Communication Electronics and Audio-Video Systems
Communication Electronics and Audio-Video Systems is examined through modulation, receivers, antennas, audio amplifiers and video signal paths.
What the exam tests
This domain covers modulation, radio receivers, antennas, audio amplifiers and video signal paths. Expect AM/FM bandwidth and frequency ranges, superheterodyne stages and IF values, antenna length calculations, amplifier class and speaker matching, and TV/HDMI resolution basics.
Core rules
- AM broadcast band in India: about 531–1602 kHz; FM: 88–108 MHz. AM bandwidth ≈ 2 × modulating frequency; FM uses Carson's rule: BW ≈ 2(Δf + fm).
- Superheterodyne receiver: RF amplifier → mixer → local oscillator → IF amplifier → detector → audio. AM IF = 455 kHz; FM IF = 10.7 MHz.
- Antenna: λ = c/f, with c = 3 × 10^8 m/s. A quarter-wave vertical is λ/4 and is common for FM.
- Audio amplifier classes: A conducts 360°, B 180°, AB between, D switches. Speaker impedance (4 Ω or 8 Ω) must match the amplifier rating; power ≈ V²/R for a sine RMS voltage.
- Video: composite/component are analog; HDMI carries digital audio/video. 1080p means 1920 × 1080 progressive; 16:9 is widescreen.
Worked example
An FM station transmits at 100 MHz. λ = 3 × 10^8 / 100 × 10^6 = 3 m. A quarter-wave antenna is 3/4 = 0.75 m. If the receiver uses an IF of 10.7 MHz and receives 100 MHz, the local oscillator for high-side injection is 100 + 10.7 = 110.7 MHz. The reasoning is: calculate wavelength from frequency, then apply the quarter-wave factor; for the receiver, add the IF because the mixer output must be the fixed IF.
Traps that cost marks
Using 300 instead of 3 × 10^8 without matching units; taking a full-wave antenna length when the question asks for quarter-wave; subtracting the IF when high-side injection is specified; confusing AM and FM IF values; rating speaker power from peak voltage instead of RMS; calling 1080i progressive.