by a 5 kHz tone. The sidebands are at 1000-5=995 kHz and 1000+5=1005 kHz, so the bandwidth is 10 kHz. If the same 5 kHz tone frequency-modulates a carrier with 75 kHz deviation, Carson's rule gives BW=2(75+5)=160 kHz. This shows why FM stations are spaced much further apart than AM stations.
Traps
Counting only one sideband; confusing modulation index with percentage modulation; and mixing up the AM and FM IF values. Also remember that a mixer does not amplify-it translates frequency, and the IF stage provides most of the gain and selectivity.